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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=xBkisH7qkt4tTks6hPY848K8Xv8nx74W1YCkO3MSs78=; b=O4OQEu/NOFNMqHXDqKifh7WIuDKT0D9mnQ18AVMSZFCUOe0nEefY9lS7siSGy2Lq+h jiL6B46S7bthTZv4dHQE87fvdzHwU9tTkwlT/7W+nDuKv5tROoJ31KOoepdbULbk2agx 2WtgS2tXb/yGkCyVExVkYqk4xe8ejXqW2sbtgkkbXUQNZat3rVi8dsSSCLbSt4R/wlI4 J7iJ31AayNohBBOnIS6hrftQb4wXenadq2Von8PqCzditIiiX/lm9yEJeFFugmfaCYuz Cc8rQ7rddRPyEvZuxBedLKq3HzWoa+5hTbjbpo5pYWhIpsNAuv0DwNThUowvoN911Cu/ nlSg== X-Gm-Message-State: AOAM533DjW9uwioZmcDMjfd01dd+CIb4K4D9FVZ3kJySzFJHWsCMQ8lz VWgcmBeQJ4J6E6NZsZe7MxdtkXKqW7/Chafutqg= X-Google-Smtp-Source: ABdhPJyDago5jgo8lT/b6tX8BLqNd48NQMnJrZtGH1jlRB2oYa35oUPd0fjyJN66ZAOSGkjfWsCl1yskmo0vd2+EJp4= X-Received: by 2002:a1c:4843:: with SMTP id v64mr6328377wma.186.1607117019180; Fri, 04 Dec 2020 13:23:39 -0800 (PST) MIME-Version: 1.0 References: <9d7a052a-6222-80ff-cbf1-612d4ca50c2a@suse.com> <17c90493-b438-fbc1-ca10-3bc4d89c4e5e@xen.org> <7a768bcd-80c1-d193-8796-7fb6720fa22a@suse.com> <1a8250f5-ea49-ac3a-e992-be7ec40deba9@xen.org> <5862eb24-d894-455a-13ac-61af54f949e7@xen.org> In-Reply-To: From: Tamas K Lengyel Date: Fri, 4 Dec 2020 16:23:03 -0500 Message-ID: Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held To: Julien Grall Cc: Jan Beulich , Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , Tamas K Lengyel , Petre Ovidiu PIRCALABU , Alexandru Isaila , "xen-devel@lists.xenproject.org" Content-Type: text/plain; charset="UTF-8" On Fri, Dec 4, 2020 at 2:22 PM Julien Grall wrote: > > On Fri, 4 Dec 2020 at 19:15, Tamas K Lengyel wrote: > > > > On Fri, Dec 4, 2020 at 10:29 AM Julien Grall wrote: > > > > > > > > > > > > On 04/12/2020 15:21, Tamas K Lengyel wrote: > > > > On Fri, Dec 4, 2020 at 6:29 AM Julien Grall wrote: > > > >> > > > >> Hi Jan, > > > >> > > > >> On 03/12/2020 10:09, Jan Beulich wrote: > > > >>> On 02.12.2020 22:10, Julien Grall wrote: > > > >>>> On 23/11/2020 13:30, Jan Beulich wrote: > > > >>>>> While there don't look to be any problems with this right now, the lock > > > >>>>> order implications from holding the lock can be very difficult to follow > > > >>>>> (and may be easy to violate unknowingly). The present callbacks don't > > > >>>>> (and no such callback should) have any need for the lock to be held. > > > >>>>> > > > >>>>> However, vm_event_disable() frees the structures used by respective > > > >>>>> callbacks and isn't otherwise synchronized with invocations of these > > > >>>>> callbacks, so maintain a count of in-progress calls, for evtchn_close() > > > >>>>> to wait to drop to zero before freeing the port (and dropping the lock). > > > >>>> > > > >>>> AFAICT, this callback is not the only place where the synchronization is > > > >>>> missing in the VM event code. > > > >>>> > > > >>>> For instance, vm_event_put_request() can also race against > > > >>>> vm_event_disable(). > > > >>>> > > > >>>> So shouldn't we handle this issue properly in VM event? > > > >>> > > > >>> I suppose that's a question to the VM event folks rather than me? > > > >> > > > >> Yes. From my understanding of Tamas's e-mail, they are relying on the > > > >> monitoring software to do the right thing. > > > >> > > > >> I will refrain to comment on this approach. However, given the race is > > > >> much wider than the event channel, I would recommend to not add more > > > >> code in the event channel to deal with such problem. > > > >> > > > >> Instead, this should be fixed in the VM event code when someone has time > > > >> to harden the subsystem. > > > > > > > > I double-checked and the disable route is actually more robust, we > > > > don't just rely on the toolstack doing the right thing. The domain > > > > gets paused before any calls to vm_event_disable. So I don't think > > > > there is really a race-condition here. > > > > > > The code will *only* pause the monitored domain. I can see two issues: > > > 1) The toolstack is still sending event while destroy is happening. > > > This is the race discussed here. > > > 2) The implement of vm_event_put_request() suggests that it can be > > > called with not-current domain. > > > > > > I don't see how just pausing the monitored domain is enough here. > > > > Requests only get generated by the monitored domain. > > If that's the case, then why is vm_event_put_request() able to > deal with a non-current domain? > > I understand that you are possibly trusting who may call it, but this > looks quite fragile. I didn't write the system. You probably want to ask that question from the original author. Tamas