From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
aarcange@redhat.com, akpm@linux-foundation.org,
christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com,
elena.reshetova@intel.com, guro@fb.com, hch@infradead.org,
james.bottomley@hansenpartnership.com, jglisse@redhat.com,
keescook@chromium.org, ldv@altlinux.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-parisc@vger.kernel.org, luto@amacapital.net,
mhocko@suse.com, mingo@kernel.org, namit@vmware.com,
peterz@infradead.org, syzkaller-bugs@googlegroups.com,
viro@zeniv.linux.org.uk, wad@chromium.org
Subject: Re: WARNING in __mmdrop
Date: Thu, 25 Jul 2019 01:52:53 -0400 [thread overview]
Message-ID: <20190725012149-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <caff362a-e208-3468-3688-63e1d093a9d3@redhat.com>
On Tue, Jul 23, 2019 at 09:31:35PM +0800, Jason Wang wrote:
>
> On 2019/7/23 下午5:26, Michael S. Tsirkin wrote:
> > On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote:
> > > On 2019/7/23 下午4:10, Michael S. Tsirkin wrote:
> > > > On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote:
> > > > > On 2019/7/23 下午3:23, Michael S. Tsirkin wrote:
> > > > > > > > Really let's just use kfree_rcu. It's way cleaner: fire and forget.
> > > > > > > Looks not, you need rate limit the fire as you've figured out?
> > > > > > See the discussion that followed. Basically no, it's good enough
> > > > > > already and is only going to be better.
> > > > > >
> > > > > > > And in fact,
> > > > > > > the synchronization is not even needed, does it help if I leave a comment to
> > > > > > > explain?
> > > > > > Let's try to figure it out in the mail first. I'm pretty sure the
> > > > > > current logic is wrong.
> > > > > Here is what the code what to achieve:
> > > > >
> > > > > - The map was protected by RCU
> > > > >
> > > > > - Writers are: MMU notifier invalidation callbacks, file operations (ioctls
> > > > > etc), meta_prefetch (datapath)
> > > > >
> > > > > - Readers are: memory accessor
> > > > >
> > > > > Writer are synchronized through mmu_lock. RCU is used to synchronized
> > > > > between writers and readers.
> > > > >
> > > > > The synchronize_rcu() in vhost_reset_vq_maps() was used to synchronized it
> > > > > with readers (memory accessors) in the path of file operations. But in this
> > > > > case, vq->mutex was already held, this means it has been serialized with
> > > > > memory accessor. That's why I think it could be removed safely.
> > > > >
> > > > > Anything I miss here?
> > > > >
> > > > So invalidate callbacks need to reset the map, and they do
> > > > not have vq mutex. How can they do this and free
> > > > the map safely? They need synchronize_rcu or kfree_rcu right?
> > > Invalidation callbacks need but file operations (e.g ioctl) not.
> > >
> > >
> > > > And I worry somewhat that synchronize_rcu in an MMU notifier
> > > > is a problem, MMU notifiers are supposed to be quick:
> > > Looks not, since it can allow to be blocked and lots of driver depends on
> > > this. (E.g mmu_notifier_range_blockable()).
> > Right, they can block. So why don't we take a VQ mutex and be
> > done with it then? No RCU tricks.
>
>
> This is how I want to go with RFC and V1. But I end up with deadlock between
> vq locks and some MM internal locks. So I decide to use RCU which is 100%
> under the control of vhost.
>
> Thanks
And I guess the deadlock is because GUP is taking mmu locks which are
taken on mmu notifier path, right? How about we add a seqlock and take
that in invalidate callbacks? We can then drop the VQ lock before GUP,
and take it again immediately after.
something like
if (!vq_meta_mapped(vq)) {
vq_meta_setup(&uaddrs);
mutex_unlock(vq->mutex)
vq_meta_map(&uaddrs);
mutex_lock(vq->mutex)
/* recheck both sock->private_data and seqlock count. */
if changed - bail out
}
And also requires that VQ uaddrs is defined like this:
- writers must have both vq mutex and dev mutex
- readers must have either vq mutex or dev mutex
That's a big change though. For now, how about switching to a per-vq SRCU?
That is only a little bit more expensive than RCU, and we
can use synchronize_srcu_expedited.
--
MST
next prev parent reply other threads:[~2019-07-25 5:53 UTC|newest]
Thread overview: 88+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-19 3:35 WARNING in __mmdrop syzbot
2019-07-20 10:08 ` syzbot
2019-07-21 10:02 ` Michael S. Tsirkin
2019-07-21 12:18 ` Michael S. Tsirkin
2019-07-22 5:24 ` Jason Wang
2019-07-22 8:08 ` Michael S. Tsirkin
2019-07-23 4:01 ` Jason Wang
2019-07-23 5:01 ` Michael S. Tsirkin
2019-07-23 5:47 ` Jason Wang
2019-07-23 7:23 ` Michael S. Tsirkin
2019-07-23 7:53 ` Jason Wang
2019-07-23 8:10 ` Michael S. Tsirkin
2019-07-23 8:49 ` Jason Wang
2019-07-23 9:26 ` Michael S. Tsirkin
2019-07-23 13:31 ` Jason Wang
2019-07-25 5:52 ` Michael S. Tsirkin [this message]
2019-07-25 7:43 ` Jason Wang
2019-07-25 8:28 ` Michael S. Tsirkin
2019-07-25 13:21 ` Jason Wang
2019-07-25 13:26 ` Michael S. Tsirkin
2019-07-25 14:25 ` Jason Wang
2019-07-26 11:49 ` Michael S. Tsirkin
2019-07-26 12:00 ` Jason Wang
2019-07-26 12:38 ` Michael S. Tsirkin
2019-07-26 12:53 ` Jason Wang
2019-07-26 13:36 ` Jason Wang
2019-07-26 13:49 ` Michael S. Tsirkin
2019-07-29 5:54 ` Jason Wang
2019-07-29 8:59 ` Michael S. Tsirkin
2019-07-29 14:24 ` Jason Wang
2019-07-29 14:44 ` Michael S. Tsirkin
2019-07-30 7:44 ` Jason Wang
2019-07-30 8:03 ` Jason Wang
2019-07-30 15:08 ` Michael S. Tsirkin
2019-07-31 8:49 ` Jason Wang
2019-07-31 23:00 ` Jason Gunthorpe
2019-07-26 13:47 ` Michael S. Tsirkin
2019-07-26 14:00 ` Jason Wang
2019-07-26 14:10 ` Michael S. Tsirkin
2019-07-26 15:03 ` Jason Gunthorpe
2019-07-29 5:56 ` Jason Wang
2019-07-21 12:28 ` RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop) Michael S. Tsirkin
2019-07-21 13:17 ` Paul E. McKenney
2019-07-21 17:53 ` Michael S. Tsirkin
2019-07-21 19:28 ` Paul E. McKenney
2019-07-22 7:56 ` Michael S. Tsirkin
2019-07-22 11:57 ` Paul E. McKenney
2019-07-21 21:08 ` Matthew Wilcox
2019-07-21 23:31 ` Paul E. McKenney
2019-07-22 7:52 ` Michael S. Tsirkin
2019-07-22 11:51 ` Paul E. McKenney
2019-07-22 13:41 ` Jason Gunthorpe
2019-07-22 15:52 ` Paul E. McKenney
2019-07-22 16:04 ` Jason Gunthorpe
2019-07-22 16:15 ` Michael S. Tsirkin
2019-07-22 16:15 ` Paul E. McKenney
2019-07-22 15:14 ` Joel Fernandes
2019-07-22 15:47 ` Michael S. Tsirkin
2019-07-22 15:55 ` Paul E. McKenney
2019-07-22 16:13 ` Michael S. Tsirkin
2019-07-22 16:25 ` Paul E. McKenney
2019-07-22 16:32 ` Michael S. Tsirkin
2019-07-22 18:58 ` Paul E. McKenney
2019-07-22 5:21 ` WARNING in __mmdrop Jason Wang
2019-07-22 8:02 ` Michael S. Tsirkin
2019-07-23 3:55 ` Jason Wang
2019-07-23 5:02 ` Michael S. Tsirkin
2019-07-23 5:48 ` Jason Wang
2019-07-23 7:25 ` Michael S. Tsirkin
2019-07-23 7:55 ` Jason Wang
2019-07-23 7:56 ` Michael S. Tsirkin
2019-07-23 8:42 ` Jason Wang
2019-07-23 10:27 ` Michael S. Tsirkin
2019-07-23 13:34 ` Jason Wang
2019-07-23 15:02 ` Michael S. Tsirkin
2019-07-24 2:17 ` Jason Wang
2019-07-24 8:05 ` Michael S. Tsirkin
2019-07-24 10:08 ` Jason Wang
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-25 3:44 ` Jason Wang
2019-07-25 5:09 ` Michael S. Tsirkin
2019-07-24 16:53 ` Jason Gunthorpe
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-23 10:42 ` Michael S. Tsirkin
2019-07-23 13:37 ` Jason Wang
2019-07-22 14:11 ` Jason Gunthorpe
2019-07-25 6:02 ` Michael S. Tsirkin
2019-07-25 7:44 ` Jason Wang
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