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Date: Fri, 17 May 2019 15:05:09 +0200 From: Ingo Molnar <mingo@...nel.org> To: Peter Zijlstra <peterz@...radead.org> Cc: yabinc@...gle.com, acme@...nel.org, alexander.shishkin@...ux.intel.com, jolsa@...hat.com, namhyung@...nel.org, mark.rutland@....com, linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com> Subject: Re: [PATCH 1/4] perf/ring_buffer: Fix exposing a temporarily decreased data_head. * Peter Zijlstra <peterz@...radead.org> wrote: > 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. > > Cc: Arnaldo Carvalho de Melo <acme@...nel.org> > Cc: Ingo Molnar <mingo@...hat.com> > Cc: Jiri Olsa <jolsa@...hat.com> > Cc: Namhyung Kim <namhyung@...nel.org> > Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com> > Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables") > Signed-off-by: Yabin Cui <yabinc@...gle.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org> > Link: https://lkml.kernel.org/r/20190516184010.167903-1-yabinc@google.com So these are missing a bunch of: From: Yabin Cui <yabinc@...gle.com> lines, right? Thanks, Ingo
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