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

Dave Chinner
2014-04-16T05:49:50Z

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@suse.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@fromorbit.com