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From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	linux-kernel@vger.kernel.org, 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, gregkh@linuxfoundation.org,
	jhansen@vmware.com, vdasa@vmware.com, aditr@vmware.com,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup
Date: Sat, 25 May 2019 00:43:40 +0200
Message-ID: <20190524224340.GA3792@andrea> (raw)
In-Reply-To: <20190524115231.GN2623@hirez.programming.kicks-ass.net>

> ---
> Subject: Documentation/atomic_t.txt: Clarify pure non-rmw usage
> 
> Clarify that pure non-RMW usage of atomic_t is pointless, there is
> nothing 'magical' about atomic_set() / atomic_read().
> 
> This is something that seems to confuse people, because I happen upon it
> semi-regularly.
> 
> Acked-by: Will Deacon <will.deacon@arm.com>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  Documentation/atomic_t.txt | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> index dca3fb0554db..89eae7f6b360 100644
> --- a/Documentation/atomic_t.txt
> +++ b/Documentation/atomic_t.txt
> @@ -81,9 +81,11 @@ SEMANTICS
>  
>  The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
>  implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
> -smp_store_release() respectively.
> +smp_store_release() respectively. Therefore, if you find yourself only using
> +the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
> +and are doing it wrong.

The counterargument (not so theoretic, just look around in the kernel!) is:
we all 'forget' to use READ_ONCE() and WRITE_ONCE(), it should be difficult
or more difficult to forget to use atomic_read() and atomic_set()...   IAC,
I wouldn't call any of them 'wrong'.

  Andrea


>  
> -The one detail to this is that atomic_set{}() should be observable to the RMW
> +A subtle detail of atomic_set{}() is that it should be observable to the RMW
>  ops. That is:
>  
>    C atomic-set

  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
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 [this message]
2019-05-28 10:47               ` Peter Zijlstra
2019-05-28 11:15                 ` Andrea Parri

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