linux-kernel - Re: [xfs-masters] RFC: Fix f_flags races without the BKL

Open Source and information security mailing list archives

Message-ID: <20081230144836.GA31439@lst.de>
Date:	Tue, 30 Dec 2008 15:48:37 +0100
From:	Christoph Hellwig <hch@....de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Christoph Hellwig <hch@....de>, Jonathan Corbet <corbet@....net>,
	Al@....sgi.com, Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, bfields@...ldses.org,
	xfs-masters@....sgi.com, Viro <viro@...IV.linux.org.uk>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [xfs-masters] RFC: Fix f_flags races without the BKL

On Tue, Dec 30, 2008 at 02:37:37PM +0100, Andi Kleen wrote:
> That's not clear. Mutexes can be much slower than a spinlock
> like BKL in some situations, mostly because they schedule more and
> have generally more overhead.
> 
> As long as you don't have another BKL user contending the BKL
> is likely faster than the mutex.

Note that I did not say faster, but better.  The subtle races the
BKL semantics introduce are nasty.

That beeing said I took another look at the patch and it seems like
most places are indeed just very quick flags setting / clearing
with the only sleeping possible inside ->fasync.  So having a
file_flags_lock spinlock, and another sleeping mutex protecting
->fasync might be another options.

Jon, do you remember what we actually need to protect in -fasync?
any reason not to take the locking inside the method?  Together with
->lock and the old ->ioctl it's pretty special in fops as none of
the others have any locking at all.

> 
> -Andi
> 
> -- 
> ak@...ux.intel.com
> 
> _______________________________________________
> xfs-masters mailing list
> xfs-masters@....sgi.com
> http://oss.sgi.com/mailman/listinfo/xfs-masters
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