Messages in this thread |  | | Subject | Re: perf events ring buffer memory barrier on powerpc | From | Victor Kaplansky <> | Date | Fri, 1 Nov 2013 16:25:42 +0200 |
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"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote on 10/31/2013 08:40:15 AM:
> > void ubuf_read(void) > > { > > u64 head, tail; > > > > tail = ACCESS_ONCE(ubuf->tail); > > head = ACCESS_ONCE(ubuf->head); > > > > /* > > * Ensure we read the buffer boundaries before the actual buffer > > * data... > > */ > > smp_rmb(); /* C, matches with B */ > > > > while (tail != head) { > > obj = ubuf->data + tail; > > /* process obj */ > > tail += obj->size; > > tail %= ubuf->size; > > } > > > > /* > > * Ensure all data reads are complete before we issue the > > * ubuf->tail update; once that update hits, kbuf_write() can > > * observe and overwrite data. > > */ > > smp_mb(); /* D, matches with A */ > > > > ubuf->tail = tail; > > }
> > Could we replace A and C with an smp_read_barrier_depends()? > > C, yes, given that you have ACCESS_ONCE() on the fetch from ->tail > and that the value fetch from ->tail feeds into the address used for > the "obj =" assignment.
No! You must to have a full smp_rmb() at C. The race on the reader side is not between fetch of @tail and read from address pointed by @tail. The real race here is between a fetch of @head and read of obj from memory pointed by @tail.
Regards, -- Victor
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