Re: [PATCH v2] cpuset: fix a deadlock due to incomplete patching of cpusets_enabled()

[Andrew Morton]

On Thu, 27 Jul 2017 09:46:08 -0700 Dima Zavin <dmitriyz@xxxxxxxxx> 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.