Re: [PATCH 4/3] locking/lockdep: Improve the deadlock scenario print for sync and read lock

[Boqun Feng]

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@xxxxxxxxx>
> > ---
> >   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
>