linux-kernel - Re: [PATCH v2] locking/semaphore: Use wake_q to wake up processes outside lock critical section

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Message-ID: <b6a32f9c-d991-408e-a80e-9a34afb23197@redhat.com>
Date: Sat, 25 Jan 2025 21:21:15 -0500
From: Waiman Long <llong@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will.deacon@....com>, Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] locking/semaphore: Use wake_q to wake up processes
 outside lock critical section


On 1/22/25 2:17 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 -> zone->lock dependency is actually another
> instance of raw_spinlock to spinlock nesting problem.
>
> [ 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
>
> We will need to make some changes in debugobjects and/or hrtimer code
> to address this problem and break the circular dependency.

I check the v6.12 debugobjects code again and I saw

static void debug_objects_fill_pool(void)
{
         /*
          * On RT enabled kernels the pool refill must happen in preemptible
          * context -- for !RT kernels we rely on the fact that 
spinlock_t and
          * raw_spinlock_t are basically the same type and this lock-type
          * inversion works just fine.
          */
         if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
                 /*
                  * Annotate away the spinlock_t inside raw_spinlock_t 
warning
                  * by temporarily raising the wait-type to WAIT_SLEEP, 
matching
                  * the preemptible() condition above.
                  */
                 static DEFINE_WAIT_OVERRIDE_MAP(fill_pool_map, 
LD_WAIT_SLEEP);
                 lock_map_acquire_try(&fill_pool_map);
                 fill_pool();
                 lock_map_release(&fill_pool_map);
         }
}

It looks like it explicitly allows raw_spinlock to spinlock nesting for 
non-PREEMPT_RT kernel and disable the lockdep nested locking check. So 
the hrtimer_bases.lock -> zone->lock dependency is legit and expected. I 
will need to update my patch description again.

Cheers,
Longman