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Re: [PATCH 13/19] MM: set PF_FSTRANS while allocating per-cpu memory to

To: NeilBrown <neilb@xxxxxxx>
Subject: Re: [PATCH 13/19] MM: set PF_FSTRANS while allocating per-cpu memory to avoid deadlock.
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 16 Apr 2014 15:49:42 +1000
Cc: linux-mm@xxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140416040336.10604.67456.stgit@xxxxxxxxxxxxxx>
References: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx> <20140416040336.10604.67456.stgit@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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@xxxxxxx>

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@xxxxxxxxxxxxx

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