linux-kernel - Re: [PATCH 13/19] MM: set PF_FSTRANS while allocating per-cpu memory to avoid deadlock.

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Message-ID: <20140416054942.GD15995@dastard>
Date:	Wed, 16 Apr 2014 15:49:42 +1000
From:	Dave Chinner <david@...morbit.com>
To:	NeilBrown <neilb@...e.de>
Cc:	linux-mm@...ck.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, xfs@....sgi.com
Subject: Re: [PATCH 13/19] MM: set PF_FSTRANS while allocating per-cpu memory
 to avoid deadlock.

On Wed, Apr 16, 2014 at 02:03:36PM +1000, NeilBrown wrote:
> lockdep reports a locking chain
> 
>   sk_lock-AF_INET --> rtnl_mutex --> pcpu_alloc_mutex
> 
> As sk_lock may be needed to reclaim memory, allowing that
> reclaim while pcu_alloc_mutex is held can lead to deadlock.
> So set PF_FSTRANS while it is help to avoid the FS reclaim.
> 
> pcpu_alloc_mutex can be taken when rtnl_mutex is held:
> 
>     [<ffffffff8117f979>] pcpu_alloc+0x49/0x960
>     [<ffffffff8118029b>] __alloc_percpu+0xb/0x10
>     [<ffffffff8193b9f7>] loopback_dev_init+0x17/0x60
>     [<ffffffff81aaf30c>] register_netdevice+0xec/0x550
>     [<ffffffff81aaf785>] register_netdev+0x15/0x30
> 
> Signed-off-by: NeilBrown <neilb@...e.de>

This looks like a workaround to avoid passing a gfp mask around to
describe the context in which the allocation is taking place.
Whether or not that's the right solution, I can't say, but spreading
this "we can turn off all reclaim of filesystem objects" mechanism
all around the kernel doesn't sit well with me...

And, again, PF_FSTRANS looks plainly wrong in this code - it sure
isn't a fs transaction context we are worried about here...


-- 
Dave Chinner
david@...morbit.com
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