* Peter Zijlstra <[email protected]> wrote:
> On Fri, May 17, 2019 at 03:05:09PM +0200, Ingo Molnar wrote: > > > > * Peter Zijlstra <[email protected]> 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 <[email protected]> > > > Cc: Ingo Molnar <[email protected]> > > > Cc: Jiri Olsa <[email protected]> > > > Cc: Namhyung Kim <[email protected]> > > > Cc: Alexander Shishkin <[email protected]> > > > Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing > > > IRQ-disables") > > > Signed-off-by: Yabin Cui <[email protected]> > > > Signed-off-by: Peter Zijlstra (Intel) <[email protected]> > > > Link: https://lkml.kernel.org/r/[email protected] > > > > So these are missing a bunch of: > > > > From: Yabin Cui <[email protected]> > > > > lines, right? > > > > The first. certainly, the rest, while inspired by his patch, is more > complete than what he did. Oh, indeed - I adjusted the first patch only. Thanks, Ingo
