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Message-ID: <221e35b8-88f5-5fc5-6961-6a8ce060a97b@redhat.com>
Date: Mon, 16 Jan 2023 17:21:09 -0500
From: Waiman Long <longman@...hat.com>
To: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
rcu@...r.kernel.org, kvm@...r.kernel.org
Cc: 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 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?
> @@ -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
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