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;
MIME-Version: 1.0
References: <20230421-seilbahn-vorpreschen-bd73ac3c88d7@brauner>
 <CAHk-=wgyL9OujQ72er7oXt_VsMeno4bMKCTydBT1WSaagZ_5CA@mail.gmail.com>
 <6882b74e-874a-c116-62ac-564104c5ad34@kernel.dk> <CAHk-=wiQ8g+B0bCPJ9fxZ+Oa0LPAUAyryw9i+-fBUe72LoA+QQ@mail.gmail.com>
 <CAHk-=wgGzwaz2yGO9_PFv4O1ke_uHg25Ab0UndK+G9vJ9V4=hw@mail.gmail.com>
 <2e7d4f63-7ddd-e4a6-e7eb-fd2a305d442e@kernel.dk> <69ec222c-1b75-cdc1-ac1b-0e9e504db6cb@kernel.dk>
 <CAHk-=wiaFUoHpztu6Zf_4pyzH-gzeJhdCU0MYNw9LzVg1-kx8g@mail.gmail.com>
 <CAHk-=wjSuGTLrmygUSNh==u81iWUtVzJ5GNSz0A-jbr4WGoZyw@mail.gmail.com> <20230425194910.GA1350354@hirez.programming.kicks-ass.net>
In-Reply-To: <20230425194910.GA1350354@hirez.programming.kicks-ass.net>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 25 Apr 2023 12:58:12 -0700
Message-ID: <CAHk-=wjNfkT1oVLGbe2=Vymp66Ht=tk+YKa9gUL4T=_hA_JLjg@mail.gmail.com>
Subject: Re: [GIT PULL] pipe: nonblocking rw for io_uring
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jens Axboe <axboe@kernel.dk>, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, Apr 25, 2023 at 12:49=E2=80=AFPM Peter Zijlstra <peterz@infradead.o=
rg> wrote:
>
> The last time this came up I shared your view however Mark argued for
> the READ_ONCE() thusly:
>
>   https://lore.kernel.org/all/Y71LoCIl+IFdy9D8@FVFF77S0Q05N/T/#u

Hmm.

Yes, I think Mark is right. It's not that 'old' might be wrong - that
doesn't matter because cmpxchg will work it out - it's just that 'new'
might not be consistent with the old value we then use.

Ok. I'll try to remember this, but maybe it might be worth documenting.

Jens - I don't think this actually matters for the f_mode value issue,
since the only thing that might change is that FMODE_NOWAIT bit, but I
was clearly wrong on READ_ONCE(). So that loop should have it, just to
have the right pattern after all.

My bad.

             Linus