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Subject[tip:perf/urgent] perf/ring_buffer: Fix exposing a temporarily decreased data_head
Commit-ID:  1b038c6e05ff70a1e66e3e571c2e6106bdb75f53
Gitweb: https://git.kernel.org/tip/1b038c6e05ff70a1e66e3e571c2e6106bdb75f53
Author: Yabin Cui <yabinc@google.com>
AuthorDate: Fri, 17 May 2019 13:52:31 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 24 May 2019 09:00:10 +0200

perf/ring_buffer: Fix exposing a temporarily decreased data_head

In perf_output_put_handle(), an IRQ/NMI can happen in below location and
write records to the same ring buffer:

...
local_dec_and_test(&rb->nest)
... <-- an IRQ/NMI can happen here
rb->user_page->data_head = head;
...

In this case, a value A is written to data_head in the IRQ, then a value
B is written to data_head after the IRQ. And A > B. As a result,
data_head is temporarily decreased from A to B. And a reader may see
data_head < data_tail if it read the buffer frequently enough, which
creates unexpected behaviors.

This can be fixed by moving dec(&rb->nest) to after updating data_head,
which prevents the IRQ/NMI above from updating data_head.

[ Split up by peterz. ]

Signed-off-by: Yabin Cui <yabinc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: mark.rutland@arm.com
Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables")
Link: http://lkml.kernel.org/r/20190517115418.224478157@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/events/ring_buffer.c | 24 ++++++++++++++++++++----
1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 674b35383491..009467a60578 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -51,11 +51,18 @@ again:
head = local_read(&rb->head);

/*
- * IRQ/NMI can happen here, which means we can miss a head update.
+ * IRQ/NMI can happen here and advance @rb->head, causing our
+ * load above to be stale.
*/

- if (!local_dec_and_test(&rb->nest))
+ /*
+ * If this isn't the outermost nesting, we don't have to update
+ * @rb->user_page->data_head.
+ */
+ if (local_read(&rb->nest) > 1) {
+ local_dec(&rb->nest);
goto out;
+ }

/*
* Since the mmap() consumer (userspace) can run on a different CPU:
@@ -87,9 +94,18 @@ again:
rb->user_page->data_head = head;

/*
- * Now check if we missed an update -- rely on previous implied
- * compiler barriers to force a re-read.
+ * We must publish the head before decrementing the nest count,
+ * otherwise an IRQ/NMI can publish a more recent head value and our
+ * write will (temporarily) publish a stale value.
+ */
+ barrier();
+ local_set(&rb->nest, 0);
+
+ /*
+ * Ensure we decrement @rb->nest before we validate the @rb->head.
+ * Otherwise we cannot be sure we caught the 'last' nested update.
*/
+ barrier();
if (unlikely(head != local_read(&rb->head))) {
local_inc(&rb->nest);
goto again;
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