kernel/printk/printk.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
Situation of schedule while atomic context can happen in a
scenario if CPU-Y executing a async probe of ufs and while
printing a line it is started spinning for console lock
after preemption disable on CPU-Y and later it got the handover
of console lock from CPU-X and in console_unlock() it get
schedule with preempt disable as console_may_schedule was one
and due to which do_cond_resched was one.
CPU-X CPU-Y
worker_thread
process_one_work
async_run_entry_fn
ufshcd_async_scan
ufshcd_device_init
really_probe+0x1c8 ufshcd_probe_hba
platform_probe+0xc0 ufshcd_config_mcq
qcom_geni_serial_probe+0x374 _dev_info
uart_add_one_port+0x10 __dev_printk
serial_ctrl_register_port+0x10 dev_printk_emit
serial_core_register_port+0x5dc dev_vprintk_emit
register_console+0x284 vprintk_emit
console_init_seq() preempt_disable();
console_trylock_spinning()
console_lock();
console_flush_all()
Hand over of console lock happen from CPU-X to CPU-Y
console_unlock()
console_lock+0x74 console_flush_all
down[jt]+0x40 __might_resched
__down+0x18
__down_common+0x68
___down_common+0xdc
schedule_timeout+0x4c
schedule+0x78
__schedule+0x6c0
Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
---
kernel/printk/printk.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f2444b581e16..8b666feff65d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3028,7 +3028,7 @@ void console_unlock(void)
* messages practically incapacitating the system. Therefore, create
* a local to use for the printing loop.
*/
- do_cond_resched = console_may_schedule;
+ do_cond_resched = in_atomic() || console_may_schedule;
do {
console_may_schedule = 0;
--
2.43.0.254.ga26002b62827On 2024-02-22, Mukesh Ojha <quic_mojha@quicinc.com> wrote: > Situation of schedule while atomic context can happen in a > scenario if CPU-Y executing a async probe of ufs and while > printing a line it is started spinning for console lock > after preemption disable on CPU-Y and later it got the handover > of console lock from CPU-X and in console_unlock() it get > schedule with preempt disable as console_may_schedule was one > and due to which do_cond_resched was one. Nice catch. But I think the below patch is the appropriate fix: John Ogness -------8<-------- Subject: [PATCH] printk: Update @console_may_schedule in console_trylock_spinning() console_trylock_spinning() may takeover the console lock from a scheduable context. Update @console_may_schedule to make sure it reflects a trylock acquire. Reported-by: Mukesh Ojha <quic_mojha@quicinc.com> Link: https://lore.kernel.org/lkml/20240222090538.23017-1-quic_mojha@quicinc.com Signed-off-by: John Ogness <john.ogness@linutronix.de> --- kernel/printk/printk.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 1685a71f3f71..1612b50b2374 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2020,6 +2020,12 @@ static int console_trylock_spinning(void) */ mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_); + /* + * Update @console_may_schedule for trylock because the previous + * owner may have been scheduable. + */ + console_may_schedule = 0; + return 1; } base-commit: e7081d5a9d976b84f61f497316d7c940a4a2e67a -- 2.30.2
On 2/22/2024 8:23 PM, John Ogness wrote: > On 2024-02-22, Mukesh Ojha <quic_mojha@quicinc.com> wrote: >> Situation of schedule while atomic context can happen in a >> scenario if CPU-Y executing a async probe of ufs and while >> printing a line it is started spinning for console lock >> after preemption disable on CPU-Y and later it got the handover >> of console lock from CPU-X and in console_unlock() it get >> schedule with preempt disable as console_may_schedule was one >> and due to which do_cond_resched was one. > > Nice catch. But I think the below patch is the appropriate fix: Thanks for the change @john, would you be sending this as proper patch. -Mukesh > > John Ogness > > -------8<-------- > Subject: [PATCH] printk: Update @console_may_schedule in > console_trylock_spinning() > > console_trylock_spinning() may takeover the console lock from a > scheduable context. Update @console_may_schedule to make sure it > reflects a trylock acquire. > > Reported-by: Mukesh Ojha <quic_mojha@quicinc.com> > Link: https://lore.kernel.org/lkml/20240222090538.23017-1-quic_mojha@quicinc.com > Signed-off-by: John Ogness <john.ogness@linutronix.de> > --- > kernel/printk/printk.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 1685a71f3f71..1612b50b2374 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -2020,6 +2020,12 @@ static int console_trylock_spinning(void) > */ > mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_); > > + /* > + * Update @console_may_schedule for trylock because the previous > + * owner may have been scheduable. > + */ > + console_may_schedule = 0; > + > return 1; > } > > > base-commit: e7081d5a9d976b84f61f497316d7c940a4a2e67a
console_trylock_spinning() may takeover the console lock from a
schedulable context. Update @console_may_schedule to make sure it
reflects a trylock acquire.
Reported-by: Mukesh Ojha <quic_mojha@quicinc.com>
Link: https://lore.kernel.org/lkml/20240222090538.23017-1-quic_mojha@quicinc.com
Fixes: dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1685a71f3f71..1612b50b2374 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2020,6 +2020,12 @@ static int console_trylock_spinning(void)
*/
mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
+ /*
+ * Update @console_may_schedule for trylock because the previous
+ * owner may have been schedulable.
+ */
+ console_may_schedule = 0;
+
return 1;
}
base-commit: e7081d5a9d976b84f61f497316d7c940a4a2e67a
--
2.30.2On Mon 2024-02-26 13:07:24, John Ogness wrote:
> console_trylock_spinning() may takeover the console lock from a
> schedulable context. Update @console_may_schedule to make sure it
> reflects a trylock acquire.
>
> Reported-by: Mukesh Ojha <quic_mojha@quicinc.com>
> Link: https://lore.kernel.org/lkml/20240222090538.23017-1-quic_mojha@quicinc.com
> Fixes: dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes")
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
Makes perfect sense:
Reviewed-by: Petr Mladek <pmladek@suse.com>
JFYI, the patch has been committed into printk/linux.git, for-6.9
branch.
I am going to give it a spin in for-next and get it into 6.9
either in the 2nd half of the merge window or in rc1.
Best Regards,
Petr
On 2/26/2024 5:31 PM, John Ogness wrote:
> console_trylock_spinning() may takeover the console lock from a
> schedulable context. Update @console_may_schedule to make sure it
> reflects a trylock acquire.
>
> Reported-by: Mukesh Ojha <quic_mojha@quicinc.com>
> Link: https://lore.kernel.org/lkml/20240222090538.23017-1-quic_mojha@quicinc.com
> Fixes: dbdda842fe96 ("printk: Add console owner and waiter logic to load balance console writes")
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
Thanks for prompt response..
Yes, this looks fine..
As spinning code runs with preemption disabled context
and should reset the console_may_schedule to 0 .
what if console_trylock_spinning() gets the lock which makes
console_may_schedule =1 and it is still schedulable ?
-Mukesh
> ---
> kernel/printk/printk.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 1685a71f3f71..1612b50b2374 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2020,6 +2020,12 @@ static int console_trylock_spinning(void)
> */
> mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
>
> + /*
> + * Update @console_may_schedule for trylock because the previous
> + * owner may have been schedulable.
> + */
> + console_may_schedule = 0;
> +
> return 1;
> }
>
>
> base-commit: e7081d5a9d976b84f61f497316d7c940a4a2e67a
On 2024-02-26, Mukesh Ojha <quic_mojha@quicinc.com> wrote: > what if console_trylock_spinning() gets the lock which makes > console_may_schedule =1 and it is still schedulable ? I am afraid I do not understand the question. console_trylock_spinning() is only called from the printk caller context. In this context, console_may_schedule is always set to 0. Only if another context acquires the console lock per sleeping wait, console_lock(), can console_may_schedule be set to 1. Note that the value of console_may_schedule is only relevant for the console lock owner when console_unlock() is called. That is why its value is set when locking the console (or, with this patch, when transferring console lock ownerhip). John
On 2/26/2024 6:32 PM, John Ogness wrote: > On 2024-02-26, Mukesh Ojha <quic_mojha@quicinc.com> wrote: >> what if console_trylock_spinning() gets the lock which makes >> console_may_schedule =1 and it is still schedulable ? > > I am afraid I do not understand the question. > > console_trylock_spinning() is only called from the printk caller > context. In this context, console_may_schedule is always set to 0. > > Only if another context acquires the console lock per sleeping wait, > console_lock(), can console_may_schedule be set to 1. > > Note that the value of console_may_schedule is only relevant for the > console lock owner when console_unlock() is called. That is why its > value is set when locking the console (or, with this patch, when > transferring console lock ownerhip). I overlooked it, thanks. Patch LGTM. Reviewed-by: Mukesh Ojha <quic_mojha@quicinc.com> -Mukesh > > John
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