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Thu, 30 Jan 2020 10:14:16 -0800 (PST) Subject: Re: [PATCH v8 13/21] main: keep rcu_atfork callback enabled for qtest To: Alexander Bulekov References: <20200129053357.27454-1-alxndr@bu.edu> <20200129053357.27454-14-alxndr@bu.edu> <20200130144224.GC180311@stefanha-x1.localdomain> <4bc49889-081e-6016-b8d5-a5d1fd615830@redhat.com> <20200130174246.fd4dcs7gntoyzx5i@mozz.bu.edu> From: Paolo Bonzini Message-ID: Date: Thu, 30 Jan 2020 19:14:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <20200130174246.fd4dcs7gntoyzx5i@mozz.bu.edu> Content-Language: en-US X-MC-Unique: fNCAENWwO5uf6glMFJLVqg-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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: "bsd@redhat.com" , "qemu-devel@nongnu.org" , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 30/01/20 18:42, Alexander Bulekov wrote: > With QTest, is this still a concern, since there are no CPU instructions > involved? Sometimes the fork-server starts after some I/O has already > occured (eg mapping BARs and setting up VQs for virtio-net). I know we > briefly talked about threads at some point, and it seems that iothreads > may be a concern, if any are started before fork. Other than that, since > there is no TCG/CPU thread, are there any other threads that could be > a concern? > -Alex There is a CPU thread, it just does not do MMIO. However, it may still execute code via run_on_cpu. It's quite unlikely to have the deadlock, but if it were possible to force an early start of the fork server (at the point of os_daemonize() would be ideal) it would be cleaner and it would allow reverting this patch. This is not a NACK, just some extra info. Paolo