[tip:perf/urgent] perf/core: Use this_cpu_ptr() when stopping AUX events

From: tip-bot for Will Deacon
Date: Wed Aug 24 2016 - 09:13:48 EST

Commit-ID: 8b6a3fe8fab97716990a3abde1a01fb5a34552a3
Gitweb: http://git.kernel.org/tip/8b6a3fe8fab97716990a3abde1a01fb5a34552a3
Author: Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Wed, 24 Aug 2016 10:07:14 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 24 Aug 2016 15:03:10 +0200

perf/core: Use this_cpu_ptr() when stopping AUX events

When tearing down an AUX buf for an event via perf_mmap_close(),
__perf_event_output_stop() is called on the event's CPU to ensure that
trace generation is halted before the process of unmapping and
freeing the buffer pages begins.

The callback is performed via cpu_function_call(), which ensures that it
runs with interrupts disabled and is therefore not preemptible.
Unfortunately, the current code grabs the per-cpu context pointer using
get_cpu_ptr(), which unnecessarily disables preemption and doesn't pair
the call with put_cpu_ptr(), leading to a preempt_count() imbalance and
a BUG when freeing the AUX buffer later on:

WARNING: CPU: 1 PID: 2249 at kernel/events/ring_buffer.c:539 __rb_free_aux+0x10c/0x120
Modules linked in:
Call Trace:
[<ffffffff813379dd>] dump_stack+0x4f/0x72
[<ffffffff81059ff6>] __warn+0xc6/0xe0
[<ffffffff8105a0c8>] warn_slowpath_null+0x18/0x20
[<ffffffff8112761c>] __rb_free_aux+0x10c/0x120
[<ffffffff81128163>] rb_free_aux+0x13/0x20
[<ffffffff8112515e>] perf_mmap_close+0x29e/0x2f0
[<ffffffff8111da30>] ? perf_iterate_ctx+0xe0/0xe0
[<ffffffff8115f685>] remove_vma+0x25/0x60
[<ffffffff81161796>] exit_mmap+0x106/0x140
[<ffffffff8105725c>] mmput+0x1c/0xd0
[<ffffffff8105cac3>] do_exit+0x253/0xbf0
[<ffffffff8105e32e>] do_group_exit+0x3e/0xb0
[<ffffffff81068d49>] get_signal+0x249/0x640
[<ffffffff8101c273>] do_signal+0x23/0x640
[<ffffffff81905f42>] ? _raw_write_unlock_irq+0x12/0x30
[<ffffffff81905f69>] ? _raw_spin_unlock_irq+0x9/0x10
[<ffffffff81901896>] ? __schedule+0x2c6/0x710
[<ffffffff810022a4>] exit_to_usermode_loop+0x74/0x90
[<ffffffff81002a56>] prepare_exit_to_usermode+0x26/0x30
[<ffffffff81906d1b>] retint_user+0x8/0x10

This patch uses this_cpu_ptr() instead of get_cpu_ptr(), since preemption is
already disabled by the caller.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Reviewed-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vince Weaver <vincent.weaver@xxxxxxxxx>
Fixes: 95ff4ca26c49 ("perf/core: Free AUX pages in unmap path")
Link: http://lkml.kernel.org/r/20160824091905.GA16944@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/events/core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5650f53..3cfabdf 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6166,7 +6166,7 @@ static int __perf_pmu_output_stop(void *info)
struct perf_event *event = info;
struct pmu *pmu = event->pmu;
- struct perf_cpu_context *cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
+ struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
struct remote_output ro = {
.rb = event->rb,