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Subject[75/85] perf: Remove WARN_ON_ONCE() check in __perf_event_enable() for valid scenario
3.2.49-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jiri Olsa <jolsa@redhat.com>

commit 06f417968beac6e6b614e17b37d347aa6a6b1d30 upstream.

The '!ctx->is_active' check has a valid scenario, so
there's no need for the warning.

The reason is that there's a time window between the
'ctx->is_active' check in the perf_event_enable() function
and the __perf_event_enable() function having:

- IRQs on
- ctx->lock unlocked

where the task could be killed and 'ctx' deactivated by
perf_event_exit_task(), ending up with the warning below.

So remove the WARN_ON_ONCE() check and add comments to
explain it all.

This addresses the following warning reported by Vince Weaver:

[ 324.983534] ------------[ cut here ]------------
[ 324.984420] WARNING: at kernel/events/core.c:1953 __perf_event_enable+0x187/0x190()
[ 324.984420] Modules linked in:
[ 324.984420] CPU: 19 PID: 2715 Comm: nmi_bug_snb Not tainted 3.10.0+ #246
[ 324.984420] Hardware name: Supermicro X8DTN/X8DTN, BIOS 4.6.3 01/08/2010
[ 324.984420] 0000000000000009 ffff88043fce3ec8 ffffffff8160ea0b ffff88043fce3f00
[ 324.984420] ffffffff81080ff0 ffff8802314fdc00 ffff880231a8f800 ffff88043fcf7860
[ 324.984420] 0000000000000286 ffff880231a8f800 ffff88043fce3f10 ffffffff8108103a
[ 324.984420] Call Trace:
[ 324.984420] <IRQ> [<ffffffff8160ea0b>] dump_stack+0x19/0x1b
[ 324.984420] [<ffffffff81080ff0>] warn_slowpath_common+0x70/0xa0
[ 324.984420] [<ffffffff8108103a>] warn_slowpath_null+0x1a/0x20
[ 324.984420] [<ffffffff81134437>] __perf_event_enable+0x187/0x190
[ 324.984420] [<ffffffff81130030>] remote_function+0x40/0x50
[ 324.984420] [<ffffffff810e51de>] generic_smp_call_function_single_interrupt+0xbe/0x130
[ 324.984420] [<ffffffff81066a47>] smp_call_function_single_interrupt+0x27/0x40
[ 324.984420] [<ffffffff8161fd2f>] call_function_single_interrupt+0x6f/0x80
[ 324.984420] <EOI> [<ffffffff816161a1>] ? _raw_spin_unlock_irqrestore+0x41/0x70
[ 324.984420] [<ffffffff8113799d>] perf_event_exit_task+0x14d/0x210
[ 324.984420] [<ffffffff810acd04>] ? switch_task_namespaces+0x24/0x60
[ 324.984420] [<ffffffff81086946>] do_exit+0x2b6/0xa40
[ 324.984420] [<ffffffff8161615c>] ? _raw_spin_unlock_irq+0x2c/0x30
[ 324.984420] [<ffffffff81087279>] do_group_exit+0x49/0xc0
[ 324.984420] [<ffffffff81096854>] get_signal_to_deliver+0x254/0x620
[ 324.984420] [<ffffffff81043057>] do_signal+0x57/0x5a0
[ 324.984420] [<ffffffff8161a164>] ? __do_page_fault+0x2a4/0x4e0
[ 324.984420] [<ffffffff8161665c>] ? retint_restore_args+0xe/0xe
[ 324.984420] [<ffffffff816166cd>] ? retint_signal+0x11/0x84
[ 324.984420] [<ffffffff81043605>] do_notify_resume+0x65/0x80
[ 324.984420] [<ffffffff81616702>] retint_signal+0x46/0x84
[ 324.984420] ---[ end trace 442ec2f04db3771a ]---

Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1373384651-6109-2-git-send-email-jolsa@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
kernel/events/core.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1687,7 +1687,16 @@ static int __perf_event_enable(void *inf
struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
int err;

- if (WARN_ON_ONCE(!ctx->is_active))
+ /*
+ * There's a time window between 'ctx->is_active' check
+ * in perf_event_enable function and this place having:
+ * - IRQs on
+ * - ctx->lock unlocked
+ *
+ * where the task could be killed and 'ctx' deactivated
+ * by perf_event_exit_task.
+ */
+ if (!ctx->is_active)
return -EINVAL;

raw_spin_lock(&ctx->lock);


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