linux-ext4 - Re: [RFC PATCH] percpu_counters: make fbc->count read atomic on 32 bit architecture

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Message-Id: <1219430002.21326.22.camel@lappy.programming.kicks-ass.net>
Date:	Fri, 22 Aug 2008 20:33:22 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	tytso@....edu, sandeen@...hat.com, linux-ext4@...r.kernel.org
Subject: Re: [RFC PATCH] percpu_counters: make fbc->count read atomic on 32
	bit architecture

On Fri, 2008-08-22 at 11:29 -0700, Mingming Cao wrote:
> 在 2008-08-22五的 19:04 +0530，Aneesh Kumar K.V写道：
> > fbc->count is of type s64. The change was introduced by
> > 0216bfcffe424a5473daa4da47440881b36c1f4 which changed the type
> > from long to s64. Moving to s64 also means on 32 bit architectures
> > we can get wrong values on fbc->count.
> > 
> > percpu_counter_read is used within interrupt context also. So
> > use the irq safe version of spinlock while reading
> > 
> 
> It's quit expensive to hold the lock to do percpu_counter_read  on 32
> bit arch, the common case.
>  
> The type of the global counter and local counter were explictly
> specified using s64 and s32 The global counter is changed from long to
> s64, while the local counter is changed from long to s32, so we could
> avoid doing 64 bit update in most cases.  After all the percpu counter
> read is not a accurate value. 

seqlocks come to mind.

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