linux-kernel - Re: [PATCH v2] cpuset: fix a deadlock due to incomplete patching of cpusets

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Message-Id: <20170727124855.aeb97ea9f74af2d3e47e1787@linux-foundation.org>
Date:   Thu, 27 Jul 2017 12:48:55 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Dima Zavin <dmitriyz@...mo.com>
Cc:     Christopher Lameter <cl@...ux.com>, Li Zefan <lizefan@...wei.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Cliff Spradlin <cspradlin@...mo.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v2] cpuset: fix a deadlock due to incomplete patching of
 cpusets_enabled()

On Thu, 27 Jul 2017 09:46:08 -0700 Dima Zavin <dmitriyz@...mo.com> 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.
> 
> The fix is to cache the value that's returned by cpusets_enabled() at the
> top of the loop, and only operate on the seqcount (both begin and retry) if
> it was true.

Tricky.  Hence we should have a nice code comment somewhere describing
all of this.

> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -16,6 +16,11 @@
>  #include <linux/mm.h>
>  #include <linux/jump_label.h>
>  
> +struct cpuset_mems_cookie {
> +	unsigned int seq;
> +	bool was_enabled;
> +};

At cpuset_mems_cookie would be a good site - why it exists, what it
does, when it is used and how.