linux-kernel - Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

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Message-ID: <20090617162521.GE3089@redhat.com>
Date:	Wed, 17 Jun 2009 19:25:21 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, paulmck@...ux.vnet.ibm.com, mingo@...e.hu
Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based
	notifier interface

On Wed, Jun 17, 2009 at 11:02:06AM -0400, Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
> > On Tue, Jun 16, 2009 at 02:09:38PM -0400, Gregory Haskins wrote:
> >   
> >>> What do you mean by copy_to_user(other->mm) here?  If you are going to switch
> >>> to another mm, then I think current->mm must be valid (I think it's not enough
> >>> that you can sleep). So preemptible() might not be enough.
> >>>   
> >>>       
> >> I dont currently use switch_mm, if that is what you mean.  I save the
> >> task_struct into the appropriate context so current->mm doesn't matter
> >> to me.  I never use it.  All I need (afaik) is to acquire the proper
> >> mutex first.  I am not an MM expert, so perhaps I have this wrong but it
> >> does appear to work properly even from kthread context.
> >>
> >> -Greg
> >>
> >>
> >>     
> >
> > I think I saw get_user_pages + memcpy in your patch. Yes, that works
> > without switching contexts but it's slower than copy to user if you are
> > in the right context, and AFAIK it's slower than get_user_pages_fast.
> >
> >   
> Yeah, understood.  It will definitely be interesting to try that
> optimization with switch_mm that you suggested.

BTW, I'm kind of confused - in your patch you do get_task_struct: does this
guarantee that mm is not going aways?

> On that front, would "if (p == current)" still work even if we don't
> have the "signal_task()" hint?
> 

Donnu.

-- 
MST
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