Re: [PATCH v2 8/8] perf record: Directly bail out for compat case

[Leo Yan]

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@xxxxxxxxx>
> 
> 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