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

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Date:	Tue, 20 Dec 2011 23:31:39 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dave Chinner <david@...morbit.com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	mc@...ux.vnet.ibm.com, Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Nick Piggin <npiggin@...nel.dk>,
	"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 Wed, Dec 21, 2011 at 09:27:34AM +1100, Dave Chinner wrote:

> Only thing that concerns me about this patch is the bitmap changing
> between lock and unlock operations. i.e.
> 
> CPU 1: lock all cpus in mask
... after having taken the spinlock
> CPU 2: brings up new cpu, notifier adds CPU to bitmask
... which would also take the same spinlock
> CPU 1: unlock all cpus in mask
... which is where that spinlock would've been released

> And in this case the unlock tries to unlock a cpu that wasn't locked
> to start with. It really seems to me that while a global lock is in
> progress, the online bitmask cannot be allowed to change.

Well, yes.  See spin_lock() in br_write_lock() before the loop and
spin_unlock() in br_write_unlock() after the loop...

> Perhaps something can be passed between the lock and unlock
> operations to be able to detect a changed mask between lock/unlock
> operations (e.g. a generation number) and then handle that via a
> slow path that unlocks only locks that are active in the online
> bitmask?  i.e. all the notifier does is bump the generation count,
> and the slow path on the unlock handles everything else?

???
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