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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id h22sm1846899eji.80.2021.03.31.14.35.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 31 Mar 2021 14:35:38 -0700 (PDT) To: Sean Christopherson Cc: Marc Zyngier , Huacai Chen , Aleksandar Markovic , Paul Mackerras , James Morse , Julien Thierry , Suzuki K Poulose , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, Ben Gardon References: <20210326021957.1424875-1-seanjc@google.com> <20210326021957.1424875-17-seanjc@google.com> <6e7dc7d0-f5dc-85d9-1c50-d23b761b5ff3@redhat.com> <56ea69fe-87b0-154b-e286-efce9233864e@redhat.com> <0e30625f-934d-9084-e293-cb3bcbc9e4b8@redhat.com> From: Paolo Bonzini Subject: Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary Message-ID: <345ab567-386f-9080-f9cb-0e17fa90a852@redhat.com> Date: Wed, 31 Mar 2021 23:35:37 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 31/03/21 23:05, Sean Christopherson wrote: >> Wouldn't it be incorrect to lock a mutex (e.g. inside*another* MMU >> notifier's invalidate callback) while holding an rwlock_t? That makes sense >> because anybody that's busy waiting in write_lock potentially cannot be >> preempted until the other task gets the mutex. This is a potential >> deadlock. > > Yes? I don't think I follow your point though. Nesting a spinlock or rwlock > inside a rwlock is ok, so long as the locks are always taken in the same order, > i.e. it's never mmu_lock -> mmu_notifier_slots_lock. *Another* MMU notifier could nest a mutex inside KVM's rwlock. But... is it correct that the MMU notifier invalidate callbacks are always called with the mmap_sem taken (sometimes for reading, e.g. try_to_merge_with_ksm_page->try_to_merge_one_page->write_protect_page)? We could take it temporarily in install_memslots, since the MMU notifier's mm is stored in kvm->mm. In this case, a pair of kvm_mmu_notifier_lock/unlock functions would be the best way to abstract it. Paolo