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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 <>
> 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 <>
> Signed-off-by: Greg Kroah-Hartman <>

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.


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