From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55822) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gDXE1-0006fF-Ki for qemu-devel@nongnu.org; Fri, 19 Oct 2018 12:02:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gDXDv-0005zN-Tr for qemu-devel@nongnu.org; Fri, 19 Oct 2018 12:02:09 -0400 References: <20181019010625.25294-1-cota@braap.org> <20181019145018.GB7279@flamenco> From: Paolo Bonzini Message-ID: <6190036c-cd1f-4549-9b7e-9e7913c972d4@redhat.com> Date: Fri, 19 Oct 2018 18:01:18 +0200 MIME-Version: 1.0 In-Reply-To: <20181019145018.GB7279@flamenco> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC v3 0/56] per-CPU locks List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Emilio G. Cota" Cc: qemu-devel@nongnu.org, Aleksandar Markovic , Alexander Graf , Alistair Francis , Andrzej Zaborowski , Anthony Green , Artyom Tarasenko , Aurelien Jarno , Bastian Koppelmann , Christian Borntraeger , Chris Wulff , Cornelia Huck , David Gibson , David Hildenbrand , "Edgar E. Iglesias" , Eduardo Habkost , Fabien Chouteau , Guan Xuetao , James Hogan , Laurent Vivier , Marek Vasut , Mark Cave-Ayland , Max Filippov , Michael Clark , Michael Walle , Palmer Dabbelt , Pavel Dovgalyuk , Peter Crosthwaite , Peter Maydell , qemu-arm@nongnu.org, qemu-ppc@nongnu.org, qemu-s390x@nongnu.org, Richard Henderson , Sagar Karandikar , Stafford Horne On 19/10/2018 16:50, Emilio G. Cota wrote: > On Fri, Oct 19, 2018 at 08:59:24 +0200, Paolo Bonzini wrote: >> On 19/10/2018 03:05, Emilio G. Cota wrote: >>> I'm calling this series a v3 because it supersedes the two series >>> I previously sent about using atomics for interrupt_request: >>> https://lists.gnu.org/archive/html/qemu-devel/2018-09/msg02013.html >>> The approach in that series cannot work reliably; using (locked) atomics >>> to set interrupt_request but not using (locked) atomics to read it >>> can lead to missed updates. >> >> The idea here was that changes to protected fields are all followed by >> kick. That may not have been the case, granted, but I wonder if the >> plan is unworkable. > > I suspect that the cpu->interrupt_request+kick mechanism is not the issue, > otherwise master should not work--we do atomic_read(cpu->interrupt_request) > and only if that read != 0 we take the BQL. > > My guess is that the problem is with other reads of cpu->interrupt_request, > e.g. those in cpu_has_work. Currently those reads happen with the > BQL held, and updates to cpu->interrupt_request take the BQL. If we drop > the BQL from the setters to instead use locked atomics (like in the > aforementioned series), those BQL-protected readers might miss updates. cpu_has_work is only needed to handle the processor's halted state (or is it?). If it is, OR+kick should work. > Given that we need a per-CPU lock anyway to remove the BQL from the > CPU loop, extending this lock to protect cpu->interrupt_request is > a simple solution that keeps the current logic and allows for > greater scalability. Sure, I was just curious what the problem was. KVM uses OR+kick with no problems. Paolo