kernel/time/tick-common.c | 16 ++++++++-------- kernel/time/tick-sched.c | 28 +++++++++++++++------------- 2 files changed, 23 insertions(+), 21 deletions(-)
mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug
reported by KCSAN.
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
---
During the rcutorture test on linux-next,
./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12"
following KCSAN BUG is reported:
[ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M
[ 35.400593] ^M
[ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M
[ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M
[ 35.407162] do_idle+0xf3/0x2a0^M
[ 35.408016] cpu_startup_entry+0x15/0x20^M
[ 35.409084] start_secondary+0x8f/0x90^M
[ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M
[ 35.411607] ^M
[ 35.412042] no locks held by swapper/3/0.^M
[ 35.413172] irq event stamp: 53048^M
[ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M
[ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M
[ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
[ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
[ 35.423685] ^M
[ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M
[ 35.425870] tick_nohz_next_event+0x233/0x2b0^M
[ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M
[ 35.428386] do_idle+0xf3/0x2a0^M
[ 35.429265] cpu_startup_entry+0x15/0x20^M
[ 35.430429] rest_init+0x20c/0x210^M
[ 35.431382] arch_call_rest_init+0xe/0x10^M
[ 35.432508] start_kernel+0x544/0x600^M
[ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M
fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE
Thanks
Zhouyi
--
kernel/time/tick-common.c | 16 ++++++++--------
kernel/time/tick-sched.c | 28 +++++++++++++++-------------
2 files changed, 23 insertions(+), 21 deletions(-)
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 46789356f856..a7753465a0be 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -84,7 +84,7 @@ int tick_is_oneshot_available(void)
*/
static void tick_periodic(int cpu)
{
- if (tick_do_timer_cpu == cpu) {
+ if (READ_ONCE(tick_do_timer_cpu) == cpu) {
raw_spin_lock(&jiffies_lock);
write_seqcount_begin(&jiffies_seq);
@@ -184,9 +184,9 @@ static void giveup_do_timer(void *info)
{
int cpu = *(unsigned int *)info;
- WARN_ON(tick_do_timer_cpu != smp_processor_id());
+ WARN_ON(READ_ONCE(tick_do_timer_cpu) != smp_processor_id());
- tick_do_timer_cpu = cpu;
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
}
static void tick_take_do_timer_from_boot(void)
@@ -217,8 +217,8 @@ static void tick_setup_device(struct tick_device *td,
* If no cpu took the do_timer update, assign it to
* this cpu:
*/
- if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
- tick_do_timer_cpu = cpu;
+ if (READ_ONCE(tick_do_timer_cpu) == TICK_DO_TIMER_BOOT) {
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
tick_next_period = ktime_get();
#ifdef CONFIG_NO_HZ_FULL
@@ -235,7 +235,7 @@ static void tick_setup_device(struct tick_device *td,
!tick_nohz_full_cpu(cpu)) {
tick_take_do_timer_from_boot();
tick_do_timer_boot_cpu = -1;
- WARN_ON(tick_do_timer_cpu != cpu);
+ WARN_ON(READ_ONCE(tick_do_timer_cpu) != cpu);
#endif
}
@@ -407,8 +407,8 @@ EXPORT_SYMBOL_GPL(tick_broadcast_oneshot_control);
*/
void tick_handover_do_timer(void)
{
- if (tick_do_timer_cpu == smp_processor_id())
- tick_do_timer_cpu = cpumask_first(cpu_online_mask);
+ if (READ_ONCE(tick_do_timer_cpu) == smp_processor_id())
+ WRITE_ONCE(tick_do_timer_cpu, cpumask_first(cpu_online_mask));
}
/*
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b0e3c9205946..2dac94b3187f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -186,16 +186,16 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
* If nohz_full is enabled, this should not happen because the
* tick_do_timer_cpu never relinquishes.
*/
- if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)) {
+ if (unlikely(READ_ONCE(tick_do_timer_cpu) == TICK_DO_TIMER_NONE)) {
#ifdef CONFIG_NO_HZ_FULL
WARN_ON_ONCE(tick_nohz_full_running);
#endif
- tick_do_timer_cpu = cpu;
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
}
#endif
/* Check, if the jiffies need an update */
- if (tick_do_timer_cpu == cpu)
+ if (READ_ONCE(tick_do_timer_cpu) == cpu)
tick_do_update_jiffies64(now);
/*
@@ -534,7 +534,7 @@ static int tick_nohz_cpu_down(unsigned int cpu)
* timers, workqueues, timekeeping, ...) on behalf of full dynticks
* CPUs. It must remain online when nohz full is enabled.
*/
- if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
+ if (tick_nohz_full_running && READ_ONCE(tick_do_timer_cpu) == cpu)
return -EBUSY;
return 0;
}
@@ -849,8 +849,9 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
* Otherwise we can sleep as long as we want.
*/
delta = timekeeping_max_deferment();
- if (cpu != tick_do_timer_cpu &&
- (tick_do_timer_cpu != TICK_DO_TIMER_NONE || !ts->do_timer_last))
+ if (cpu != READ_ONCE(tick_do_timer_cpu) &&
+ (READ_ONCE(tick_do_timer_cpu) != TICK_DO_TIMER_NONE
+ || !ts->do_timer_last))
delta = KTIME_MAX;
/* Calculate the next expiry time */
@@ -883,10 +884,10 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
* do_timer() never invoked. Keep track of the fact that it
* was the one which had the do_timer() duty last.
*/
- if (cpu == tick_do_timer_cpu) {
- tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ if (cpu == READ_ONCE(tick_do_timer_cpu)) {
+ WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
ts->do_timer_last = 1;
- } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
+ } else if (READ_ONCE(tick_do_timer_cpu) != TICK_DO_TIMER_NONE) {
ts->do_timer_last = 0;
}
@@ -1049,8 +1050,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
* invoked.
*/
if (unlikely(!cpu_online(cpu))) {
- if (cpu == tick_do_timer_cpu)
- tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ if (cpu == READ_ONCE(tick_do_timer_cpu))
+ WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
/*
* Make sure the CPU doesn't get fooled by obsolete tick
* deadline if it comes back online later.
@@ -1073,11 +1074,12 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
* Keep the tick alive to guarantee timekeeping progression
* if there are full dynticks CPUs around
*/
- if (tick_do_timer_cpu == cpu)
+ if (READ_ONCE(tick_do_timer_cpu) == cpu)
return false;
/* Should not happen for nohz-full */
- if (WARN_ON_ONCE(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
+ if (WARN_ON_ONCE(READ_ONCE(tick_do_timer_cpu)
+ == TICK_DO_TIMER_NONE))
return false;
}
--
2.34.1On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote: > mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug > reported by KCSAN. > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > --- > During the rcutorture test on linux-next, > ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12" > following KCSAN BUG is reported: > [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M > [ 35.400593] ^M > [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M > [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M > [ 35.407162] do_idle+0xf3/0x2a0^M > [ 35.408016] cpu_startup_entry+0x15/0x20^M > [ 35.409084] start_secondary+0x8f/0x90^M > [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M > [ 35.411607] ^M > [ 35.412042] no locks held by swapper/3/0.^M > [ 35.413172] irq event stamp: 53048^M > [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M > [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M > [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > [ 35.423685] ^M > [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M > [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M > [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M > [ 35.428386] do_idle+0xf3/0x2a0^M > [ 35.429265] cpu_startup_entry+0x15/0x20^M > [ 35.430429] rest_init+0x20c/0x210^M > [ 35.431382] arch_call_rest_init+0xe/0x10^M > [ 35.432508] start_kernel+0x544/0x600^M > [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M > > fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE This has been discussed before with passion: http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/ To me data_race() would be more appropriate but that would need a changelog with proper analysis of the tick_do_timer_cpu state machine. One more thing on my TODO list, but feel free to beat me at it :-) Thanks.
On Mon, Jan 09 2023 at 13:51, Frederic Weisbecker wrote:
> On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote:
>> fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE
>
> This has been discussed before with passion:
>
> http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/
>
> To me data_race() would be more appropriate but that would need a changelog with
> proper analysis of the tick_do_timer_cpu state machine.
Proper analysis and comments in the code are required independent of
READ/WRITE_ONCE() or data_race().
Thanks,
tglx
On Mon, Jan 09, 2023 at 01:51:29PM +0100, Frederic Weisbecker wrote: > On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote: > > mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug > > reported by KCSAN. > > > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > > --- > > During the rcutorture test on linux-next, > > ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12" > > following KCSAN BUG is reported: > > [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M > > [ 35.400593] ^M > > [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M > > [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M > > [ 35.407162] do_idle+0xf3/0x2a0^M > > [ 35.408016] cpu_startup_entry+0x15/0x20^M > > [ 35.409084] start_secondary+0x8f/0x90^M > > [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M > > [ 35.411607] ^M > > [ 35.412042] no locks held by swapper/3/0.^M > > [ 35.413172] irq event stamp: 53048^M > > [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M > > [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M > > [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > [ 35.423685] ^M > > [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M > > [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M > > [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M > > [ 35.428386] do_idle+0xf3/0x2a0^M > > [ 35.429265] cpu_startup_entry+0x15/0x20^M > > [ 35.430429] rest_init+0x20c/0x210^M > > [ 35.431382] arch_call_rest_init+0xe/0x10^M > > [ 35.432508] start_kernel+0x544/0x600^M > > [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M > > > > fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE > > This has been discussed before with passion: > > http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/ > > To me data_race() would be more appropriate but that would need a changelog with > proper analysis of the tick_do_timer_cpu state machine. Please also an analysis of why the compiler cannot do any destructive optimizations in this case. Maybe also comments. > One more thing on my TODO list, but feel free to beat me at it :-) I know that feeling! ;-) Thanx, Paul
On Mon, Jan 9, 2023 at 11:25 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > On Mon, Jan 09, 2023 at 01:51:29PM +0100, Frederic Weisbecker wrote: > > On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote: > > > mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug > > > reported by KCSAN. > > > > > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > > > --- > > > During the rcutorture test on linux-next, > > > ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12" > > > following KCSAN BUG is reported: > > > [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M > > > [ 35.400593] ^M > > > [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M > > > [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M > > > [ 35.407162] do_idle+0xf3/0x2a0^M > > > [ 35.408016] cpu_startup_entry+0x15/0x20^M > > > [ 35.409084] start_secondary+0x8f/0x90^M > > > [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M > > > [ 35.411607] ^M > > > [ 35.412042] no locks held by swapper/3/0.^M > > > [ 35.413172] irq event stamp: 53048^M > > > [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M > > > [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M > > > [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > > [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > > [ 35.423685] ^M > > > [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M > > > [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M > > > [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M > > > [ 35.428386] do_idle+0xf3/0x2a0^M > > > [ 35.429265] cpu_startup_entry+0x15/0x20^M > > > [ 35.430429] rest_init+0x20c/0x210^M > > > [ 35.431382] arch_call_rest_init+0xe/0x10^M > > > [ 35.432508] start_kernel+0x544/0x600^M > > > [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M > > > > > > fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE > > > > This has been discussed before with passion: > > > > http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/ > > > > To me data_race() would be more appropriate but that would need a changelog with > > proper analysis of the tick_do_timer_cpu state machine. > > Please also an analysis of why the compiler cannot do any destructive > optimizations in this case. Maybe also comments. I want to try the analysis above, as a newbie I have taught myself LLVM for 3 years in my spare time ;-) > > > One more thing on my TODO list, but feel free to beat me at it :-) Please take your time ;-) Please don't look my next possible email as a reminder ;-) > > I know that feeling! ;-) > Thanx, Zhouyi > Thanx, Paul
On Mon, Jan 9, 2023 at 11:59 PM Zhouyi Zhou <zhouzhouyi@gmail.com> wrote: > > On Mon, Jan 9, 2023 at 11:25 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > > > On Mon, Jan 09, 2023 at 01:51:29PM +0100, Frederic Weisbecker wrote: > > > On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote: > > > > mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug > > > > reported by KCSAN. > > > > > > > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > > > > --- > > > > During the rcutorture test on linux-next, > > > > ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12" > > > > following KCSAN BUG is reported: > > > > [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M > > > > [ 35.400593] ^M > > > > [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M > > > > [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M > > > > [ 35.407162] do_idle+0xf3/0x2a0^M > > > > [ 35.408016] cpu_startup_entry+0x15/0x20^M > > > > [ 35.409084] start_secondary+0x8f/0x90^M > > > > [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M > > > > [ 35.411607] ^M > > > > [ 35.412042] no locks held by swapper/3/0.^M > > > > [ 35.413172] irq event stamp: 53048^M > > > > [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M > > > > [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M > > > > [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > > > [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > > > [ 35.423685] ^M > > > > [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M > > > > [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M > > > > [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M > > > > [ 35.428386] do_idle+0xf3/0x2a0^M > > > > [ 35.429265] cpu_startup_entry+0x15/0x20^M > > > > [ 35.430429] rest_init+0x20c/0x210^M > > > > [ 35.431382] arch_call_rest_init+0xe/0x10^M > > > > [ 35.432508] start_kernel+0x544/0x600^M > > > > [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M > > > > > > > > fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE > > > > > > This has been discussed before with passion: > > > > > > http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/ > > > > > > To me data_race() would be more appropriate but that would need a changelog with > > > proper analysis of the tick_do_timer_cpu state machine. > > > > Please also an analysis of why the compiler cannot do any destructive > > optimizations in this case. Maybe also comments. > I want to try the analysis above, as a newbie I have taught myself > LLVM for 3 years in my spare time ;-) I am continuing to study LLVM/Clang's optimization behaviour on tick_do_timer_cpu, it is very interesting and fascinating, but as a newbie, I still need 3-4 week to go, could you wait for me a little while? ;-) Many Thanks Zhouyi > > > > > One more thing on my TODO list, but feel free to beat me at it :-) > Please take your time ;-) Please don't look my next possible email > as a reminder ;-) > > > > I know that feeling! ;-) > > > Thanx, Zhouyi > > Thanx, Paul
On Mon, Jan 23, 2023 at 08:44:25AM +0800, Zhouyi Zhou wrote: > On Mon, Jan 9, 2023 at 11:59 PM Zhouyi Zhou <zhouzhouyi@gmail.com> wrote: > > > > On Mon, Jan 9, 2023 at 11:25 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > > > > > On Mon, Jan 09, 2023 at 01:51:29PM +0100, Frederic Weisbecker wrote: > > > > On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote: > > > > > mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug > > > > > reported by KCSAN. > > > > > > > > > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > > > > > --- > > > > > During the rcutorture test on linux-next, > > > > > ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12" > > > > > following KCSAN BUG is reported: > > > > > [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M > > > > > [ 35.400593] ^M > > > > > [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M > > > > > [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M > > > > > [ 35.407162] do_idle+0xf3/0x2a0^M > > > > > [ 35.408016] cpu_startup_entry+0x15/0x20^M > > > > > [ 35.409084] start_secondary+0x8f/0x90^M > > > > > [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M > > > > > [ 35.411607] ^M > > > > > [ 35.412042] no locks held by swapper/3/0.^M > > > > > [ 35.413172] irq event stamp: 53048^M > > > > > [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M > > > > > [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M > > > > > [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > > > > [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > > > > [ 35.423685] ^M > > > > > [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M > > > > > [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M > > > > > [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M > > > > > [ 35.428386] do_idle+0xf3/0x2a0^M > > > > > [ 35.429265] cpu_startup_entry+0x15/0x20^M > > > > > [ 35.430429] rest_init+0x20c/0x210^M > > > > > [ 35.431382] arch_call_rest_init+0xe/0x10^M > > > > > [ 35.432508] start_kernel+0x544/0x600^M > > > > > [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M > > > > > > > > > > fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE > > > > > > > > This has been discussed before with passion: > > > > > > > > http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/ > > > > > > > > To me data_race() would be more appropriate but that would need a changelog with > > > > proper analysis of the tick_do_timer_cpu state machine. > > > > > > Please also an analysis of why the compiler cannot do any destructive > > > optimizations in this case. Maybe also comments. > > I want to try the analysis above, as a newbie I have taught myself > > LLVM for 3 years in my spare time ;-) > > I am continuing to study LLVM/Clang's optimization behaviour on > tick_do_timer_cpu, it is very interesting and fascinating, but as a > newbie, I still need 3-4 week to go, could you wait for me a little > while? ;-) Please do take the time to get it right and to get it correctly documented. ;-) Thanx, Paul > Many Thanks > Zhouyi > > > > > > > One more thing on my TODO list, but feel free to beat me at it :-) > > Please take your time ;-) Please don't look my next possible email > > as a reminder ;-) > > > > > > I know that feeling! ;-) > > > > > Thanx, Zhouyi > > > Thanx, Paul
On Mon, Jan 9, 2023 at 8:51 PM Frederic Weisbecker <frederic@kernel.org> wrote: > > On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote: > > mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug > > reported by KCSAN. > > > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > > --- > > During the rcutorture test on linux-next, > > ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12" > > following KCSAN BUG is reported: > > [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M > > [ 35.400593] ^M > > [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M > > [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M > > [ 35.407162] do_idle+0xf3/0x2a0^M > > [ 35.408016] cpu_startup_entry+0x15/0x20^M > > [ 35.409084] start_secondary+0x8f/0x90^M > > [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M > > [ 35.411607] ^M > > [ 35.412042] no locks held by swapper/3/0.^M > > [ 35.413172] irq event stamp: 53048^M > > [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M > > [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M > > [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M > > [ 35.423685] ^M > > [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M > > [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M > > [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M > > [ 35.428386] do_idle+0xf3/0x2a0^M > > [ 35.429265] cpu_startup_entry+0x15/0x20^M > > [ 35.430429] rest_init+0x20c/0x210^M > > [ 35.431382] arch_call_rest_init+0xe/0x10^M > > [ 35.432508] start_kernel+0x544/0x600^M > > [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M > > > > fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE > > This has been discussed before with passion: > > http://archive.lwn.net:8080/linux-kernel/1C65422C-FFA4-4651-893B-300FAF9C49DE@lca.pw/T/ > Thank you for your guidance, yes, the discussion is very passionate ;-) and I learned a lot from the above thread ;-) > To me data_race() would be more appropriate but that would need a changelog with > proper analysis of the tick_do_timer_cpu state machine. I Agree, and also I learned to do more rigorous technology research. > > One more thing on my TODO list, but feel free to beat me at it :-) Thank you for your concern ;-) Thanks Zhouyi > > Thanks.
On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote:
> mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug
> reported by KCSAN.
>
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
This does indeed get rid of some KCSAN complaints, thank you!!!
Tested-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> During the rcutorture test on linux-next,
> ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12"
> following KCSAN BUG is reported:
> [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M
> [ 35.400593] ^M
> [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M
> [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M
> [ 35.407162] do_idle+0xf3/0x2a0^M
> [ 35.408016] cpu_startup_entry+0x15/0x20^M
> [ 35.409084] start_secondary+0x8f/0x90^M
> [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M
> [ 35.411607] ^M
> [ 35.412042] no locks held by swapper/3/0.^M
> [ 35.413172] irq event stamp: 53048^M
> [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M
> [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M
> [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
> [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
> [ 35.423685] ^M
> [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M
> [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M
> [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M
> [ 35.428386] do_idle+0xf3/0x2a0^M
> [ 35.429265] cpu_startup_entry+0x15/0x20^M
> [ 35.430429] rest_init+0x20c/0x210^M
> [ 35.431382] arch_call_rest_init+0xe/0x10^M
> [ 35.432508] start_kernel+0x544/0x600^M
> [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M
>
> fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE
>
> Thanks
> Zhouyi
> --
> kernel/time/tick-common.c | 16 ++++++++--------
> kernel/time/tick-sched.c | 28 +++++++++++++++-------------
> 2 files changed, 23 insertions(+), 21 deletions(-)
>
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index 46789356f856..a7753465a0be 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -84,7 +84,7 @@ int tick_is_oneshot_available(void)
> */
> static void tick_periodic(int cpu)
> {
> - if (tick_do_timer_cpu == cpu) {
> + if (READ_ONCE(tick_do_timer_cpu) == cpu) {
> raw_spin_lock(&jiffies_lock);
> write_seqcount_begin(&jiffies_seq);
>
> @@ -184,9 +184,9 @@ static void giveup_do_timer(void *info)
> {
> int cpu = *(unsigned int *)info;
>
> - WARN_ON(tick_do_timer_cpu != smp_processor_id());
> + WARN_ON(READ_ONCE(tick_do_timer_cpu) != smp_processor_id());
>
> - tick_do_timer_cpu = cpu;
> + WRITE_ONCE(tick_do_timer_cpu, cpu);
> }
>
> static void tick_take_do_timer_from_boot(void)
> @@ -217,8 +217,8 @@ static void tick_setup_device(struct tick_device *td,
> * If no cpu took the do_timer update, assign it to
> * this cpu:
> */
> - if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
> - tick_do_timer_cpu = cpu;
> + if (READ_ONCE(tick_do_timer_cpu) == TICK_DO_TIMER_BOOT) {
> + WRITE_ONCE(tick_do_timer_cpu, cpu);
>
> tick_next_period = ktime_get();
> #ifdef CONFIG_NO_HZ_FULL
> @@ -235,7 +235,7 @@ static void tick_setup_device(struct tick_device *td,
> !tick_nohz_full_cpu(cpu)) {
> tick_take_do_timer_from_boot();
> tick_do_timer_boot_cpu = -1;
> - WARN_ON(tick_do_timer_cpu != cpu);
> + WARN_ON(READ_ONCE(tick_do_timer_cpu) != cpu);
> #endif
> }
>
> @@ -407,8 +407,8 @@ EXPORT_SYMBOL_GPL(tick_broadcast_oneshot_control);
> */
> void tick_handover_do_timer(void)
> {
> - if (tick_do_timer_cpu == smp_processor_id())
> - tick_do_timer_cpu = cpumask_first(cpu_online_mask);
> + if (READ_ONCE(tick_do_timer_cpu) == smp_processor_id())
> + WRITE_ONCE(tick_do_timer_cpu, cpumask_first(cpu_online_mask));
> }
>
> /*
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index b0e3c9205946..2dac94b3187f 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -186,16 +186,16 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
> * If nohz_full is enabled, this should not happen because the
> * tick_do_timer_cpu never relinquishes.
> */
> - if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)) {
> + if (unlikely(READ_ONCE(tick_do_timer_cpu) == TICK_DO_TIMER_NONE)) {
> #ifdef CONFIG_NO_HZ_FULL
> WARN_ON_ONCE(tick_nohz_full_running);
> #endif
> - tick_do_timer_cpu = cpu;
> + WRITE_ONCE(tick_do_timer_cpu, cpu);
> }
> #endif
>
> /* Check, if the jiffies need an update */
> - if (tick_do_timer_cpu == cpu)
> + if (READ_ONCE(tick_do_timer_cpu) == cpu)
> tick_do_update_jiffies64(now);
>
> /*
> @@ -534,7 +534,7 @@ static int tick_nohz_cpu_down(unsigned int cpu)
> * timers, workqueues, timekeeping, ...) on behalf of full dynticks
> * CPUs. It must remain online when nohz full is enabled.
> */
> - if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
> + if (tick_nohz_full_running && READ_ONCE(tick_do_timer_cpu) == cpu)
> return -EBUSY;
> return 0;
> }
> @@ -849,8 +849,9 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
> * Otherwise we can sleep as long as we want.
> */
> delta = timekeeping_max_deferment();
> - if (cpu != tick_do_timer_cpu &&
> - (tick_do_timer_cpu != TICK_DO_TIMER_NONE || !ts->do_timer_last))
> + if (cpu != READ_ONCE(tick_do_timer_cpu) &&
> + (READ_ONCE(tick_do_timer_cpu) != TICK_DO_TIMER_NONE
> + || !ts->do_timer_last))
> delta = KTIME_MAX;
>
> /* Calculate the next expiry time */
> @@ -883,10 +884,10 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> * do_timer() never invoked. Keep track of the fact that it
> * was the one which had the do_timer() duty last.
> */
> - if (cpu == tick_do_timer_cpu) {
> - tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> + if (cpu == READ_ONCE(tick_do_timer_cpu)) {
> + WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
> ts->do_timer_last = 1;
> - } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
> + } else if (READ_ONCE(tick_do_timer_cpu) != TICK_DO_TIMER_NONE) {
> ts->do_timer_last = 0;
> }
>
> @@ -1049,8 +1050,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
> * invoked.
> */
> if (unlikely(!cpu_online(cpu))) {
> - if (cpu == tick_do_timer_cpu)
> - tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> + if (cpu == READ_ONCE(tick_do_timer_cpu))
> + WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
> /*
> * Make sure the CPU doesn't get fooled by obsolete tick
> * deadline if it comes back online later.
> @@ -1073,11 +1074,12 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
> * Keep the tick alive to guarantee timekeeping progression
> * if there are full dynticks CPUs around
> */
> - if (tick_do_timer_cpu == cpu)
> + if (READ_ONCE(tick_do_timer_cpu) == cpu)
> return false;
>
> /* Should not happen for nohz-full */
> - if (WARN_ON_ONCE(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
> + if (WARN_ON_ONCE(READ_ONCE(tick_do_timer_cpu)
> + == TICK_DO_TIMER_NONE))
> return false;
> }
>
> --
> 2.34.1
>
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