linux-kernel - Re: [greybus-dev] [PATCH] staging: greybus: Add missing rwsem around snd_ctl

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Date:   Thu, 18 Nov 2021 07:55:56 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Vaibhav Agarwal <vaibhav.sr@...il.com>
Cc:     Alex Elder <elder@...aro.org>, Takashi Iwai <tiwai@...e.de>,
        "moderated list:GREYBUS SUBSYSTEM" <greybus-dev@...ts.linaro.org>,
        Alex Elder <elder@...nel.org>, Johan Hovold <johan@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [greybus-dev] [PATCH] staging: greybus: Add missing rwsem around snd_ctl_remove() calls

On Thu, 18 Nov 2021 04:32:40 +0100,
Vaibhav Agarwal wrote:
> 
> On Thu, Nov 18, 2021 at 3:25 AM Alex Elder <elder@...aro.org> wrote:
> >
> > On 11/17/21 3:02 PM, Takashi Iwai wrote:
> > > On Wed, 17 Nov 2021 20:56:14 +0100,
> > > Alex Elder wrote:
> > >>
> > >> On 11/16/21 1:20 AM, Takashi Iwai wrote:
> > >>> snd_ctl_remove() has to be called with card->controls_rwsem held (when
> > >>> called after the card instantiation).  This patch adds the missing
> > >>> rwsem calls around it.
> > >>
> > >> I see the comment above snd_ctl_remove() that says you must hold
> > >> the write lock.  And given that, this seems correct to me.
> > >>
> > >> I understand why you want to take the lock just once, rather
> > >> than each time snd_ctl_remove() is called.
> > >>
> > >> However I believe the acquisition and release of the lock
> > >> belongs inside gbaudio_remove_controls(), not in its caller.
> > >>
> > >> If you disagree, can you please explain why?
> > >
> > > In general if the function returns an error and has a loop inside,
> > > taking a lock in the caller side avoids the forgotten unlock.
> >
> > But taking the lock in the called function makes the
> > caller not need to take the lock (which would be even
> > more valuable if there were more than one caller).
> >
> > I prefer having the lock acquisition in the called
> > function.  Please send version 2, as I suggested.
> 
> 
> Hi Takashi,
> 
> Thanks for sharing this patch. In reference to the suggestion from Alex,
> do you think replacing snd_ctl_find_id(), snd_ctl_remove() with
> snd_ctl_remove_id() inside gbaudio_remove_controls() would be an even
> better choice without worrying about locks?

Yeah, that sounds like a better plan, indeed.


Takashi