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