linux-kernel - Re: BUG?: super.c:sget spin

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Message-ID: <20140529231921.GO18016@ZenIV.linux.org.uk>
Date:	Fri, 30 May 2014 00:19:22 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Kai <wildex999@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: BUG?: super.c:sget spin_lock not released?

On Fri, May 30, 2014 at 12:49:56AM +0200, Kai wrote:
> Hello,
> 
> I have recently been playing around with writing a custom filesystem
> for Linux, and have been reading the source of other filesystems, and
> the functions they call into the linux kernel.
> While looking at the function sget in super.c I found something strange.
> 
> before if(test) it takes a spinlock, however there seems to be two
> cases where it does not unlock it.
> (Kernel 3.14, super.c line 441)
> 
> One:
> if(!grab_super(old)) goto retry;
> Will jump to before spin_lock(), without releasing it first.

Er...  So either grab_super() releases that spinlock, or that thing
is screwed.  The next obvious step would be to look into grab_super()...

> Two:
> And if it finds an old sb, it will return without releasing the lock.

... unless something it calls does just that.
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