ring-buffer: Do not try to put back write_stamp
commit dd939425 upstream. If an update to an event is interrupted by another event between the time the initial event allocated its buffer and where it wrote to the write_stamp, the code try to reset the write stamp back to the what it had just overwritten. It knows that it was overwritten via checking the before_stamp, and if it didn't match what it wrote to the before_stamp before it allocated its space, it knows it was overwritten. To put back the write_stamp, it uses the before_stamp it read. The problem here is that by writing the before_stamp to the write_stamp it makes the two equal again, which means that the write_stamp can be considered valid as the last timestamp written to the ring buffer. But this is not necessarily true. The event that interrupted the event could have been interrupted in a way that it was interrupted as well, and can end up leaving with an invalid write_stamp. But if this happens and returns to this context that uses the before_stamp to update the write_stamp again, it can possibly incorrectly make it valid, causing later events to have in correct time stamps. As it is OK to leave this function with an invalid write_stamp (one that doesn't match the before_stamp), there's no reason to try to make it valid again in this case. If this race happens, then just leave with the invalid write_stamp and the next event to come along will just add a absolute timestamp and validate everything again. Bonus points: This gets rid of another cmpxchg64! Link: https://lore.kernel.org/linux-trace-kernel/20231214222921.193037a7@gandalf.local.home Cc: stable@vger.kernel.org Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: Joel Fernandes <joel@joelfernandes.org> Cc: Vincent Donnefort <vdonnefort@google.com> Fixes: a389d86f ("ring-buffer: Have nested events still record running time stamp") Signed-off-by:Steven Rostedt (Google) <rostedt@goodmis.org> Signed-off-by:
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
parent
bc17bc96
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