linux-kernel - Re: [PATCH] locking/rtmutex: Flush the plug before entering the slowpath.

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Message-ID: <20230418151838.jNcnnDqL@linutronix.de>
Date:   Tue, 18 Apr 2023 17:18:38 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Cc:     Crystal Wood <swood@...hat.com>, John Keeping <john@...anate.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH] locking/rtmutex: Flush the plug before entering the
 slowpath.

On 2023-03-22 17:27:21 [+0100], To Thomas Gleixner wrote:
> blk_flush_plug() is invoked on schedule() to flush out the IO progress
> that has been made so far so that it is globally visible. This is
> important to avoid deadlocks because a lock owner can wait for IO.
> Therefore the IO must be first flushed before a thread can block on a
> lock.
> 
> The plug flush routine can acquire a sleeping lock which is contended.
> Blocking on a lock requires an assignment to task_struct::pi_blocked_on.
> If blk_flush_plug() is invoked from the slow path on schedule() then the
> variable is already set and will be overwritten by the lock in
> blk_flush_plug().
> Therefore it is needed to invoke blk_flush_plug() (and block on
> potential locks in the process) before the blocking on the actual lock.
> 
> Invoke blk_flush_plug() before blocking on a sleeping lock. The
> PREEMPT_RT only sleeping locks (spinlock_t and rwlock_t) are excluded
> because their slow path does not invoke blk_flush_plug().
> 
> Fixes: e17ba59b7e8e1 ("locking/rtmutex: Guard regular sleeping locks specific functions")
> Reported-by: Crystal Wood <swood@...hat.com>
> Link: https://lore.kernel.org/4b4ab374d3e24e6ea8df5cadc4297619a6d945af.camel@redhat.com
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

polite ping.

Sebastian