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Date: Mon, 16 Jan 2023 14:35:33 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Waiman Long <longman@...hat.com>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
kvm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
"Paul E. McKenney" <paulmck@...nel.org>,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
David Woodhouse <dwmw2@...radead.org>,
Paolo Bonzini <pbonzini@...hat.com>, seanjc@...gle.com,
Joel Fernandes <joel@...lfernandes.org>,
Matthew Wilcox <willy@...radead.org>,
Michal Luczaj <mhal@...x.co>
Subject: Re: [PATCH 4/3] locking/lockdep: Improve the deadlock scenario print
for sync and read lock
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@...il.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
>
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