linux-kernel - Re: [PATCH v2 3/3] kernel/watch_queue: Remove wqueue->defunct and use pipe for clear check

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Date:   Fri, 5 Aug 2022 18:33:20 +0000
From:   Eric Biggers <ebiggers@...nel.org>
To:     Siddh Raman Pant <code@...dh.me>
Cc:     "christophe.jaillet" <christophe.jaillet@...adoo.fr>,
        corbet <corbet@....net>, dhowells <dhowells@...hat.com>,
        edumazet <edumazet@...gle.com>,
        linux-kernel-mentees 
        <linux-kernel-mentees@...ts.linuxfoundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        mchehab <mchehab@...nel.org>, rdunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v2 3/3] kernel/watch_queue: Remove wqueue->defunct and
 use pipe for clear check

On Fri, Aug 05, 2022 at 03:05:41PM +0530, Siddh Raman Pant wrote:
> On Fri, 05 Aug 2022 12:54:31 +0530  Eric Biggers  wrote:
> > Why is the READ_ONCE() needed?  Doesn't wqueue->lock protect wqueue->pipe?
> 
> We are changing the pointer while a notification can be potentially waiting to
> be posted to the pipe. So a barrier is needed to prevent compiler magic from
> reloading the value.
> 

wqueue->pipe is only read or written while wqueue->lock is held, so that is not
an issue at all.

- Eric