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Message-ID: <221e35b8-88f5-5fc5-6961-6a8ce060a97b@redhat.com>
Date:   Mon, 16 Jan 2023 17:21:09 -0500
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Subject: Re: [PATCH 4/3] locking/lockdep: Improve the deadlock scenario print
 for sync and read lock
Content-Language: en-US
To:     Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
        rcu@vger.kernel.org, kvm@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>, seanjc@google.com,
        Joel Fernandes <joel@joelfernandes.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Luczaj <mhal@rbox.co>
References: <20230113065955.815667-1-boqun.feng@gmail.com>
 <20230113235722.1226525-1-boqun.feng@gmail.com>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20230113235722.1226525-1-boqun.feng@gmail.com>
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Precedence: bulk

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?


> @@ -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