linux-kernel - Re: spinlock contention of files->file

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Message-ID: <CA+55aFz3gJE_DpZufTDNoRZ=vgJS3OwMspR7ZSqzGY_rWMq4gQ@mail.gmail.com>
Date:	Mon, 30 Sep 2013 19:02:23 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: spinlock contention of files->file_lock

On Mon, Sep 30, 2013 at 6:53 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> The problem is dup2()

Shouldn't a cmpxchg() in just the dup2 code solve that?

If the old value was NULL, you'd have to repeat and go back and see if
the open_fds[] bit had been cleared in the meantime (ie it's NULL not
because somebody else is busy installing it, but because somebody just
uninstalled it).

But yeah, I do agree that that sounds nasty and a complication I
hadn't even thought about. dup2() does violate our normal "let's
pre-allocate the fd slot" rule. Ugh.

              Linus
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