linux-kernel - Re: [PATCH 1/10] Add generic helpers for arch IPI function calls

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20080503054930.GA19664@wotan.suse.de>
Date:	Sat, 3 May 2008 07:49:30 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ux.vnet.ibm.com, Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	jeremy@...p.org, mingo@...e.hu
Subject: Re: [PATCH 1/10] Add generic helpers for arch IPI function calls

On Fri, May 02, 2008 at 02:59:29PM +0200, Peter Zijlstra wrote:
> On Fri, 2008-05-02 at 05:42 -0700, Paul E. McKenney wrote:
> 
> > And here is one scenario that makes me doubt that my imagination is
> > faulty:
> > 
> > 1.	CPU 0 disables irqs.
> > 
> > 2.	CPU 1 disables irqs.
> > 
> > 3.	CPU 0 invokes smp_call_function().  But CPU 1 will never respond
> > 	because its irqs are disabled.
> > 
> > 4.	CPU 1 invokes smp_call_function().  But CPU 0 will never respond
> > 	because its irqs are disabled.
> > 
> > Looks like inherent deadlock to me, requiring that smp_call_function()
> > be invoked with irqs enabled.
> > 
> > So, what am I missing here?
> 
> The wish to do it anyway ;-)
> 
> I can imagine some situations where I'd like to try anyway and fall back
> to a slower path when failing.
> 
> With the initial design we would simply allocate data, stick it on the
> queue and call the ipi (when needed).
> 
> This is perfectly deadlock free when wait=0 and it just returns -ENOMEM
> on allocation failure.

Yeah, I'm just talking about the wait=0 case. (btw. I'd rather the core
API takes some data rather than allocates some itself, eg because you
might want to have it on the stack).

For the wait=1 case, something very clever such as processing pending
requests in a polling loop might be cool... however I'd rather not add
such complexity until someone needs it (you could stick a comment in
there outlining your algorithm). But I'd just rather not have peole rely
on it yet.

 
> It it doesn't return -ENOMEM I know its been queued and will be
> processed at some point, if it does fail, I can deal with it in another
> way.

At least with IPIs I think we can guarantee they will be processed on
the target after we queue them.

 
> I know I'd like to do that and I suspect Nick has a few use cases up his
> sleeve as well.

It would be handy. The "quickly kick something off on another CPU" is
pretty nice in mm/ when you have per-cpu queues or caches that might
want to be flushed.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/