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From: Mark Rutland <mark.rutland@arm.com>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
	will.deacon@arm.com, aou@eecs.berkeley.edu, arnd@arndb.de,
	bp@alien8.de, catalin.marinas@arm.com, davem@davemloft.net,
	fenghua.yu@intel.com, heiko.carstens@de.ibm.com,
	herbert@gondor.apana.org.au, ink@jurassic.park.msu.ru,
	jhogan@kernel.org, linux@armlinux.org.uk, mattst88@gmail.com,
	mingo@kernel.org, mpe@ellerman.id.au, palmer@sifive.com,
	paul.burton@mips.com, paulus@samba.org, ralf@linux-mips.org,
	rth@twiddle.net, stable@vger.kernel.org, tglx@linutronix.de,
	tony.luck@intel.com, vgupta@synopsys.com
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup
Date: Thu, 23 May 2019 11:19:26 +0100
Message-ID: <20190523101926.GA3370@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20190523083013.GA4616@andrea>

On Thu, May 23, 2019 at 10:30:13AM +0200, Andrea Parri wrote:
> Hi Mark,

Hi Andrea,

> On Wed, May 22, 2019 at 02:22:32PM +0100, Mark Rutland wrote:
> > Currently architectures return inconsistent types for atomic64 ops. Some return
> > long (e..g. powerpc), some return long long (e.g. arc), and some return s64
> > (e.g. x86).
> 
> (only partially related, but probably worth asking:)
> 
> While reading the series, I realized that the following expression:
> 
> 	atomic64_t v;
>         ...
> 	typeof(v.counter) my_val = atomic64_set(&v, VAL);
> 
> is a valid expression on some architectures (in part., on architectures
> which #define atomic64_set() to WRITE_ONCE()) but is invalid on others.
> (This is due to the fact that WRITE_ONCE() can be used as an rvalue in
> the above assignment; TBH, I ignore the reasons for having such rvalue?)
> 
> IIUC, similar considerations hold for atomic_set().
> 
> The question is whether this is a known/"expected" inconsistency in the
> implementation of atomic64_set() or if this would also need to be fixed
> /addressed (say in a different patchset)?

In either case, I don't think the intent is that they should be used that way,
and from a quick scan, I can only fine a single relevant instance today:

[mark@lakrids:~/src/linux]% git grep '\(return\|=\)\s\+atomic\(64\)\?_set'
include/linux/vmw_vmci_defs.h:  return atomic_set((atomic_t *)var, (u32)new_val);
include/linux/vmw_vmci_defs.h:  return atomic64_set(var, new_val);


[mark@lakrids:~/src/linux]% git grep '=\s+atomic_set' | wc -l
0
[mark@lakrids:~/src/linux]% git grep '=\s+atomic64_set' | wc -l
0

Any architectures implementing arch_atomic_* will have both of these functions
returning void. Currently that's x86 and arm64, but (time permitting) I intend
to migrate other architectures, so I guess we'll have to fix the above up as
required.

I think it's best to avoid the construct above.

Thanks,
Mark.

  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-22 13:22 Mark Rutland
2019-05-22 13:22 ` [PATCH 01/18] locking/atomic: crypto: nx: prepare for atomic64_read() conversion Mark Rutland
2019-05-22 13:22 ` [PATCH 02/18] locking/atomic: s390/pci: " Mark Rutland
2019-05-22 13:22 ` [PATCH 03/18] locking/atomic: generic: use s64 for atomic64 Mark Rutland
2019-05-22 21:16   ` Arnd Bergmann
2019-05-22 13:22 ` [PATCH 04/18] locking/atomic: alpha: " Mark Rutland
2019-05-22 13:22 ` [PATCH 05/18] locking/atomic: arc: " Mark Rutland
2019-05-23 23:10   ` Vineet Gupta
2019-05-22 13:22 ` [PATCH 06/18] locking/atomic: arm: " Mark Rutland
2019-05-22 13:22 ` [PATCH 07/18] locking/atomic: arm64: " Mark Rutland
2019-05-22 13:22 ` [PATCH 08/18] locking/atomic: ia64: " Mark Rutland
2019-05-22 13:22 ` [PATCH 09/18] locking/atomic: mips: " Mark Rutland
2019-05-22 13:22 ` [PATCH 10/18] locking/atomic: powerpc: " Mark Rutland
2019-05-23 13:27   ` Michael Ellerman
2019-05-22 13:22 ` [PATCH 11/18] locking/atomic: riscv: fix atomic64_sub_if_positive() offset argument Mark Rutland
2019-05-22 19:06   ` Palmer Dabbelt
2019-05-22 13:22 ` [PATCH 12/18] locking/atomic: riscv: use s64 for atomic64 Mark Rutland
2019-05-22 19:06   ` Palmer Dabbelt
2019-05-23 10:23     ` Mark Rutland
2019-05-22 13:22 ` [PATCH 13/18] locking/atomic: s390: " Mark Rutland
2019-05-22 13:22 ` [PATCH 14/18] locking/atomic: sparc: " Mark Rutland
2019-05-22 13:22 ` [PATCH 15/18] locking/atomic: x86: " Mark Rutland
2019-05-22 13:22 ` [PATCH 16/18] locking/atomic: use s64 for atomic64_t on 64-bit Mark Rutland
2019-05-22 13:22 ` [PATCH 17/18] locking/atomic: crypto: nx: remove redundant casts Mark Rutland
2019-05-22 13:22 ` [PATCH 18/18] locking/atomic: s390/pci: " Mark Rutland
2019-05-22 21:18 ` [PATCH 00/18] locking/atomic: atomic64 type cleanup Arnd Bergmann
2019-05-23 10:28   ` Mark Rutland
2019-05-23  8:30 ` Andrea Parri
2019-05-23 10:19   ` Mark Rutland [this message]
2019-05-23 11:20     ` Andrea Parri
2019-05-24 10:37     ` Peter Zijlstra
2019-05-24 11:18       ` Peter Zijlstra
2019-05-24 11:38         ` Greg KH
2019-05-24 11:42         ` Will Deacon
2019-05-24 11:52           ` Peter Zijlstra
2019-05-24 22:43             ` Andrea Parri
2019-05-28 10:47               ` Peter Zijlstra
2019-05-28 11:15                 ` Andrea Parri

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