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

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Message-ID: <4EF09D34.1060607@linux.vnet.ibm.com>
Date:	Tue, 20 Dec 2011 20:05:32 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	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>, 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 12/20/2011 07:36 PM, Al Viro wrote:

> On Tue, Dec 20, 2011 at 06:20:44PM +0530, Srivatsa S. Bhat wrote:
> 
>>> Hey, actually there is a simple solution: just nip it (or rather delay it)
>>> in the bud ;) That is, we watch out for CPU_UP_PREPARE event and lock it
>>> up there itself using our spinlock.. that way, that cpu will not come up
>>> until we are done executing br_write_unlock(). In fact, we can even fail
>>> the onlining of that cpu by returning appropriate value from our callback,
>>> but that would be too harsh.. so we can settle for delaying the cpu online
> 
> Eeeek...  Are you serious about leaving a spinlock grabbed by notifier
> callback and not released until another callback call?  That would be one
> hell of a constraint on what these notifiers can do - _nothing_ between
> these calls (including other notifier callbacks, etc.) would be allowed
> to block.
> 
> That sounds extremely brittle...
> 

Sorry but I didn't quite get your point...
No two cpu hotplug operations can race because of the cpu_hotplug lock they
use. Hence, if a cpu online operation begins, it has to succeed or fail
eventually. No other cpu hotplug operation can intervene. Ditto for cpu offline
operations.

Hence a CPU_UP_PREPARE event *will* be followed by a corresponding
CPU_UP_CANCELED or CPU_ONLINE event for the same cpu. (And we ignore the
CPU_STARTING event that comes in between, on purpose, so as to avoid the race
with cpu_online_mask). Similar is the story for offline operation.

And if the notifier grabs the spinlock and keeps it locked between these 2
points of a cpu hotplug operation, it ensures that our br locks will spin,
instead of block till the cpu hotplug operation is complete. Isn't this what
we desired all along? "A non-blocking way to sync br locks with cpu hotplug"?

Or am I missing something?

Regards,
Srivatsa S. Bhat

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