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[79.181.91.42]) by smtp.gmail.com with ESMTPSA id z19sm22080696qkg.28.2019.07.24.11.25.18 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Wed, 24 Jul 2019 11:25:24 -0700 (PDT) Date: Wed, 24 Jul 2019 14:25:15 -0400 From: "Michael S. Tsirkin" To: Jason Gunthorpe Cc: Jason Wang , syzbot , 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> <20190723062221-mutt-send-email-mst@kernel.org> <9baa4214-67fd-7ad2-cbad-aadf90bbfc20@redhat.com> <20190723110219-mutt-send-email-mst@kernel.org> <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> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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