Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 5 Aug 2022 18:33:20 +0000
From:   Eric Biggers <ebiggers@kernel.org>
To:     Siddh Raman Pant <code@siddh.me>
Cc:     "christophe.jaillet" <christophe.jaillet@wanadoo.fr>,
        corbet <corbet@lwn.net>, dhowells <dhowells@redhat.com>,
        edumazet <edumazet@google.com>,
        linux-kernel-mentees 
        <linux-kernel-mentees@lists.linuxfoundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        mchehab <mchehab@kernel.org>, rdunlap <rdunlap@infradead.org>
Subject: Re: [PATCH v2 3/3] kernel/watch_queue: Remove wqueue->defunct and
 use pipe for clear check
Message-ID: <Yu1icG/o8VcfsQdy@gmail.com>
References: <cd2a10ffd8cb11c79754f43d0d20fd3cd4ba9b7e.1659618705.git.code@siddh.me>
 <20220804144152.468916-1-code@siddh.me>
 <YuzFrzrTnTtUHMn/@sol.localdomain>
 <1826d5c4a90.282ac4bf118702.5300662644268737477@siddh.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1826d5c4a90.282ac4bf118702.5300662644268737477@siddh.me>
Precedence: bulk

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