linux-kernel - Re: [PATCH] ALSA: dummy: Fix &dpcm->lock deadlock issues

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Message-ID: <87edlzwgti.wl-tiwai@suse.de>
Date:   Sun, 25 Jun 2023 20:13:45 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     YE Chengfeng <cyeaa@...nect.ust.hk>
Cc:     "perex@...ex.cz" <perex@...ex.cz>,
        "tiwai@...e.com" <tiwai@...e.com>,
        "yunjunlee@...omium.org" <yunjunlee@...omium.org>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ALSA: dummy: Fix &dpcm->lock deadlock issues

On Sun, 25 Jun 2023 17:35:48 +0200,
YE Chengfeng wrote:
> 
> The timer dummy_systimer_callback is executed under softirq
> context, thus other process context code requiring the same lock
> should disable interrupt. Otherwise there would be potential
> deadlock issues when the code executing under process context
> (i.e., dummy_systimer_pointer, dummy_systimer_start,
> dummy_systimer_stop) is preempted by the timer while holding
> the lock.
> 
> Deadlock scenario:
> dummy_systimer_pointer
>     -> spin_lock(&dpcm->lock);
>         <timer interrupt>
>         -> dummy_systimer_callback
>         -> spin_lock_irqsave(&dpcm->lock, flags);
> 
> Fix the potential deadlock by using spin_lock_irqsave.

Did you really trigger this deadlock, or is just your hypothesis?
I'm asking it because basically the deadlock above shouldn't happen;
those are called only via PCM trigger and pointer callbacks, and they
are always called inside the PCM stream lock, and already
irq-disabled.


thanks,

Takashi