On Thu, Nov 07, 2013 at 04:43:15PM +1100, Michael Neuling wrote: > Upstream commit: bf378d341e4873ed928dc3c636252e6895a21f50 Since this seems to be applicable to old kernels, I'm queuing it for 3.5 (with a file rename: include/uapi/linux/perf_event.h -> include/linux/perf_event.h). Cheers, -- Luis > > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > perf: Fix perf ring buffer memory ordering > > The PPC64 people noticed a missing memory barrier and crufty old > comments in the perf ring buffer code. So update all the comments and > add the missing barrier. > > When the architecture implements local_t using atomic_long_t there > will be double barriers issued; but short of introducing more > conditional barrier primitives this is the best we can do. > > Reported-by: Victor Kaplansky <victork@xxxxxxxxxx> > Tested-by: Victor Kaplansky <victork@xxxxxxxxxx> > Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> > Cc: michael@xxxxxxxxxxxxxx > Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > Cc: Michael Neuling <mikey@xxxxxxxxxxx> > Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> > Cc: anton@xxxxxxxxx > Cc: benh@xxxxxxxxxxxxxxxxxxx > Link: http://lkml.kernel.org/r/20131025173749.GG19466@xxxxxxxxxx > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> > --- > include/uapi/linux/perf_event.h | 12 +++++++----- > kernel/events/ring_buffer.c | 31 +++++++++++++++++++++++++++---- > 2 files changed, 34 insertions(+), 9 deletions(-) > > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h > index 009a655..2fc1602 100644 > --- a/include/uapi/linux/perf_event.h > +++ b/include/uapi/linux/perf_event.h > @@ -456,13 +456,15 @@ struct perf_event_mmap_page { > /* > * Control data for the mmap() data buffer. > * > - * User-space reading the @data_head value should issue an rmb(), on > - * SMP capable platforms, after reading this value -- see > - * perf_event_wakeup(). > + * User-space reading the @data_head value should issue an smp_rmb(), > + * after reading this value. > * > * When the mapping is PROT_WRITE the @data_tail value should be > - * written by userspace to reflect the last read data. In this case > - * the kernel will not over-write unread data. > + * written by userspace to reflect the last read data, after issueing > + * an smp_mb() to separate the data read from the ->data_tail store. > + * In this case the kernel will not over-write unread data. > + * > + * See perf_output_put_handle() for the data ordering. > */ > __u64 data_head; /* head in the data section */ > __u64 data_tail; /* user-space written tail */ > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > index cd55144..9c2ddfb 100644 > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -87,10 +87,31 @@ again: > goto out; > > /* > - * Publish the known good head. Rely on the full barrier implied > - * by atomic_dec_and_test() order the rb->head read and this > - * write. > + * Since the mmap() consumer (userspace) can run on a different CPU: > + * > + * kernel user > + * > + * READ ->data_tail READ ->data_head > + * smp_mb() (A) smp_rmb() (C) > + * WRITE $data READ $data > + * smp_wmb() (B) smp_mb() (D) > + * STORE ->data_head WRITE ->data_tail > + * > + * Where A pairs with D, and B pairs with C. > + * > + * I don't think A needs to be a full barrier because we won't in fact > + * write data until we see the store from userspace. So we simply don't > + * issue the data WRITE until we observe it. Be conservative for now. > + * > + * OTOH, D needs to be a full barrier since it separates the data READ > + * from the tail WRITE. > + * > + * For B a WMB is sufficient since it separates two WRITEs, and for C > + * an RMB is sufficient since it separates two READs. > + * > + * See perf_output_begin(). > */ > + smp_wmb(); > rb->user_page->data_head = head; > > /* > @@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output_handle *handle, > * Userspace could choose to issue a mb() before updating the > * tail pointer. So that all reads will be completed before the > * write is issued. > + * > + * See perf_output_put_handle(). > */ > tail = ACCESS_ONCE(rb->user_page->data_tail); > - smp_rmb(); > + smp_mb(); > offset = head = local_read(&rb->head); > head += size; > if (unlikely(!perf_output_space(rb, tail, offset, head))) > -- > 1.8.3.2 > > -- > To unsubscribe from this list: send the line "unsubscribe stable" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html