netdev - Re: [PATCH] vhost: locking/rcu cleanup

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]

Date:	Fri, 30 Jul 2010 16:49:54 +0200
From:	Tejun Heo <tj@...nel.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Sridhar Samudrala <samudrala@...ibm.com>,
	Jeff Dike <jdike@...ux.intel.com>,
	Juan Quintela <quintela@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>,
	David Stevens <dlstevens@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, virtualization@...ts.osdl.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost: locking/rcu cleanup

Hello,

On 07/29/2010 02:23 PM, Michael S. Tsirkin wrote:
> I saw WARN_ON(!list_empty(&dev->work_list)) trigger
> so our custom flush is not as airtight as need be.

Could be but it's also possible that something has queued something
after the last flush?  Is the problem reproducible?

> This patch switches to a simple atomic counter + srcu instead of
> the custom locked queue + flush implementation.
> 
> This will slow down the setup ioctls, which should not matter -
> it's slow path anyway. We use the expedited flush to at least
> make sure it has a sane time bound.
> 
> Works fine for me. I got reports that with many guests,
> work lock is highly contended, and this patch should in theory
> fix this as well - but I haven't tested this yet.

Hmmm... vhost_poll_flush() becomes synchronize_srcu_expedited().  Can
you please explain how it works?  synchronize_srcu_expedited() is an
extremely heavy operation involving scheduling the cpu_stop task on
all cpus.  I'm not quite sure whether doing it from every flush is a
good idea.  Is flush supposed to be a very rare operation?

Having custom implementation is fine too but let's try to implement
something generic if at all possible.

Thanks.

-- 
tejun
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html