linux-kernel - Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs

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Date:	Mon, 19 Dec 2011 20:52:51 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Stephen Boyd <sboyd@...eaurora.org>, mc@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Nick Piggin <npiggin@...nel.dk>, david@...morbit.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>
Subject: Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than
 online CPUs

On Tue, Dec 20, 2011 at 01:53:42AM +0530, Srivatsa S. Bhat wrote:
> If this new definition of our requirement is acceptable (correct me if I am
> wrong), then we can do something like the following patch, while still
> retaining br locks as non-blocking.
> 
> We make a copy of the current cpu_online_mask, and lock the per-cpu locks of
> all those cpus. Then while unlocking, we unlock the per-cpu locks of these cpus
> (by using that temporary copy of cpu_online_mask we created earlier), without
> caring about the cpus actually online at that moment.
> IOW, we do lock-unlock on the same set of cpus, and that too, without missing
> the complete lock-unlock sequence in any of them. Guaranteed.

	And what's to stop a process on a newly added CPU from _not_
spinning in br_read_lock(), even though br_write_unlock() hadn't been
done yet?
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