linux-kernel - Re: [PATCH 0/6] locking/rtmutex: Avoid PI state recursion through sched_submit

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Date:   Wed, 16 Aug 2023 15:46:30 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bsegall@...gle.com, boqun.feng@...il.com, swood@...hat.com,
        bristot@...hat.com, dietmar.eggemann@....com, mingo@...hat.com,
        jstultz@...gle.com, juri.lelli@...hat.com, mgorman@...e.de,
        rostedt@...dmis.org, vschneid@...hat.com,
        vincent.guittot@...aro.org, longman@...hat.com, will@...nel.org
Subject: Re: [PATCH 0/6] locking/rtmutex: Avoid PI state recursion through
 sched_submit_work()

On 2023-08-16 12:19:04 [+0200], To Peter Zijlstra wrote:
> On 2023-08-16 11:42:57 [+0200], Peter Zijlstra wrote:
> > Not the same -- this is namespace_lock(), right? That's a regular rwsem
> > afaict and that *should* be good. Clearly I messed something up.
> 
> Most likely. I do see it also fom inode_lock() which does down_write().
> I see it only to originate from rwbase_write_lock().

I've been looking at what you did and what we had.
I'm not sure if your additional debug/assert code figured it out or me
looking at it, but in rwbase_write_lock() for down_write(), we had this
beauty with a comment that you made go away:

|        * Take the rtmutex as a first step. For rwsem this will also
|        * invoke sched_submit_work() to flush IO and workers.
|	 */
|         if (rwbase_rtmutex_lock_state(rtm, state))

for rw_semaphore we don't have any explicit rwbase_sched_submit_work()
but relied on this one. Now that I look at it again,
rwbase_rtmutex_lock_state() can succeed in the fast path so we don't
flush/ invoke rwbase_pre_schedule().
So you rightfully removed the comment as it was misleading but we do
need that rwbase_pre_schedule() thingy before
raw_spin_lock_irqsave(&rtm->wait_lock).

Sebastian