linux-kernel - Re: [PATCH] fix race in AF

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Date:	Mon, 18 Jun 2007 12:39:07 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	tgraf@...g.ch
CC:	davem@...emloft.net, akpm@...ux-foundation.org,
	viro@....linux.org.uk, alan@...rguk.ukuu.org.uk,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix race in AF_UNIX

> * Miklos Szeredi <miklos@...redi.hu> 2007-06-18 11:44
> > Garbage collection only ever happens, if the app is sending AF_UNIX
> > sockets over AF_UNIX sockets.  Which is a rather rare case.  And which
> > is basically why this bug went unnoticed for so long.
> > 
> > So my second patch only affects the performance of _exactly_ those
> > apps which might well be bitten by the bug itself.
> 
> That's not entirely the truth. It affects all applications using
> AF_UNIX sockets while file descriptors are being transfered. I
> agree that the performance impact is not severe on most systems
> but if file descriptors are being transfered continously by just
> a single application it can become rather severe.

You are wrong.  Look in unix_release_sock():

	if (atomic_read(&unix_tot_inflight))
		unix_gc();		/* Garbage collect fds */


unix_tot_inflight is the number of AF_UNIX sockets currently being
transferred over some AF_UNIX sockets.

That means that just sending (non-unix socket) fds over unix sockets
will never invoke the gc.

Miklos
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