lkml.org 
[lkml]   [2019]   [Jun]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH AUTOSEL 5.1 21/70] perf/ring_buffer: Fix exposing a temporarily decreased data_head
    Date
    From: Yabin Cui <yabinc@google.com>

    [ Upstream commit 1b038c6e05ff70a1e66e3e571c2e6106bdb75f53 ]

    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: Sasha Levin <sashal@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;
    --
    2.20.1
    \
     
     \ /
      Last update: 2019-06-08 13:40    [W:4.135 / U:0.948 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site