RE: [PATCH 1/2]AArch64 Fix 128-bit sequential consistency atomic operations.

[Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>]

Hi Tamar,

Let me be the latest to offer my apologies for the slow review.

> -----Original Message-----
> From: Tamar Christina <Tamar.Christina@arm.com>
> Sent: Wednesday, June 8, 2022 3:49 PM
> To: gcc-patches@gcc.gnu.org
> Cc: nd <nd@arm.com>; Richard Earnshaw <Richard.Earnshaw@arm.com>;
> Marcus Shawcroft <Marcus.Shawcroft@arm.com>; Kyrylo Tkachov
> <Kyrylo.Tkachov@arm.com>; Richard Sandiford
> <Richard.Sandiford@arm.com>
> Subject: [PATCH 1/2]AArch64 Fix 128-bit sequential consistency atomic
> operations.
> 
> Hi All,
> 
> The AArch64 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.  To remedy this performance hit I think we should revisit using a
> similar approach to out-line-atomics for the 128-bit atomics.
> 
> Note that I believe I need the empty file due to the include_next chain but
> I am not entirely sure.  I have hand verified that the barriers are inserted
> for atomic seq cst.
> 
> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> 
> Ok for master? and for backporting to GCC 12, 11 and 10?

I'll admit I'm not too familiar with the mechanics of libatomic but...

> 
> Thanks,
> Tamar
> 
> libatomic/ChangeLog:
> 
> 	PR target/102218
> 	* config/aarch64/aarch64-config.h: New file.
> 	* config/aarch64/host-config.h: New file.
> 
> --- inline copy of patch --
> diff --git a/libatomic/config/aarch64/aarch64-config.h
> b/libatomic/config/aarch64/aarch64-config.h
> new file mode 100644
> index
> 0000000000000000000000000000000000000000..d3474fa8ff80cb0c3ddbf8c4
> 8acd931d2339d33d
> --- /dev/null
> +++ b/libatomic/config/aarch64/aarch64-config.h
> @@ -0,0 +1,23 @@
> +/* Copyright (C) 2022 Free Software Foundation, Inc.
> +
> +   This file is part of the GNU Atomic Library (libatomic).
> +
> +   Libatomic is free software; you can redistribute it and/or modify it
> +   under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   Libatomic is distributed in the hope that it will be useful, but WITHOUT
> ANY
> +   WARRANTY; without even the implied warranty of MERCHANTABILITY or
> FITNESS
> +   FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> +   more details.
> +
> +   Under Section 7 of GPL version 3, you are granted additional
> +   permissions described in the GCC Runtime Library Exception, version
> +   3.1, as published by the Free Software Foundation.
> +
> +   You should have received a copy of the GNU General Public License and
> +   a copy of the GCC Runtime Library Exception along with this program;
> +   see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> diff --git a/libatomic/config/aarch64/host-config.h
> b/libatomic/config/aarch64/host-config.h
> new file mode 100644
> index
> 0000000000000000000000000000000000000000..f445a47d25ef5cc51cd21670
> 69500245d07bf1bc
> --- /dev/null
> +++ b/libatomic/config/aarch64/host-config.h
> @@ -0,0 +1,46 @@
> +/* Copyright (C) 2022 Free Software Foundation, Inc.
> +
> +   This file is part of the GNU Atomic Library (libatomic).
> +
> +   Libatomic is free software; you can redistribute it and/or modify it
> +   under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   Libatomic is distributed in the hope that it will be useful, but WITHOUT
> ANY
> +   WARRANTY; without even the implied warranty of MERCHANTABILITY or
> FITNESS
> +   FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> +   more details.
> +
> +   Under Section 7 of GPL version 3, you are granted additional
> +   permissions described in the GCC Runtime Library Exception, version
> +   3.1, as published by the Free Software Foundation.
> +
> +   You should have received a copy of the GNU General Public License and
> +   a copy of the GCC Runtime Library Exception along with this program;
> +   see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +/* 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>

... This does looks sensible and similar to what's done on powerpc, which is similar to the aarch64 target in this regard.
However, there is already a host-config.h in config/linux/aarch64/host-config.h . Does this file end up including the one in config/linux?
If so, does this mean that this works correctly (i.e. was tested) for aarch64-none-elf as well as Linux? 

Thanks,
Kyrill

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