linux-kernel - Re: [PATCH] loop: fput() called in loop_clr_fd() may cause bd

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Message-ID: <20111217225846.GC2203@ZenIV.linux.org.uk>
Date:	Sat, 17 Dec 2011 22:58:46 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Namhyung Kim <namhyung@...il.com>,
	Lukas Czerner <lczerner@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] loop: fput() called in loop_clr_fd() may cause bd_mutex
 recursive locking

On Sun, Dec 18, 2011 at 01:37:45AM +0300, Sergey Senozhatsky wrote:
> > Think of it for a minute - if we could run into the
> > same bdev in that recursion, what would have happened on read() from
> > that sucker? So yes, it is a false positive. 
> 
> I've tried read()/write() some time ago and it worked. Perhaps, I just
> wasn't "lucky" enough to hit any problems.

Sure - exactly because of that loop prevention logics.  *If* we really
had been able to set a loop0 -> loop1 -> loop2 -> loop0 or something of
that sort, this warning wouldn't be a false positive.  But on any
such setup, where would IO attempts end up doing?  IOW, we have to
prevent such setups anyway and not just because of problems on close() -
they would be deadly on read() and write()...
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