linux-kernel - Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd

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Message-ID: <1429650413.18561.10.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 21 Apr 2015 14:06:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in
 fd_install

On Tue, 2015-04-21 at 22:12 +0200, Mateusz Guzik wrote:

> in dup_fd:
>        for (i = open_files; i != 0; i--) {
>                 struct file *f = *old_fds++;
>                 if (f) {
>                         get_file(f);
> 

I see no new requirement here. f is either NULL or not.
multi threaded programs never had a guarantee dup_fd() would catch a non
NULL pointer here.


> at least a data dependency barrier, or maybe smp_rmb for peace of mind
> 
> similarly in do_dup2:
>         tofree = fdt->fd[fd];
>         if (!tofree && fd_is_open(fd, fdt))
>                 goto Ebusy;

Same here.


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