Commit 1b038c6e authored May 17, 2019 by Yabin Cui Committed by Ingo Molnar May 24, 2019

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: ef60777c ("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>

parent 23e3983a

kernel/events/ring_buffer.c

+20 −4

Original line number	Diff line number	Diff line
		@@ -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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