Messages in this thread | | | Date | Wed, 2 Jun 2021 20:38:47 +0800 | From | Leo Yan <> | Subject | Re: [PATCH v2 8/8] perf record: Directly bail out for compat case |
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Hi Adrain,
On Wed, Jun 02, 2021 at 02:18:47PM +0300, Adrian Hunter wrote: > On 2/06/21 1:30 pm, Leo Yan wrote: > > Since the 64-bit atomicity is not promised in 32-bit perf, directly > > report the error and bail out for this case. > > > > Now only applies on x86_64 and Arm64 platforms. > > > > Suggested-by: Adrian Hunter <adrian.hunter@intel.com> > > Maybe we can do better for the compat case. > > We can assume the upper 32-bits change very seldom, > and always increase. So for the 'read' case: > > u64 first, second, last; > u64 mask = (u64)((u32)-1) << 32; > > do { > first = READ_ONCE(pc->aux_head); > rmb(); > second = READ_ONCE(pc->aux_head); > rmb(); > last = READ_ONCE(pc->aux_head); > } while ((first & mask) != (last & mask)); > return second; > > For the write case, we can cause a fatal error only if the new > tail has non-zero upper 32-bits. That gives up to 4GiB of data > before aborting: > > if (tail & mask) > return -1; > smp_mb(); > WRITE_ONCE(pc->aux_tail, tail);
Seems to me, it's pointless to only support aux_head for 64-bit and support aux_tail for 32-bit. I understand this can be helpful for the snapshot mode which only uses aux_head, but it still fails to support the normal case for AUX ring buffer using 64-bit head/tail.
Thanks, Leo
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