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

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Message-ID: <1826d5c4a90.282ac4bf118702.5300662644268737477@siddh.me>
Date:   Fri, 05 Aug 2022 15:05:41 +0530
From:   Siddh Raman Pant <code@...dh.me>
To:     "Eric Biggers" <ebiggers@...nel.org>
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, 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.

This was remarked by David Howells here:
https://lore.kernel.org/lkml/3558070.1658933200@warthog.procyon.org.uk/

> This isn't the correct block comment format; it should look like:
> 
>      /*
>       * This pipe will get freed by the caller free_pipe_info().
>       * Removing this reference also prevents new notifications.
>       */
> 
> - Eric
> 

Okay, will make the change.

Thanks,
Siddh