Date: Fri, 28 Jul 2017 11:30:47 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Dima Zavin <dmitriyz@waymo.com>, Christopher Lameter <cl@linux.com>,
        Li Zefan <lizefan@huawei.com>, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Cliff Spradlin <cspradlin@waymo.com>,
        Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH v2] cpuset: fix a deadlock due to incomplete patching of
 cpusets_enabled()
Message-ID: <20170728093047.ykgbufjj74xa5x3r@hirez.programming.kicks-ass.net>
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 <20170727164608.12701-1-dmitriyz@waymo.com>
 <41954034-9de1-de8e-f915-51a4b0334f98@suse.cz>
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On Fri, Jul 28, 2017 at 09:45:16AM +0200, Vlastimil Babka wrote:
> [+CC PeterZ]
> 
> On 07/27/2017 06:46 PM, Dima Zavin wrote:
> > In codepaths that use the begin/retry interface for reading
> > mems_allowed_seq with irqs disabled, there exists a race condition that
> > stalls the patch process after only modifying a subset of the
> > static_branch call sites.
> > 
> > This problem manifested itself as a dead lock in the slub
> > allocator, inside get_any_partial. The loop reads
> > mems_allowed_seq value (via read_mems_allowed_begin),
> > performs the defrag operation, and then verifies the consistency
> > of mem_allowed via the read_mems_allowed_retry and the cookie
> > returned by xxx_begin. The issue here is that both begin and retry
> > first check if cpusets are enabled via cpusets_enabled() static branch.
> > This branch can be rewritted dynamically (via cpuset_inc) if a new
> > cpuset is created. The x86 jump label code fully synchronizes across
> > all CPUs for every entry it rewrites. If it rewrites only one of the
> > callsites (specifically the one in read_mems_allowed_retry) and then
> > waits for the smp_call_function(do_sync_core) to complete while a CPU is
> > inside the begin/retry section with IRQs off and the mems_allowed value
> > is changed, we can hang. This is because begin() will always return 0
> > (since it wasn't patched yet) while retry() will test the 0 against
> > the actual value of the seq counter.
> 
> Hm I wonder if there are other static branch users potentially having
> similar problem. Then it would be best to fix this at static branch
> level. Any idea, Peter? An inelegant solution would be to have indicate
> static_branch_(un)likely() callsites ordering for the patching. I.e.
> here we would make sure that read_mems_allowed_begin() callsites are
> patched before read_mems_allowed_retry() when enabling the static key,
> and the opposite order when disabling the static key.

I'm not aware of any other sure ordering requirements. But you can
manually create this order by using 2 static keys. Then flip them in the
desired order.