The order of accesses to ring buffer's aux_mmap_count and aux_refcount
has to be preserved across the users, namely perf_mmap_close() and
perf_aux_output_begin(), otherwise the inversion can result in the latter
holding the last reference to the aux buffer and subsequently free'ing
it in atomic context, triggering a warning.

> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 257 at kernel/events/ring_buffer.c:541 __rb_free_aux+0x11a/0x130
> CPU: 0 PID: 257 Comm: stopbug Not tainted 4.8.0-rc1+ #2596
> Call Trace:
>  [<ffffffff810f3e0b>] __warn+0xcb/0xf0
>  [<ffffffff810f3f3d>] warn_slowpath_null+0x1d/0x20
>  [<ffffffff8121182a>] __rb_free_aux+0x11a/0x130
>  [<ffffffff812127a8>] rb_free_aux+0x18/0x20
>  [<ffffffff81212913>] perf_aux_output_begin+0x163/0x1e0
>  [<ffffffff8100c33a>] bts_event_start+0x3a/0xd0
>  [<ffffffff8100c42d>] bts_event_add+0x5d/0x80
>  [<ffffffff81203646>] event_sched_in.isra.104+0xf6/0x2f0
>  [<ffffffff8120652e>] group_sched_in+0x6e/0x190
>  [<ffffffff8120694e>] ctx_sched_in+0x2fe/0x5f0
>  [<ffffffff81206ca0>] perf_event_sched_in+0x60/0x80
>  [<ffffffff81206d1b>] ctx_resched+0x5b/0x90
>  [<ffffffff81207281>] __perf_event_enable+0x1e1/0x240
>  [<ffffffff81200639>] event_function+0xa9/0x180
>  [<ffffffff81202000>] ? perf_cgroup_attach+0x70/0x70
>  [<ffffffff8120203f>] remote_function+0x3f/0x50
>  [<ffffffff811971f3>] flush_smp_call_function_queue+0x83/0x150
>  [<ffffffff81197bd3>] generic_smp_call_function_single_interrupt+0x13/0x60
>  [<ffffffff810a6477>] smp_call_function_single_interrupt+0x27/0x40
>  [<ffffffff81a26ea9>] call_function_single_interrupt+0x89/0x90
>  [<ffffffff81120056>] finish_task_switch+0xa6/0x210
>  [<ffffffff81120017>] ? finish_task_switch+0x67/0x210
>  [<ffffffff81a1e83d>] __schedule+0x3dd/0xb50
>  [<ffffffff81a1efe5>] schedule+0x35/0x80
>  [<ffffffff81128031>] sys_sched_yield+0x61/0x70
>  [<ffffffff81a25be5>] entry_SYSCALL_64_fastpath+0x18/0xa8
> ---[ end trace 6235f556f5ea83a9 ]---

This patch puts the checks in perf_aux_output_begin() in the same order
as that of perf_mmap_close().
Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: vince@deater.net
Link: http://lkml.kernel.org/r/20160906132353.19887-3-alexander.shishkin@linux.intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>