linux-kernel - Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu

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Date:	Thu, 24 Aug 2006 14:25:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	rusty@...tcorp.com.au, torvalds@...l.org, akpm@...l.org,
	linux-kernel@...r.kernel.org, arjan@...ux.intel.com,
	davej@...hat.com, vatsa@...ibm.com, dipankar@...ibm.com,
	ashok.raj@...el.com
Subject: Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking"


* Gautham R Shenoy <ego@...ibm.com> wrote:

> This was the approach I tried to make it cache friendly.
> These are the problems I faced.
> 
> - Reader checks the write_active flag. If set, he waits in the global read
> queue. else, he gets the lock and increments percpu refcount.
> 
> - the writer would have to check each cpu's read refcount, and ensure that
> read refcount =0 on all of them before he sets write_active and 
> begins a write operation.
> This will create a big race window - a writer is checking
> for a refcount on cpu(j), a reader comes on cpu(i) where i<j;
> Let's assume the writer checks refcounts in increasing order of cpus.
> Should the reader on cpu(i) wait or go ahead? If we use a global
> lock to serialize this operation, we the whole purpose of maintaining
> per cpu data is lost.

no. The writer first sets the global write_active flag, and _then_ goes 
on to wait for all readers (if any) to get out of their critical 
sections. (That's the purpose of the per-cpu waitqueue that readers use 
to wake up a writer waiting for the refcount to go to 0.)

can you still see problems with this scheme?

(the 'write_active' flag is probably best implemented as a mutex, where 
readers check mutex_is_locked(), and writers try to take it.)

	Ingo
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