> -----Original Message-----
> From: Tamar Christina <tamar.christ...@arm.com>
> Sent: Wednesday, June 8, 2022 3:50 PM
> To: gcc-patches@gcc.gnu.org
> Cc: nd <n...@arm.com>; Ramana Radhakrishnan
> <ramana.radhakrish...@arm.com>; Richard Earnshaw
> <richard.earns...@arm.com>; ni...@redhat.com; Kyrylo Tkachov
> <kyrylo.tkac...@arm.com>
> Subject: [PATCH 2/2][AArch32] Fix 128-bit sequential consistency atomic
> operations.
>
> Hi All,
>
> Similar to AArch64 the Arm implementation of 128-bit atomics is broken.
>
> For 128-bit atomics we rely on pthread barriers to correct guard the address
> in the pointer to get correct memory ordering. However for 128-bit atomics
> the
> address under the lock is different from the original pointer.
>
> This means that one of the values under the atomic operation is not
> protected
> properly and so we fail during when the user has requested sequential
> consistency as there's no barrier to enforce this requirement.
>
> As such users have resorted to adding an
>
> #ifdef GCC
> <emit barrier>
> #endif
>
> around the use of these atomics.
>
> This corrects the issue by issuing a barrier only when __ATOMIC_SEQ_CST
> was
> requested. I have hand verified that the barriers are inserted
> for atomic seq cst.
>
>
> Bootstrapped Regtested on arm-none-linux-gnueabihf and no issues.
>
> Ok for master? and for backporting to GCC 12, 11 and 10?
Ok, with backports after a couple weeks on master.
Thanks,
Kyrill
>
> Thanks,
> Tamar
>
> libatomic/ChangeLog:
>
> PR target/102218
> * config/arm/host-config.h (pre_seq_barrier, post_seq_barrier,
> pre_post_seq_barrier): Require barrier on __ATOMIC_SEQ_CST.
>
> --- inline copy of patch --
> diff --git a/libatomic/config/arm/host-config.h b/libatomic/config/arm/host-
> config.h
> index
> bbf4a3f84c3f3ae21fb2162aab68bdedf3fbdcb4..ef16fad2a35ec9055e918849e
> 69a1a0e23b62838 100644
> --- a/libatomic/config/arm/host-config.h
> +++ b/libatomic/config/arm/host-config.h
> @@ -1,4 +1,23 @@
> /* Avoiding the DMB (or kernel helper) can be a good thing. */
> #define WANT_SPECIALCASE_RELAXED
>
> +/* Glibc, at least, uses acq_rel in its pthread mutex
> + implementation. If the user is asking for seq_cst,
> + this is insufficient. */
> +
> +static inline void __attribute__((always_inline, artificial))
> +pre_seq_barrier(int model)
> +{
> + if (model == __ATOMIC_SEQ_CST)
> + __atomic_thread_fence (__ATOMIC_SEQ_CST);
> +}
> +
> +static inline void __attribute__((always_inline, artificial))
> +post_seq_barrier(int model)
> +{
> + pre_seq_barrier(model);
> +}
> +
> +#define pre_post_seq_barrier 1
> +
> #include_next <host-config.h>
>
>
>
>
> --
