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Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Shuah Khan , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Vishal Annapurve , Yu Zhang , "Kirill A . Shutemov" , luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com, david@redhat.com, aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com, Muchun Song Subject: Re: [PATCH v7 08/14] KVM: Rename mmu_notifier_* Message-ID: <20230524203336.GC3447678@hirez.programming.kicks-ass.net> References: <20220706082016.2603916-1-chao.p.peng@linux.intel.com> <20220706082016.2603916-9-chao.p.peng@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: B35631A0022 X-Rspam-User: X-Stat-Signature: hqngb936aeyyb6gyay9rujs459k4a6j1 X-Rspamd-Server: rspam01 X-HE-Tag: 1684960457-616308 X-HE-Meta: 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 IGJRHjnj HSD4hWM9yB/h0RiuJ8VDHbOa+wLpJYAER/ItzCkVRmQIwB+nFJ1s/OrWBj+DsYTCRTSqkDWlAjeo20EnjPzUnpNEpZtKaSt7Z93sL+C5kDS3l1WUqQir2YvqsQIPyLbNZtLNEg76UgFtqY9yf9fE0RlBz44AncnfsZSxEx3Rf/eiHc1t580McX20avw== 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, May 24, 2023 at 01:16:03PM -0700, Sean Christopherson wrote: > Atomics aren't memory barriers on all architectures, e.g. see the various > definitions of smp_mb__after_atomic(). > > Even if atomic operations did provide barriers, using an atomic would be overkill > and a net negative. On strongly ordered architectures like x86, memory barriers are > just compiler barriers, whereas atomics may be more expensive. Not quite, smp_{r,w}mb() and smp_mb__{before,after}_atomic() are compiler barriers on the TSO archs, but smp_mb() very much isn't. TSO still allows stores to be delayed vs later loads (iow it doesn't pretend to hide the store buffer). > Of course, the only > accesses outside of mmu_lock are reads, so on x86 that "atomic" access is just a > READ_ONCE() load, but that's not the case for all architectures. This is true on *all* archs. atomic_set() and atomic_read() are no more and no less than WRITE_ONCE() / READ_ONCE(). > Anyways, the point is that atomics and memory barriers are different things that > serve different purposes. This is true; esp. on the weakly ordered architectures where atomics do not naturally imply any ordering.