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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <2b1d9549-5564-94e6-a55f-ca80996c6ef9@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , kvm@vger.kernel.org, "Dr . David Alan Gilbert" , Peter Xu , Igor Mammedov , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 06.03.20 15:39, Paolo Bonzini wrote: > On 06/03/20 15:30, David Hildenbrand wrote: >>> Assuming we're only talking about CPU ioctls (seems like a good >>> approximation) maybe you could use start_exclusive/end_exclusive? The >>> current_cpu->in_exclusive_context assignments can be made conditional on >>> "if (current_cpu)". >>> >>> However that means you have to drop the BQL, see >>> process_queued_cpu_work. It may be a problem. >>> >> Yeah, start_exclusive() is expected to be called without the BQL, >> otherwise the other CPUs would not be able to make progress and can >> eventually be "caught". >> >> It's essentially the same reason why I can't use high-level >> pause_all_vcpus()/resume_all_vcpus(). Will drop the BQL which is very >> bad for resizing code. > > But any other synchronization primitive that you do which blocks all > vCPUs will have the same issue, otherwise you get a deadlock. This is essentially what this patch solves. The lock essentially blocks anybody from entering, but not leaving a KVM ioctl. An inhibitor only waits for all IOCTLs to be left. No other lock prohibits that, so I don't think there can ever be a deadlock. -- Thanks, David / dhildenb