From: jamie@shareable•org (Jamie Lokier)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH 1/1] [RFCv2] arm: add half-word __xchg
Date: Sun, 28 Mar 2010 00:14:29 +0000 [thread overview]
Message-ID: <20100328001429.GA16697@shareable.org> (raw)
In-Reply-To: <20100327225240.GC13203@n2100.arm.linux.org.uk>
Russell King - ARM Linux wrote:
> I wonder if we should be using __alignof__ here.
>
> unsigned long *ptrbig = (unsigned long *)((unsigned long)ptr &
> (__alignof__(unsigned long) - 1));
Are there ARM targets with a smaller value from __alignof__()?
I think you meant:
unsigned long *ptrbig = (unsigned long *)((unsigned long)ptr &
~(unsigned long)(__alignof__(unsigned long) - 1));
Perhaps in asm-generic that has a place. It would simplify the asm if
the alignment is 1 on some machine.
But I don't think it'd happen anyway. There are machines which don't
require full alignment of long, but insufficiently aligned *atomic*
accesses like cmpxchg are *not atomic*. x86 is one such machine.
Fortunately GCC aligns the types sufficiently well - and we rely on
that all over the kernel.
I'm not sure about ARM, when doing a 64-bit cmpxchg, if the doubleword
must be 64-bit aligned to get atomicity.
Note that the posted code doesn't work as is for 64-bit longs: the
"mask" calculation overflows if called with size >= 4.
But seeing as this is for ARM only at present, I'd just change the
types to u32 and be done with it. It does seem like a good thing to
go to asm-generic eventually though.
-- Jamie
next prev parent reply other threads:[~2010-03-28 0:14 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-03-10 16:22 __xchg for sizes other than 32bit Imre Deak
2010-03-10 17:35 ` Russell King - ARM Linux
2010-03-10 20:02 ` [PATCH] ARM support single byte cmpxchg and cmpxchg_local on ARMv6 Mathieu Desnoyers
2010-03-10 20:30 ` Russell King - ARM Linux
2010-03-10 21:15 ` Mathieu Desnoyers
2010-03-10 23:16 ` __xchg for sizes other than 32bit Jamie Lokier
2010-03-18 9:29 ` [PATCH 1/1] [RFC] arm: add half-word __xchg Alexander Shishkin
2010-03-18 12:32 ` Mathieu Desnoyers
2010-03-18 12:37 ` Alexander Shishkin
2010-03-18 13:33 ` Alexander Shishkin
2010-03-18 13:50 ` Mathieu Desnoyers
2010-03-18 16:33 ` Imre Deak
2010-03-18 17:21 ` Mathieu Desnoyers
2010-03-18 19:00 ` Imre Deak
2010-03-18 19:30 ` Mathieu Desnoyers
2010-03-19 1:49 ` Jamie Lokier
2010-03-19 2:12 ` Mathieu Desnoyers
2010-03-19 3:36 ` Jamie Lokier
2010-03-25 15:52 ` [PATCH 1/1] [RFCv2] " Alexander Shishkin
2010-03-25 16:42 ` Alexander Shishkin
2010-03-27 22:52 ` Russell King - ARM Linux
2010-03-28 0:14 ` Jamie Lokier [this message]
2010-03-28 0:18 ` Russell King - ARM Linux
2010-03-28 1:00 ` Mathieu Desnoyers
2010-03-28 14:39 ` Jamie Lokier
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