linux-kernel - Re: processes hung after sys_renameat, and 'missing' processes

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Message-ID: <20120611121723.GD7654@fieldses.org>
Date:	Mon, 11 Jun 2012 08:17:23 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>, Jan Kara <jack@...e.cz>
Subject: Re: processes hung after sys_renameat, and 'missing' processes

On Fri, Jun 08, 2012 at 05:08:34PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-06-08 at 10:46 -0400, J. Bruce Fields wrote:
> > > > > Sadly, if you get that annotation wrong you can annotate an actual
> > > > > deadlock away.
> > 
> > What's a (contrived as you want) example where that happens?
> 
> spinlock_t lock_array[10];
> 
> void init_array(void)
> {
> 	int i;
> 
> 	for (i = 0; i < ARRAY_SIZE(lock_array); i++)
> 		spin_lock_init(array + i);
> }
> 
> void double_lock(int a, int b)
> {
> 	spin_lock(lock_array + a);
> 	spin_lock_nested(lock_array + b, SINGLE_DEPTH_NESTING);
> }
> 
> The above places all locks in the array in the same class, it then does
> a double lock without order, but tells lockdep the nesting is ok.
> 
> A correct version of the double_lock() function would look like:
> 
> void double_lock(int a, int b)
> {
> 	if (b < a)
> 		swap(a, b);
> 
> 	spin_lock(lock_array + a);
> 	spin_lock_nested(lock_array + b, SINGLE_DEPTH_NESTING);
> }
> 
> This orders the locks in array order.

Got it, thanks!

--b.

> 
> > > > > This the reason you have to be very careful when
> > > > > annotating stuff.
> > 
> > Or alternatively--what do I need to check before I call
> > mutex_lock_nested? 
> 
> That the lock order you tell lockdep is ok, is indeed correct.
> 
> 
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