linux-kernel - Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current) with fsfreeze (4.19.25-rt16)

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Message-ID: <20190503154654.GE2606@hirez.programming.kicks-ass.net>
Date:   Fri, 3 May 2019 17:46:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>, jack@...e.com,
        Waiman Long <longman@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [RT WARNING] DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) !=
 current) with fsfreeze (4.19.25-rt16)

On Fri, May 03, 2019 at 05:37:48PM +0200, Oleg Nesterov wrote:
> (And if we change this code to use wait_event(xchg(readers_block) == 0) we
>  can remove rw_sem altogether).

That patch you just saw and didn't look at did just that.

> The main problem is that this is sub-optimal. We can have a lot of readers
> sleeping in __down_read() when percpu_down_write() succeeds, then after
> percpu_down_write_non_owner() does up_write() they all will be woken just
> to hang in readers_block(). Plus the new readers will need to pass the
> lock-check-unlock-schedule path.

Yes, that's gone. Still, write side locking on percpu-rwsem _should_ be
relatively rare and is certainly not a fast path.

> Peter, just in case... I see another patch from you but I need to run away
> till Monday.

n/p, enjoy the weekend!