diff options
authorYabin Cui <yabinc@google.com>2019-05-17 13:52:31 +0200
committerPaul Gortmaker <paul.gortmaker@windriver.com>2019-09-16 12:21:35 -0400
commite1fdf22e0fffa8265aa0275a450942d7f113c0cb (patch)
parent5fc3dab04a699f4b399abef5cdfcddc84abda9e2 (diff)
perf/ring_buffer: Fix exposing a temporarily decreased data_head
commit 1b038c6e05ff70a1e66e3e571c2e6106bdb75f53 upstream. 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> Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
1 files changed, 20 insertions, 4 deletions
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 99c7f199f2d4..31edf1f39cca 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -52,11 +52,18 @@ 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:
@@ -88,9 +95,18 @@ 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))) {
goto again;