linux-kernel - Re: rtmutex, pi_blocked_on, and blk_flush

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Date:   Mon, 20 Feb 2023 19:21:51 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Crystal Wood <swood@...hat.com>, John Keeping <john@...anate.com>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: rtmutex, pi_blocked_on, and blk_flush_plug()

On Mon, Feb 20 2023 at 12:42, Sebastian Andrzej Siewior wrote:
> On 2023-02-20 12:04:56 [+0100], To Thomas Gleixner wrote:
>> The ->pi_blocked_on field is set by __rwbase_read_lock() before
>> schedule() is invoked while blocking on the sleeping lock. By doing this
>> we avoid __blk_flush_plug() and as such will may deadlock because we are
>> going to sleep and made I/O progress earlier which is not globally
>> visibly but might be (s/might be/is/ in the deadlock case) expected by
>> the owner of the lock.

Fair enough.

> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1700,6 +1700,13 @@ static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock,
>  	if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
>  		return 0;
>  
> +	if (state != TASK_RTLOCK_WAIT) {
> +		/*
> +		 * If we are going to sleep and we have plugged IO queued,
> +		 * make sure to submit it to avoid deadlocks.
> +		 */
> +		blk_flush_plug(tsk->plug, true);

This still leaves the problem vs. io_wq_worker_sleeping() and it's
running() counterpart after schedule().

Aside of that for CONFIG_DEBUG_RT_MUTEXES=y builds it flushes on every
lock operation whether the lock is contended or not.

Grmbl.