Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 24 Jul 2019 14:25:15 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Jason Wang <jasowang@redhat.com>,
        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
Message-ID: <20190724142417-mutt-send-email-mst@kernel.org>
References: <20190723010156-mutt-send-email-mst@kernel.org>
 <124be1a2-1c53-8e65-0f06-ee2294710822@redhat.com>
 <20190723032800-mutt-send-email-mst@kernel.org>
 <e2e01a05-63d8-4388-2bcd-b2be3c865486@redhat.com>
 <20190723062221-mutt-send-email-mst@kernel.org>
 <9baa4214-67fd-7ad2-cbad-aadf90bbfc20@redhat.com>
 <20190723110219-mutt-send-email-mst@kernel.org>
 <e0c91b89-d1e8-9831-00fe-23fe92d79fa2@redhat.com>
 <20190724040238-mutt-send-email-mst@kernel.org>
 <20190724165317.GD28493@ziepe.ca>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190724165317.GD28493@ziepe.ca>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jul 24, 2019 at 01:53:17PM -0300, Jason Gunthorpe wrote:
> On Wed, Jul 24, 2019 at 04:05:17AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Jul 24, 2019 at 10:17:14AM +0800, Jason Wang wrote:
> > > So even PTE is read speculatively before reading invalidate_count (only in
> > > the case of invalidate_count is zero). The spinlock has guaranteed that we
> > > won't read any stale PTEs.
> > 
> > I'm sorry I just do not get the argument.
> > If you want to order two reads you need an smp_rmb
> > or stronger between them executed on the same CPU.
> 
> No, that is only for unlocked algorithms.
> 
> In this case the spinlock provides all the 'or stronger' ordering
> required.
> 
> For invalidate_count going 0->1 the spin_lock ensures that any
> following PTE update during invalidation does not order before the
> spin_lock()
> 
> While holding the lock and observing 1 in invalidate_count the PTE
> values might be changing, but are ignored. C's rules about sequencing
> make this safe.
> 
> For invalidate_count going 1->0 the spin_unlock ensures that any
> preceeding PTE update during invalidation does not order after the
> spin_unlock
> 
> While holding the lock and observing 0 in invalidating_count the PTE
> values cannot be changing.
> 
> Jason

Oh right. So prefetch holds the spinlock the whole time.
Sorry about the noise.

-- 
MST