Re: [PATCH v3] locking/semaphore: Use wake_q to wake up processes outside lock critical section

[Waiman Long <llong@redhat.com>]

On 1/26/25 8:31 PM, Waiman Long wrote:
> A circular lock dependency splat has been seen involving down_trylock().
>
> [ 4011.795602] ======================================================
> [ 4011.795603] WARNING: possible circular locking dependency detected
> [ 4011.795607] 6.12.0-41.el10.s390x+debug
> [ 4011.795612] ------------------------------------------------------
> [ 4011.795613] dd/32479 is trying to acquire lock:
> [ 4011.795617] 0015a20accd0d4f8 ((console_sem).lock){-.-.}-{2:2}, at: down_trylock+0x26/0x90
> [ 4011.795636]
> [ 4011.795636] but task is already holding lock:
> [ 4011.795637] 000000017e461698 (&zone->lock){-.-.}-{2:2}, at: rmqueue_bulk+0xac/0x8f0
>
>    the existing dependency chain (in reverse order) is:
>    -> #4 (&zone->lock){-.-.}-{2:2}:
>    -> #3 (hrtimer_bases.lock){-.-.}-{2:2}:
>    -> #2 (&rq->__lock){-.-.}-{2:2}:
>    -> #1 (&p->pi_lock){-.-.}-{2:2}:
>    -> #0 ((console_sem).lock){-.-.}-{2:2}:
>
> The console_sem -> pi_lock dependency is due to calling try_to_wake_up()
> while holding the console.sem raw_spinlock. This dependency can be broken
> by using wake_q to do the wakeup instead of calling try_to_wake_up()
> under the console_sem lock. This will also make the semaphore's
> raw_spinlock become a terminal lock without taking any further locks
> underneath it.
>
> The hrtimer_bases.lock is a raw_spinlock while zone->lock is a
> spinlock. The hrtimer_bases.lock -> zone->lock dependency happens via
> the debug_objects_fill_pool() helper function in the debugobjects code.
>
> [ 4011.795646] -> #4 (&zone->lock){-.-.}-{2:2}:
> [ 4011.795650]        __lock_acquire+0xe86/0x1cc0
> [ 4011.795655]        lock_acquire.part.0+0x258/0x630
> [ 4011.795657]        lock_acquire+0xb8/0xe0
> [ 4011.795659]        _raw_spin_lock_irqsave+0xb4/0x120
> [ 4011.795663]        rmqueue_bulk+0xac/0x8f0
> [ 4011.795665]        __rmqueue_pcplist+0x580/0x830
> [ 4011.795667]        rmqueue_pcplist+0xfc/0x470
> [ 4011.795669]        rmqueue.isra.0+0xdec/0x11b0
> [ 4011.795671]        get_page_from_freelist+0x2ee/0xeb0
> [ 4011.795673]        __alloc_pages_noprof+0x2c2/0x520
> [ 4011.795676]        alloc_pages_mpol_noprof+0x1fc/0x4d0
> [ 4011.795681]        alloc_pages_noprof+0x8c/0xe0
> [ 4011.795684]        allocate_slab+0x320/0x460
> [ 4011.795686]        ___slab_alloc+0xa58/0x12b0
> [ 4011.795688]        __slab_alloc.isra.0+0x42/0x60
> [ 4011.795690]        kmem_cache_alloc_noprof+0x304/0x350
> [ 4011.795692]        fill_pool+0xf6/0x450
> [ 4011.795697]        debug_object_activate+0xfe/0x360
> [ 4011.795700]        enqueue_hrtimer+0x34/0x190
> [ 4011.795703]        __run_hrtimer+0x3c8/0x4c0
> [ 4011.795705]        __hrtimer_run_queues+0x1b2/0x260
> [ 4011.795707]        hrtimer_interrupt+0x316/0x760
> [ 4011.795709]        do_IRQ+0x9a/0xe0
> [ 4011.795712]        do_irq_async+0xf6/0x160
>
> Normally raw_spinlock to spinlock dependency is not legit
> and will be warned if PROVE_RAW_LOCK_NESTING is enabled,
> but debug_objects_fill_pool() is an exception as it explicitly
> allows this dependency for non-PREEMPT_RT kernel without causing
> PROVE_RAW_LOCK_NESTING lockdep splat. As a result, this dependency is
> legit and not a bug.
>
> Anyway, semaphore is the only locking primitive left that is still
> using try_to_wake_up() to do wakeup inside critical section, all the
> other locking primitives had been migrated to use wake_q to do wakeup
> outside of the critical section. It is also possible that there are
> other circular locking dependencies involving printk/console_sem or
> other existing/new semaphores lurking somewhere which may show up in
> the future. Let just do the migration now to wake_q to avoid headache
> like this.

I can also add the following as another instance where deadlock can happen.

Reported-by:syzbot+ed801a886dfdbfe7136d@syzkaller.appspotmail.com

Cheers,
Longman

> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/locking/semaphore.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> index 34bfae72f295..de9117c0e671 100644
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -29,6 +29,7 @@
>   #include <linux/export.h>
>   #include <linux/sched.h>
>   #include <linux/sched/debug.h>
> +#include <linux/sched/wake_q.h>
>   #include <linux/semaphore.h>
>   #include <linux/spinlock.h>
>   #include <linux/ftrace.h>
> @@ -38,7 +39,7 @@ static noinline void __down(struct semaphore *sem);
>   static noinline int __down_interruptible(struct semaphore *sem);
>   static noinline int __down_killable(struct semaphore *sem);
>   static noinline int __down_timeout(struct semaphore *sem, long timeout);
> -static noinline void __up(struct semaphore *sem);
> +static noinline void __up(struct semaphore *sem, struct wake_q_head *wake_q);
>   
>   /**
>    * down - acquire the semaphore
> @@ -183,13 +184,16 @@ EXPORT_SYMBOL(down_timeout);
>   void __sched up(struct semaphore *sem)
>   {
>   	unsigned long flags;
> +	DEFINE_WAKE_Q(wake_q);
>   
>   	raw_spin_lock_irqsave(&sem->lock, flags);
>   	if (likely(list_empty(&sem->wait_list)))
>   		sem->count++;
>   	else
> -		__up(sem);
> +		__up(sem, &wake_q);
>   	raw_spin_unlock_irqrestore(&sem->lock, flags);
> +	if (!wake_q_empty(&wake_q))
> +		wake_up_q(&wake_q);
>   }
>   EXPORT_SYMBOL(up);
>   
> @@ -269,11 +273,12 @@ static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
>   	return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
>   }
>   
> -static noinline void __sched __up(struct semaphore *sem)
> +static noinline void __sched __up(struct semaphore *sem,
> +				  struct wake_q_head *wake_q)
>   {
>   	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
>   						struct semaphore_waiter, list);
>   	list_del(&waiter->list);
>   	waiter->up = true;
> -	wake_up_process(waiter->task);
> +	wake_q_add(wake_q, waiter->task);
>   }