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

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Message-ID: <20111220140628.GD23916@ZenIV.linux.org.uk>
Date:	Tue, 20 Dec 2011 14:06:28 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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 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...
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