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    SubjectRe: [ 24/71] ring-buffer: Fix race between integrity check and readers
    On Tue, Jan 15, 2013 at 02:54:21PM -0800, Greg Kroah-Hartman wrote:
    > 3.0-stable review patch. If anyone has any objections, please let me know.
    >
    > ------------------
    >
    > From: Steven Rostedt <srostedt@redhat.com>
    >
    > commit 9366c1ba13fbc41bdb57702e75ca4382f209c82f upstream.
    >
    > The function rb_check_pages() was added to make sure the ring buffer's
    > pages were sane. This check is done when the ring buffer size is modified
    > as well as when the iterator is released (closing the "trace" file),
    > as that was considered a non fast path and a good place to do a sanity
    > check.
    >
    > The problem is that the check does not have any locks around it.
    > If one process were to read the trace file, and another were to read
    > the raw binary file, the check could happen while the reader is reading
    > the file.
    >
    > The issues with this is that the check requires to clear the HEAD page
    > before doing the full check and it restores it afterward. But readers
    > require the HEAD page to exist before it can read the buffer, otherwise
    > it gives a nasty warning and disables the buffer.
    >
    > By adding the reader lock around the check, this keeps the race from
    > happening.
    >
    > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    Too late now (and harmless), but the commit id and changelog are wrong, the
    real commit for this backport is 54f7be5b831254199522523ccab4c3d954bbf576
    (ring-buffer: Fix NULL pointer if rb_set_head_page() fails). The same
    happened with the patch applied on 3.4 this cycle.

    --
    []'s
    Herton


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