Lock scenario print is always a weak spot of lockdep splats. Improvement
can be made if we rework the dependency search and the error printing.
However without touching the graph search, we can improve a little for
the circular deadlock case, since we have the to-be-added lock
dependency, and know whether these two locks are read/write/sync.
In order to know whether a held_lock is sync or not, a bit was
"stolen" from ->references, which reduce our limit for the same lock
class nesting from 2^12 to 2^11, and it should still be good enough.
Besides, since we now have bit in held_lock for sync, we don't need the
"hardirqoffs being 1" trick, and also we can avoid the __lock_release()
if we jump out of __lock_acquire() before the held_lock stored.
With these changes, a deadlock case evolved with read lock and sync gets
a better print-out from:
[...] Possible unsafe locking scenario:
[...]
[...] CPU0 CPU1
[...] ---- ----
[...] lock(srcuA);
[...] lock(srcuB);
[...] lock(srcuA);
[...] lock(srcuB);
to
[...] Possible unsafe locking scenario:
[...]
[...] CPU0 CPU1
[...] ---- ----
[...] rlock(srcuA);
[...] lock(srcuB);
[...] lock(srcuA);
[...] sync(srcuB);
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
include/linux/lockdep.h | 3 ++-
kernel/locking/lockdep.c | 48 ++++++++++++++++++++++++++--------------
2 files changed, 34 insertions(+), 17 deletions(-)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index ba09df6a0872..febd7ecc225c 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -134,7 +134,8 @@ struct held_lock {
unsigned int read:2; /* see lock_acquire() comment */
unsigned int check:1; /* see lock_acquire() comment */
unsigned int hardirqs_off:1;
- unsigned int references:12; /* 32 bits */
+ unsigned int sync:1;
+ unsigned int references:11; /* 32 bits */
unsigned int pin_count;
};
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index cffa026a765f..4031d87f6829 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1880,6 +1880,8 @@ print_circular_lock_scenario(struct held_lock *src,
struct lock_class *source = hlock_class(src);
struct lock_class *target = hlock_class(tgt);
struct lock_class *parent = prt->class;
+ int src_read = src->read;
+ int tgt_read = tgt->read;
/*
* A direct locking problem where unsafe_class lock is taken
@@ -1907,7 +1909,10 @@ print_circular_lock_scenario(struct held_lock *src,
printk(" Possible unsafe locking scenario:\n\n");
printk(" CPU0 CPU1\n");
printk(" ---- ----\n");
- printk(" lock(");
+ if (tgt_read != 0)
+ printk(" rlock(");
+ else
+ printk(" lock(");
__print_lock_name(target);
printk(KERN_CONT ");\n");
printk(" lock(");
@@ -1916,7 +1921,12 @@ print_circular_lock_scenario(struct held_lock *src,
printk(" lock(");
__print_lock_name(target);
printk(KERN_CONT ");\n");
- printk(" lock(");
+ if (src_read != 0)
+ printk(" rlock(");
+ else if (src->sync)
+ printk(" sync(");
+ else
+ printk(" lock(");
__print_lock_name(source);
printk(KERN_CONT ");\n");
printk("\n *** DEADLOCK ***\n\n");
@@ -4530,7 +4540,13 @@ mark_usage(struct task_struct *curr, struct held_lock *hlock, int check)
return 0;
}
}
- if (!hlock->hardirqs_off) {
+
+ /*
+ * For lock_sync(), don't mark the ENABLED usage, since lock_sync()
+ * creates no critical section and no extra dependency can be introduced
+ * by interrupts
+ */
+ if (!hlock->hardirqs_off && !hlock->sync) {
if (hlock->read) {
if (!mark_lock(curr, hlock,
LOCK_ENABLED_HARDIRQ_READ))
@@ -4909,7 +4925,7 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
int trylock, int read, int check, int hardirqs_off,
struct lockdep_map *nest_lock, unsigned long ip,
- int references, int pin_count)
+ int references, int pin_count, int sync)
{
struct task_struct *curr = current;
struct lock_class *class = NULL;
@@ -4960,7 +4976,8 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
class_idx = class - lock_classes;
- if (depth) { /* we're holding locks */
+ if (depth && !sync) {
+ /* we're holding locks and the new held lock is not a sync */
hlock = curr->held_locks + depth - 1;
if (hlock->class_idx == class_idx && nest_lock) {
if (!references)
@@ -4994,6 +5011,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
hlock->trylock = trylock;
hlock->read = read;
hlock->check = check;
+ hlock->sync = !!sync;
hlock->hardirqs_off = !!hardirqs_off;
hlock->references = references;
#ifdef CONFIG_LOCK_STAT
@@ -5055,6 +5073,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
if (!validate_chain(curr, hlock, chain_head, chain_key))
return 0;
+ /* For lock_sync(), we are done here since no actual critical section */
+ if (hlock->sync)
+ return 1;
+
curr->curr_chain_key = chain_key;
curr->lockdep_depth++;
check_chain_key(curr);
@@ -5196,7 +5218,7 @@ static int reacquire_held_locks(struct task_struct *curr, unsigned int depth,
hlock->read, hlock->check,
hlock->hardirqs_off,
hlock->nest_lock, hlock->acquire_ip,
- hlock->references, hlock->pin_count)) {
+ hlock->references, hlock->pin_count, 0)) {
case 0:
return 1;
case 1:
@@ -5666,7 +5688,7 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
lockdep_recursion_inc();
__lock_acquire(lock, subclass, trylock, read, check,
- irqs_disabled_flags(flags), nest_lock, ip, 0, 0);
+ irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 0);
lockdep_recursion_finish();
raw_local_irq_restore(flags);
}
@@ -5699,11 +5721,6 @@ EXPORT_SYMBOL_GPL(lock_release);
* APIs are used to wait for one or multiple critical sections (on other CPUs
* or threads), and it means that calling these APIs inside these critical
* sections is potential deadlock.
- *
- * This annotation acts as an acqurie+release anontation pair with hardirqoff
- * being 1. Since there's no critical section, no interrupt can create extra
- * dependencies "inside" the annotation, hardirqoff == 1 allows us to avoid
- * false positives.
*/
void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
int check, struct lockdep_map *nest_lock, unsigned long ip)
@@ -5717,10 +5734,9 @@ void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
check_flags(flags);
lockdep_recursion_inc();
- __lock_acquire(lock, subclass, 0, read, check, 1, nest_lock, ip, 0, 0);
-
- if (__lock_release(lock, ip))
- check_chain_key(current);
+ __lock_acquire(lock, subclass, 0, read, check,
+ irqs_disabled_flags(flags), nest_lock, ip, 0, 0, 1);
+ check_chain_key(current);
lockdep_recursion_finish();
raw_local_irq_restore(flags);
}
--
2.38.1On 1/13/23 18:57, Boqun Feng wrote:
> Lock scenario print is always a weak spot of lockdep splats. Improvement
> can be made if we rework the dependency search and the error printing.
>
> However without touching the graph search, we can improve a little for
> the circular deadlock case, since we have the to-be-added lock
> dependency, and know whether these two locks are read/write/sync.
>
> In order to know whether a held_lock is sync or not, a bit was
> "stolen" from ->references, which reduce our limit for the same lock
> class nesting from 2^12 to 2^11, and it should still be good enough.
>
> Besides, since we now have bit in held_lock for sync, we don't need the
> "hardirqoffs being 1" trick, and also we can avoid the __lock_release()
> if we jump out of __lock_acquire() before the held_lock stored.
>
> With these changes, a deadlock case evolved with read lock and sync gets
> a better print-out from:
>
> [...] Possible unsafe locking scenario:
> [...]
> [...] CPU0 CPU1
> [...] ---- ----
> [...] lock(srcuA);
> [...] lock(srcuB);
> [...] lock(srcuA);
> [...] lock(srcuB);
>
> to
>
> [...] Possible unsafe locking scenario:
> [...]
> [...] CPU0 CPU1
> [...] ---- ----
> [...] rlock(srcuA);
> [...] lock(srcuB);
> [...] lock(srcuA);
> [...] sync(srcuB);
>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> include/linux/lockdep.h | 3 ++-
> kernel/locking/lockdep.c | 48 ++++++++++++++++++++++++++--------------
> 2 files changed, 34 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index ba09df6a0872..febd7ecc225c 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -134,7 +134,8 @@ struct held_lock {
> unsigned int read:2; /* see lock_acquire() comment */
> unsigned int check:1; /* see lock_acquire() comment */
> unsigned int hardirqs_off:1;
> - unsigned int references:12; /* 32 bits */
> + unsigned int sync:1;
> + unsigned int references:11; /* 32 bits */
> unsigned int pin_count;
> };
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index cffa026a765f..4031d87f6829 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1880,6 +1880,8 @@ print_circular_lock_scenario(struct held_lock *src,
> struct lock_class *source = hlock_class(src);
> struct lock_class *target = hlock_class(tgt);
> struct lock_class *parent = prt->class;
> + int src_read = src->read;
> + int tgt_read = tgt->read;
>
> /*
> * A direct locking problem where unsafe_class lock is taken
> @@ -1907,7 +1909,10 @@ print_circular_lock_scenario(struct held_lock *src,
> printk(" Possible unsafe locking scenario:\n\n");
> printk(" CPU0 CPU1\n");
> printk(" ---- ----\n");
> - printk(" lock(");
> + if (tgt_read != 0)
> + printk(" rlock(");
> + else
> + printk(" lock(");
> __print_lock_name(target);
> printk(KERN_CONT ");\n");
> printk(" lock(");
> @@ -1916,7 +1921,12 @@ print_circular_lock_scenario(struct held_lock *src,
> printk(" lock(");
> __print_lock_name(target);
> printk(KERN_CONT ");\n");
> - printk(" lock(");
> + if (src_read != 0)
> + printk(" rlock(");
> + else if (src->sync)
> + printk(" sync(");
> + else
> + printk(" lock(");
> __print_lock_name(source);
> printk(KERN_CONT ");\n");
> printk("\n *** DEADLOCK ***\n\n");
src can be sync() but not the target. Is there a reason why that is the
case?
> @@ -4530,7 +4540,13 @@ mark_usage(struct task_struct *curr, struct held_lock *hlock, int check)
> return 0;
> }
> }
> - if (!hlock->hardirqs_off) {
> +
> + /*
> + * For lock_sync(), don't mark the ENABLED usage, since lock_sync()
> + * creates no critical section and no extra dependency can be introduced
> + * by interrupts
> + */
> + if (!hlock->hardirqs_off && !hlock->sync) {
> if (hlock->read) {
> if (!mark_lock(curr, hlock,
> LOCK_ENABLED_HARDIRQ_READ))
> @@ -4909,7 +4925,7 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
> static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> int trylock, int read, int check, int hardirqs_off,
> struct lockdep_map *nest_lock, unsigned long ip,
> - int references, int pin_count)
> + int references, int pin_count, int sync)
> {
> struct task_struct *curr = current;
> struct lock_class *class = NULL;
> @@ -4960,7 +4976,8 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>
> class_idx = class - lock_classes;
>
> - if (depth) { /* we're holding locks */
> + if (depth && !sync) {
> + /* we're holding locks and the new held lock is not a sync */
> hlock = curr->held_locks + depth - 1;
> if (hlock->class_idx == class_idx && nest_lock) {
> if (!references)
> @@ -4994,6 +5011,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> hlock->trylock = trylock;
> hlock->read = read;
> hlock->check = check;
> + hlock->sync = !!sync;
> hlock->hardirqs_off = !!hardirqs_off;
> hlock->references = references;
> #ifdef CONFIG_LOCK_STAT
> @@ -5055,6 +5073,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> if (!validate_chain(curr, hlock, chain_head, chain_key))
> return 0;
>
> + /* For lock_sync(), we are done here since no actual critical section */
> + if (hlock->sync)
> + return 1;
> +
> curr->curr_chain_key = chain_key;
> curr->lockdep_depth++;
> check_chain_key(curr);
Even with sync, there is still a corresponding lock_acquire() and
lock_release(), you can't exit here without increasing lockdep_depth.
That can cause underflow.
Cheers,
Longman
On Mon, Jan 16, 2023 at 05:21:09PM -0500, Waiman Long wrote:
> On 1/13/23 18:57, Boqun Feng wrote:
> > Lock scenario print is always a weak spot of lockdep splats. Improvement
> > can be made if we rework the dependency search and the error printing.
> >
> > However without touching the graph search, we can improve a little for
> > the circular deadlock case, since we have the to-be-added lock
> > dependency, and know whether these two locks are read/write/sync.
> >
> > In order to know whether a held_lock is sync or not, a bit was
> > "stolen" from ->references, which reduce our limit for the same lock
> > class nesting from 2^12 to 2^11, and it should still be good enough.
> >
> > Besides, since we now have bit in held_lock for sync, we don't need the
> > "hardirqoffs being 1" trick, and also we can avoid the __lock_release()
> > if we jump out of __lock_acquire() before the held_lock stored.
> >
> > With these changes, a deadlock case evolved with read lock and sync gets
> > a better print-out from:
> >
> > [...] Possible unsafe locking scenario:
> > [...]
> > [...] CPU0 CPU1
> > [...] ---- ----
> > [...] lock(srcuA);
> > [...] lock(srcuB);
> > [...] lock(srcuA);
> > [...] lock(srcuB);
> >
> > to
> >
> > [...] Possible unsafe locking scenario:
> > [...]
> > [...] CPU0 CPU1
> > [...] ---- ----
> > [...] rlock(srcuA);
> > [...] lock(srcuB);
> > [...] lock(srcuA);
> > [...] sync(srcuB);
> >
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> > include/linux/lockdep.h | 3 ++-
> > kernel/locking/lockdep.c | 48 ++++++++++++++++++++++++++--------------
> > 2 files changed, 34 insertions(+), 17 deletions(-)
> >
> > diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> > index ba09df6a0872..febd7ecc225c 100644
> > --- a/include/linux/lockdep.h
> > +++ b/include/linux/lockdep.h
> > @@ -134,7 +134,8 @@ struct held_lock {
> > unsigned int read:2; /* see lock_acquire() comment */
> > unsigned int check:1; /* see lock_acquire() comment */
> > unsigned int hardirqs_off:1;
> > - unsigned int references:12; /* 32 bits */
> > + unsigned int sync:1;
> > + unsigned int references:11; /* 32 bits */
> > unsigned int pin_count;
> > };
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index cffa026a765f..4031d87f6829 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -1880,6 +1880,8 @@ print_circular_lock_scenario(struct held_lock *src,
> > struct lock_class *source = hlock_class(src);
> > struct lock_class *target = hlock_class(tgt);
> > struct lock_class *parent = prt->class;
> > + int src_read = src->read;
> > + int tgt_read = tgt->read;
> > /*
> > * A direct locking problem where unsafe_class lock is taken
> > @@ -1907,7 +1909,10 @@ print_circular_lock_scenario(struct held_lock *src,
> > printk(" Possible unsafe locking scenario:\n\n");
> > printk(" CPU0 CPU1\n");
> > printk(" ---- ----\n");
> > - printk(" lock(");
> > + if (tgt_read != 0)
> > + printk(" rlock(");
> > + else
> > + printk(" lock(");
> > __print_lock_name(target);
> > printk(KERN_CONT ");\n");
> > printk(" lock(");
> > @@ -1916,7 +1921,12 @@ print_circular_lock_scenario(struct held_lock *src,
> > printk(" lock(");
> > __print_lock_name(target);
> > printk(KERN_CONT ");\n");
> > - printk(" lock(");
> > + if (src_read != 0)
> > + printk(" rlock(");
> > + else if (src->sync)
> > + printk(" sync(");
> > + else
> > + printk(" lock(");
> > __print_lock_name(source);
> > printk(KERN_CONT ");\n");
> > printk("\n *** DEADLOCK ***\n\n");
>
> src can be sync() but not the target. Is there a reason why that is the
> case?
>
The functions annotated by sync() don't create real critical sections,
so no lock dependency can be created from a sync(), for example:
synchronize_srcu(A);
mutex_lock(B);
no dependency from A to B. In the scenario case, if we see a dependency
target -> source, the target cannot be a lock_sync(). I will add better
documentation later.
>
> > @@ -4530,7 +4540,13 @@ mark_usage(struct task_struct *curr, struct held_lock *hlock, int check)
> > return 0;
> > }
> > }
> > - if (!hlock->hardirqs_off) {
> > +
> > + /*
> > + * For lock_sync(), don't mark the ENABLED usage, since lock_sync()
> > + * creates no critical section and no extra dependency can be introduced
> > + * by interrupts
> > + */
> > + if (!hlock->hardirqs_off && !hlock->sync) {
> > if (hlock->read) {
> > if (!mark_lock(curr, hlock,
> > LOCK_ENABLED_HARDIRQ_READ))
> > @@ -4909,7 +4925,7 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
> > static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> > int trylock, int read, int check, int hardirqs_off,
> > struct lockdep_map *nest_lock, unsigned long ip,
> > - int references, int pin_count)
> > + int references, int pin_count, int sync)
> > {
> > struct task_struct *curr = current;
> > struct lock_class *class = NULL;
> > @@ -4960,7 +4976,8 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> > class_idx = class - lock_classes;
> > - if (depth) { /* we're holding locks */
> > + if (depth && !sync) {
> > + /* we're holding locks and the new held lock is not a sync */
> > hlock = curr->held_locks + depth - 1;
> > if (hlock->class_idx == class_idx && nest_lock) {
> > if (!references)
> > @@ -4994,6 +5011,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> > hlock->trylock = trylock;
> > hlock->read = read;
> > hlock->check = check;
> > + hlock->sync = !!sync;
> > hlock->hardirqs_off = !!hardirqs_off;
> > hlock->references = references;
> > #ifdef CONFIG_LOCK_STAT
> > @@ -5055,6 +5073,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> > if (!validate_chain(curr, hlock, chain_head, chain_key))
> > return 0;
> > + /* For lock_sync(), we are done here since no actual critical section */
> > + if (hlock->sync)
> > + return 1;
> > +
> > curr->curr_chain_key = chain_key;
> > curr->lockdep_depth++;
> > check_chain_key(curr);
>
> Even with sync, there is still a corresponding lock_acquire() and
> lock_release(), you can't exit here without increasing lockdep_depth. That
> can cause underflow.
>
I actually remove the __lock_release() in lock_sync() in this patch, so
I think it's OK. But I must admit the whole submission is to give David
something to see whether the output is an improvement, so I probably
should separate the output changes and the lock_sync() internall into
two patches (and the later can also be folded into the introduction
patch).
Regards,
Boqun
> Cheers,
> Longman
>
On 1/16/23 17:35, Boqun Feng wrote:
> On Mon, Jan 16, 2023 at 05:21:09PM -0500, Waiman Long wrote:
>> On 1/13/23 18:57, Boqun Feng wrote:
>>> Lock scenario print is always a weak spot of lockdep splats. Improvement
>>> can be made if we rework the dependency search and the error printing.
>>>
>>> However without touching the graph search, we can improve a little for
>>> the circular deadlock case, since we have the to-be-added lock
>>> dependency, and know whether these two locks are read/write/sync.
>>>
>>> In order to know whether a held_lock is sync or not, a bit was
>>> "stolen" from ->references, which reduce our limit for the same lock
>>> class nesting from 2^12 to 2^11, and it should still be good enough.
>>>
>>> Besides, since we now have bit in held_lock for sync, we don't need the
>>> "hardirqoffs being 1" trick, and also we can avoid the __lock_release()
>>> if we jump out of __lock_acquire() before the held_lock stored.
>>>
>>> With these changes, a deadlock case evolved with read lock and sync gets
>>> a better print-out from:
>>>
>>> [...] Possible unsafe locking scenario:
>>> [...]
>>> [...] CPU0 CPU1
>>> [...] ---- ----
>>> [...] lock(srcuA);
>>> [...] lock(srcuB);
>>> [...] lock(srcuA);
>>> [...] lock(srcuB);
>>>
>>> to
>>>
>>> [...] Possible unsafe locking scenario:
>>> [...]
>>> [...] CPU0 CPU1
>>> [...] ---- ----
>>> [...] rlock(srcuA);
>>> [...] lock(srcuB);
>>> [...] lock(srcuA);
>>> [...] sync(srcuB);
>>>
>>> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
>>> ---
>>> include/linux/lockdep.h | 3 ++-
>>> kernel/locking/lockdep.c | 48 ++++++++++++++++++++++++++--------------
>>> 2 files changed, 34 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
>>> index ba09df6a0872..febd7ecc225c 100644
>>> --- a/include/linux/lockdep.h
>>> +++ b/include/linux/lockdep.h
>>> @@ -134,7 +134,8 @@ struct held_lock {
>>> unsigned int read:2; /* see lock_acquire() comment */
>>> unsigned int check:1; /* see lock_acquire() comment */
>>> unsigned int hardirqs_off:1;
>>> - unsigned int references:12; /* 32 bits */
>>> + unsigned int sync:1;
>>> + unsigned int references:11; /* 32 bits */
>>> unsigned int pin_count;
>>> };
>>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>>> index cffa026a765f..4031d87f6829 100644
>>> --- a/kernel/locking/lockdep.c
>>> +++ b/kernel/locking/lockdep.c
>>> @@ -1880,6 +1880,8 @@ print_circular_lock_scenario(struct held_lock *src,
>>> struct lock_class *source = hlock_class(src);
>>> struct lock_class *target = hlock_class(tgt);
>>> struct lock_class *parent = prt->class;
>>> + int src_read = src->read;
>>> + int tgt_read = tgt->read;
>>> /*
>>> * A direct locking problem where unsafe_class lock is taken
>>> @@ -1907,7 +1909,10 @@ print_circular_lock_scenario(struct held_lock *src,
>>> printk(" Possible unsafe locking scenario:\n\n");
>>> printk(" CPU0 CPU1\n");
>>> printk(" ---- ----\n");
>>> - printk(" lock(");
>>> + if (tgt_read != 0)
>>> + printk(" rlock(");
>>> + else
>>> + printk(" lock(");
>>> __print_lock_name(target);
>>> printk(KERN_CONT ");\n");
>>> printk(" lock(");
>>> @@ -1916,7 +1921,12 @@ print_circular_lock_scenario(struct held_lock *src,
>>> printk(" lock(");
>>> __print_lock_name(target);
>>> printk(KERN_CONT ");\n");
>>> - printk(" lock(");
>>> + if (src_read != 0)
>>> + printk(" rlock(");
>>> + else if (src->sync)
>>> + printk(" sync(");
>>> + else
>>> + printk(" lock(");
>>> __print_lock_name(source);
>>> printk(KERN_CONT ");\n");
>>> printk("\n *** DEADLOCK ***\n\n");
>> src can be sync() but not the target. Is there a reason why that is the
>> case?
>>
> The functions annotated by sync() don't create real critical sections,
> so no lock dependency can be created from a sync(), for example:
>
> synchronize_srcu(A);
> mutex_lock(B);
>
> no dependency from A to B. In the scenario case, if we see a dependency
> target -> source, the target cannot be a lock_sync(). I will add better
> documentation later.
Right, the dependency won't happen since you reduce lock_sync() to
mostly do validate_chain() without actually storing it in the lock chain
which I did miss in my initial review. Without that, a dependency may
happen if an NMI happens between lock_acquire() and lock_release() in
lock_sync().
>>> @@ -4530,7 +4540,13 @@ mark_usage(struct task_struct *curr, struct held_lock *hlock, int check)
>>> return 0;
>>> }
>>> }
>>> - if (!hlock->hardirqs_off) {
>>> +
>>> + /*
>>> + * For lock_sync(), don't mark the ENABLED usage, since lock_sync()
>>> + * creates no critical section and no extra dependency can be introduced
>>> + * by interrupts
>>> + */
>>> + if (!hlock->hardirqs_off && !hlock->sync) {
>>> if (hlock->read) {
>>> if (!mark_lock(curr, hlock,
>>> LOCK_ENABLED_HARDIRQ_READ))
>>> @@ -4909,7 +4925,7 @@ static int __lock_is_held(const struct lockdep_map *lock, int read);
>>> static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>>> int trylock, int read, int check, int hardirqs_off,
>>> struct lockdep_map *nest_lock, unsigned long ip,
>>> - int references, int pin_count)
>>> + int references, int pin_count, int sync)
>>> {
>>> struct task_struct *curr = current;
>>> struct lock_class *class = NULL;
>>> @@ -4960,7 +4976,8 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>>> class_idx = class - lock_classes;
>>> - if (depth) { /* we're holding locks */
>>> + if (depth && !sync) {
>>> + /* we're holding locks and the new held lock is not a sync */
>>> hlock = curr->held_locks + depth - 1;
>>> if (hlock->class_idx == class_idx && nest_lock) {
>>> if (!references)
>>> @@ -4994,6 +5011,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>>> hlock->trylock = trylock;
>>> hlock->read = read;
>>> hlock->check = check;
>>> + hlock->sync = !!sync;
>>> hlock->hardirqs_off = !!hardirqs_off;
>>> hlock->references = references;
>>> #ifdef CONFIG_LOCK_STAT
>>> @@ -5055,6 +5073,10 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>>> if (!validate_chain(curr, hlock, chain_head, chain_key))
>>> return 0;
>>> + /* For lock_sync(), we are done here since no actual critical section */
>>> + if (hlock->sync)
>>> + return 1;
>>> +
>>> curr->curr_chain_key = chain_key;
>>> curr->lockdep_depth++;
>>> check_chain_key(curr);
>> Even with sync, there is still a corresponding lock_acquire() and
>> lock_release(), you can't exit here without increasing lockdep_depth. That
>> can cause underflow.
>>
> I actually remove the __lock_release() in lock_sync() in this patch, so
> I think it's OK. But I must admit the whole submission is to give David
> something to see whether the output is an improvement, so I probably
> should separate the output changes and the lock_sync() internall into
> two patches (and the later can also be folded into the introduction
> patch).
I saw that now. You may not need to separate it into 2 patches since
there is some dependency between the two. You do have to document the 2
different changes in your patch description.
Cheers,
Longman
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