From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Boqun Feng <boqun.feng@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/semaphore: Use wake_q to wake up processes outside lock critical section
Date: Wed, 9 Feb 2022 21:07:46 -0500 [thread overview]
Message-ID: <97416d9f-abce-3dba-80fd-3f5d03a8e4d5@redhat.com> (raw)
In-Reply-To: <20220118153254.358748-1-longman@redhat.com>
On 1/18/22 10:32, Waiman Long wrote:
> The following lockdep splat was observed:
>
> [ 9776.459819] ======================================================
> [ 9776.459820] WARNING: possible circular locking dependency detected
> [ 9776.459821] 5.14.0-0.rc4.35.el9.x86_64+debug #1 Not tainted
> [ 9776.459823] ------------------------------------------------------
> [ 9776.459824] stress-ng/117708 is trying to acquire lock:
> [ 9776.459825] ffffffff892d41d8 ((console_sem).lock){-...}-{2:2}, at: down_trylock+0x13/0x70
>
> [ 9776.459831] but task is already holding lock:
> [ 9776.459832] ffff888e005f6d18 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x27/0x130
>
> [ 9776.459837] which lock already depends on the new lock.
> :
> [ 9776.459857] -> #1 (&p->pi_lock){-.-.}-{2:2}:
> [ 9776.459860] __lock_acquire+0xb72/0x1870
> [ 9776.459861] lock_acquire+0x1ca/0x570
> [ 9776.459862] _raw_spin_lock_irqsave+0x40/0x90
> [ 9776.459863] try_to_wake_up+0x9d/0x1210
> [ 9776.459864] up+0x7a/0xb0
> [ 9776.459864] __up_console_sem+0x33/0x70
> [ 9776.459865] console_unlock+0x3a1/0x5f0
> [ 9776.459866] vprintk_emit+0x23b/0x2b0
> [ 9776.459867] devkmsg_emit.constprop.0+0xab/0xdc
> [ 9776.459868] devkmsg_write.cold+0x4e/0x78
> [ 9776.459869] do_iter_readv_writev+0x343/0x690
> [ 9776.459870] do_iter_write+0x123/0x340
> [ 9776.459871] vfs_writev+0x19d/0x520
> [ 9776.459871] do_writev+0x110/0x290
> [ 9776.459872] do_syscall_64+0x3b/0x90
> [ 9776.459873] entry_SYSCALL_64_after_hwframe+0x44/0xae
> :
> [ 9776.459905] Chain exists of:
> [ 9776.459906] (console_sem).lock --> &p->pi_lock --> &rq->__lock
>
> [ 9776.459911] Possible unsafe locking scenario:
>
> [ 9776.459913] CPU0 CPU1
> [ 9776.459914] ---- ----
> [ 9776.459914] lock(&rq->__lock);
> [ 9776.459917] lock(&p->pi_lock);
> [ 9776.459919] lock(&rq->__lock);
> [ 9776.459921] lock((console_sem).lock);
>
> [ 9776.459923] *** DEADLOCK ***
>
> The problematic locking sequence ((console_sem).lock --> &p->pi_lock)
> was caused by the fact the semaphore up() function is calling
> wake_up_process() while holding the semaphore raw spinlock.
>
> By moving the wake_up_processs() call out of the raw spinlock critical
> section using wake_q, it will break the problematic locking sequence as
> well as reducing raw spinlock hold time which will be good for
> PREEMPT_RT.
>
> 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 9ee381e4d2a4..a26c915430ba 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>
> @@ -37,7 +38,7 @@ 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
> @@ -182,13 +183,16 @@ EXPORT_SYMBOL(down_timeout);
> void 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);
>
> @@ -256,11 +260,12 @@ 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);
> }
Peter, is this patch good enough to be merged?
Regards,
Longman
next prev parent reply other threads:[~2022-02-10 2:09 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-18 15:32 [PATCH] locking/semaphore: Use wake_q to wake up processes outside lock critical section Waiman Long
2022-02-10 2:07 ` Waiman Long [this message]
2022-02-10 10:19 ` Peter Zijlstra
2022-02-10 17:04 ` Waiman Long
2022-02-11 10:51 ` Peter Zijlstra
2022-02-11 15:08 ` Waiman Long
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