linux-kernel - Re: [RFC PATCH] fs: elide the smp_rmb fence in fd

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Message-ID: <CAGudoHGyFVCjSTjenyO8Y+uPHyhkOCwZrvBW=FyQRDundntFdw@mail.gmail.com>
Date: Thu, 5 Dec 2024 16:36:40 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: paulmck@...nel.org, brauner@...nel.org, viro@...iv.linux.org.uk, 
	linux-fsdevel@...r.kernel.org, torvalds@...ux-foundation.org, 
	edumazet@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: elide the smp_rmb fence in fd_install()

On Thu, Dec 5, 2024 at 4:29 PM Jan Kara <jack@...e.cz> wrote:
> On Thu 05-12-24 16:01:07, Mateusz Guzik wrote:
> > Suppose the CPU reordered loads of the flag and the fd table. There is
> > no ordering in which it can see both the old table and the unset flag.
>
> But I disagree here. If the reads are reordered, then the fd table read can
> happen during the "flag is true and the fd table is old" state and the flag
> read can happen later in "flag is false and the fd table is new" state.
> Just as I outlined above...

In your example all the work happens *after* synchronize_rcu(). The
thread resizing the table already published the result even before
calling into it. Furthermore by the time synchronize_rcu returns
everyone is guaranteed to have issued a full fence. Meaning nobody can
see the flag as unset.

-- 
Mateusz Guzik <mjguzik gmail.com>