Re: [PATCH v10 00/12] barrier: Add smp_cond_load_{relaxed,acquire}_timeout()

[David Laight]

On Mon, 16 Mar 2026 15:08:07 -0700
Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:

...
> However, as David Laight pointed out in this thread
> (https://lore.kernel.org/lkml/20260214113122.70627a8b@pumpkin/)
> that this would be fine so long as the polling is on memory, but would
> need some work to handle MMIO.

I'm not sure the current code works with MMIO on arm64.
I was looking at the osq_lock() code, it uses smp_cond_load() with 'expr'
being 'VAL || need_resched()' expecting to get woken by the IPI associated
with the preemption being requested.
But the arm64 code relies on 'wfe' being woken when the memory write
'breaks' the 'ldx' for the monitored location.
That will only work for cached addresses.

For osq_lock(), while an IPI will wake it up, there is also a small timing
window where the IPI can happen before the ldx and so not actually wake up it.
This is true whenever 'expr' is non-trivial.
On arm64 I think you could use explicit sev and wfe - but that will wake all
'sleeping' cpu; and you may not want the 'thundering herd'.

	David