[PATCH v3] Avoid memory barrier in read_seqcount() through load acquire
Christoph Lameter (Ampere)
cl at gentwo.org
Wed Sep 18 04:03:44 PDT 2024
On Tue, 17 Sep 2024, Will Deacon wrote:
> > +config ARCH_HAS_ACQUIRE_RELEASE
> > + bool
> > + help
> > + Setting ARCH_HAS_ACQUIRE_RELEASE indicates that the architecture
> > + supports load acquire and release. Typically these are more effective
> > + than memory barriers. Code will prefer the use of load acquire and
> > + store release over memory barriers if this option is enabled.
> > +
>
> Unsurprisingly, I'd be in favour of making this unconditional rather than
> adding a new Kconfig option. Would that actually hurt any architectures
> where we care about the last few shreds of performance?
Other arches do not have acquire / release and will create additional
barriers in the fallback implementation of smp_load_acquire. So it needs
to be an arch config option.
> > + if (USE_COND_LOAD_ACQUIRE)
> > + return smp_cond_load_acquire((unsigned int *)&s->sequence, (s->sequence & 1) == 0);
>
> This looks wrong to me.
>
> The conditional expression passed to smp_cond_load_acquire() should be
> written in terms of 'VAL', otherwise you're introducing an additional
> non-atomic access to the sequence counter.
Hmmm... The compiler seems to have optimized that out. Will use VAL in
next rollup.
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