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

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

> -----Original Message-----
> From: Tamar Christina <Tamar.Christina@arm.com>
> Sent: Monday, August 8, 2022 10:28 AM
> To: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>; gcc-patches@gcc.gnu.org
> Cc: nd <nd@arm.com>; Richard Earnshaw <Richard.Earnshaw@arm.com>;
> Marcus Shawcroft <Marcus.Shawcroft@arm.com>; Richard Sandiford
> <Richard.Sandiford@arm.com>
> Subject: RE: [PATCH 1/2]AArch64 Fix 128-bit sequential consistency atomic
> operations.
> 
> 
> > -----Original Message-----
> > From: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
> > Sent: Tuesday, July 12, 2022 2:46 PM
> > To: Tamar Christina <Tamar.Christina@arm.com>; gcc-
> patches@gcc.gnu.org
> > Cc: nd <nd@arm.com>; Richard Earnshaw <Richard.Earnshaw@arm.com>;
> > Marcus Shawcroft <Marcus.Shawcroft@arm.com>; Richard Sandiford
> > <Richard.Sandiford@arm.com>
> > Subject: RE: [PATCH 1/2]AArch64 Fix 128-bit sequential consistency atomic
> > operations.
> >
> > 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?
> >
> 
> Hi,
> 
> We don't build libatomic on any elf platforms.  It has a default unsupported
> flag which we don't override.  Indeed we don't produce libatomic.a for elf
> and a simple example fails to link as well..

Ok, then I think this patch is a step in the right direction.
Ok for trunk.
Thanks,
Kyrill

> 
> Regards,
> Tamar
> 
> > Thanks,
> > Kyrill
> >
> > >
> > >
> > >
> > >
> > > --