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

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Mar 16, 2026 at 11:37:12PM +0000, David Laight wrote:
> On Mon, 16 Mar 2026 15:08:07 -0700
> Ankur Arora <ankur.a.arora@oracle.com> 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.

It won't but also passing an MMIO pointer to smp_cond_load() is wrong in
general. You'd need a new API that takes an __iomem pointer.

> 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.

Even worse, depending on the hardware, you may even get a data abort
when attempting LDXR on Device memory.

> 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.

Hmm, I thought this is fine because of the implicit SEVL on exception
return but the arm64 __cmpwait_relaxed() does a SEVL+WFE which clears
any prior event, it can wait in theory forever when the event stream is
disabled.

Expanding smp_cond_load_relaxed() into asm, we have something like:

	LDR	X0, [PTR]
	condition check for VAL || need_resched() with branch out
	SEVL
	WFE
	LDXR	X1, [PTR]
	EOR	X1, X1, X0
	CBNZ	out
	WFE
  out:

If the condition is updated to become true (need_resched()) after the
condition check but before the first WFE while *PTR remains unchanged,
the IPI won't do anything. Maybe we should revert 1cfc63b5ae60 ("arm64:
cmpwait: Clear event register before arming exclusive monitor"). Not
great but probably better than reverting f5bfdc8e3947 ("locking/osq: Use
optimized spinning loop for arm64")).

Using SEV instead of IPI would have the same problem.

-- 
Catalin