From: Andrew Morton <akpm@osdl.org>
To: David Howells <dhowells@redhat.com>
Cc: torvalds@osdl.org, davem@davemloft.com, wli@holomorphy.com,
matthew@wil.cx, linux-kernel@vger.kernel.org,
linux-arch@vger.kernel.org
Subject: Re: [PATCH 3/3] WorkStruct: Use direct assignment rather than cmpxchg()
Date: Thu, 7 Dec 2006 13:06:30 -0800 [thread overview]
Message-ID: <20061207130630.6c1a8d32.akpm@osdl.org> (raw)
In-Reply-To: <639.1165521999@redhat.com>
On Thu, 07 Dec 2006 20:06:39 +0000
David Howells <dhowells@redhat.com> wrote:
> Andrew Morton <akpm@osdl.org> wrote:
>
> > and we can assume (and ensure) that a failing test_and_set_bit() will not
> > write to the affected word at all.
>
> You may not assume that; and indeed that is not so in the generic
> spinlock-based bitops or ARM pre-v6 or PA-RISC or sparc32 or ...
Ah. How obnoxious of them.
> Remember: if you have to put a conditional jump in there, it's going to fail
> one way or the other a certain percentage of the time, and that's going to
> cause a pipeline stall, and these ops are used quite a lot.
>
> OTOH, I don't know that the stall would be that bad since the spin_lock and
> spin_unlock may cause a stall anyway.
>
Yes, the branch would cost. But in not uncommon cases that branch will save
the machine from dirtying a cacheline.
And if we add those branches, we bring those architectures' semantics in
line with all the other architectures. And we get better semantics
overall.
So I don't think we should rule this out.
next prev parent reply other threads:[~2006-12-07 21:07 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-12-07 15:31 [PATCH 1/3] WorkStruct: Fix up some PA-RISC work items David Howells
2006-12-07 15:31 ` [PATCH 2/3] WorkStruct: Add assign_bits() to give an atomic-bitops safe assignment David Howells
2006-12-07 15:31 ` [PATCH 3/3] WorkStruct: Use direct assignment rather than cmpxchg() David Howells
2006-12-07 16:54 ` Andrew Morton
2006-12-07 20:06 ` David Howells
2006-12-07 21:06 ` Andrew Morton [this message]
2006-12-07 21:06 ` James Bottomley
2006-12-07 21:16 ` David Howells
2006-12-07 22:11 ` David Miller
2006-12-07 23:42 ` Russell King
2006-12-07 23:58 ` David Howells
2006-12-08 11:14 ` Russell King
2006-12-08 13:57 ` David Howells
2006-12-08 3:42 ` [PATCH 1/3] WorkStruct: Fix up some PA-RISC work items Kyle McMartin
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