Messages in this thread | | | From | Uros Bizjak <> | Date | Tue, 9 Apr 2024 14:50:19 +0200 | Subject | Re: [PATCH 2/6] locking/atomic/x86: Rewrite x86_32 arch_atomic64_{,fetch}_{and,or,xor}() functions |
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On Tue, Apr 9, 2024 at 2:03 PM Uros Bizjak <ubizjak@gmail.com> wrote: > > On Tue, Apr 9, 2024 at 1:13 PM Mark Rutland <mark.rutland@arm.com> wrote: > > > > static __always_inline void arch_atomic64_and(s64 i, atomic64_t *v) > > > { > > > - s64 old, c = 0; > > > + s64 val = __READ_ONCE(v->counter); > > > > I reckon it's worth placing this in a helper with a big comment, e.g. > > > > static __always_inline s64 arch_atomic64_read_tearable(atomic64_t *v) > > { > > /* > > * TODO: explain that this might be torn, but it occurs *once*, and can > > * safely be consumed by atomic64_try_cmpxchg(). > > * > > * TODO: point to the existing commentary regarding why we use > > * __READ_ONCE() for KASAN reasons. > > */ > > return __READ_ONCE(v->counter); > > } > > > > ... and then use that in each of the instances below. > > > > That way the subtlety is clearly documented, and it'd more clearly align with > > the x86_64 verions. > > This is an excellent idea. The separate definitions needs to be placed > in atomic64_32.h and atomic_64_64.h (due to use of atomic64_t > typedef), but it will allow the same unification of functions between > x64_32 and x64_64 as the approach with __READ_ONCE().
Something like this:
--cut here-- /* * This function is intended to preload the value from atomic64_t * location in a non-atomic way. The read might be torn, but can * safely be consumed by the compare-and-swap loop. */ static __always_inline s64 arch_atomic64_read_tearable(atomic64_t *v) { /* * See the comment in arch_atomic_read() on why we use * __READ_ONCE() instead of READ_ONCE_NOCHECK() here. */ return __READ_ONCE(v->counter); } --cut here--
Thanks, Uros.
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