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

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

On Wed, Mar 25, 2026 at 08:23:57PM +0000, David Laight wrote:
> On Wed, 25 Mar 2026 16:32:49 +0000
> Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> > On Wed, Mar 25, 2026 at 03:42:10PM +0000, David Laight wrote:
> ...
> > > Looking at the code I think the "sevl; wfe" pair should be higher up.  
> > 
> > Yes, I replied to your other message. We could move it higher indeed,
> > before the condition check, but I can't get my head around the ordering.
> > Can need_resched() check be speculated before the WFE? I need to think
> > some more.
> 
> I don't think speculation can matter.
> Both SEVL and WFE must be serialised against any other instructions
> that can change the event flag (as well as each other) otherwise
> everything is broken.

Welcome to the Arm memory model. We don't have any guarantee that an LDR
will only access memory after SEVL+WFE. They are not serialising.

> Apart from that it doesn't matter, what matters is the instruction
> boundary the interrupt happens at.

True. If an interrupt is taken before the LDR (that would be a
need_resched() check for example), then a prior WFE would not matter.
This won't work if we replace the IPI with a SEV though (suggested
somewhere in this thread).

> Actually both SEVL and WFE may be synchronising instructions and very slow.

Most likely not.

> So you may not want to put them in the fast path where the condition
> is true on entry (or even true after a retry).
> So the code might have to look like:
> 	for (;;) {
> 		VAL = mem;

If we only waited on the location passed to LDXR, things would have been
much simpler. But the osq_lock() also wants to wait on the TIF flags via
need_resched() (and vcpu_is_preempted()).

> 		if (cond(VAL)) return;

So the cond(VAL) here is actually a series of other memory loads
unrelated to 'mem'

> 		SEVL; WFE;
> 		if (cond(VAL)) return;

I think this will work in principle even if 'cond' accesses other memory
locations, though I wouldn't bother with an additional 'cond' call, I'd
expect SEVL+WFE to be mostly NOPs. However, 'cond' must not set a local
event, otherwise the power saving on waiting is gone.

> 		v1 = LDX(mem);
> 		if (v1 == VAL)
> 			WFE;
> 	}

I think it's cleaner to use Ankur's timeout API here for the very rare
case where an IPI hits at the wrong time. We then keep
smp_cond_load_relaxed() intact as it's really not meant to wait on
multiple memory locations to change. Any changes of
smp_cond_load_relaxed() with moving the WFE around are just hacks, not
the intended use of this API.

-- 
Catalin