From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:48348) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QKSfz-0007nZ-Cu for qemu-devel@nongnu.org; Thu, 12 May 2011 05:59:28 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QKSft-0001h6-PQ for qemu-devel@nongnu.org; Thu, 12 May 2011 05:59:23 -0400 Received: from mail-gy0-f173.google.com ([209.85.160.173]:39236) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QKSft-0001fe-7w for qemu-devel@nongnu.org; Thu, 12 May 2011 05:59:17 -0400 Received: by gyg4 with SMTP id 4so543366gyg.4 for ; Thu, 12 May 2011 02:59:15 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <4DCBADA4.6030401@siemens.com> References: <1305108925-26048-1-git-send-email-stefanha@linux.vnet.ibm.com> <1305108925-26048-2-git-send-email-stefanha@linux.vnet.ibm.com> <4DCBADA4.6030401@siemens.com> Date: Thu, 12 May 2011 10:59:15 +0100 Message-ID: From: Stefan Hajnoczi Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 1/2] coroutine: introduce coroutines List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Kevin Wolf , Anthony Liguori , Venkateswararao Jujjuri , Stefan Hajnoczi , qemu-devel@nongnu.org On Thu, May 12, 2011 at 10:51 AM, Jan Kiszka wrote= : > On 2011-05-11 12:15, Stefan Hajnoczi wrote: >> From: Kevin Wolf >> >> Asynchronous code is becoming very complex. =A0At the same time >> synchronous code is growing because it is convenient to write. >> Sometimes duplicate code paths are even added, one synchronous and the >> other asynchronous. =A0This patch introduces coroutines which allow code >> that looks synchronous but is asynchronous under the covers. >> >> A coroutine has its own stack and is therefore able to preserve state >> across blocking operations, which traditionally require callback >> functions and manual marshalling of parameters. >> >> Creating and starting a coroutine is easy: >> >> =A0 coroutine =3D qemu_coroutine_create(my_coroutine); >> =A0 qemu_coroutine_enter(coroutine, my_data); >> >> The coroutine then executes until it returns or yields: >> >> =A0 void coroutine_fn my_coroutine(void *opaque) { >> =A0 =A0 =A0 MyData *my_data =3D opaque; >> >> =A0 =A0 =A0 /* do some work */ >> >> =A0 =A0 =A0 qemu_coroutine_yield(); >> >> =A0 =A0 =A0 /* do some more work */ >> =A0 } >> >> Yielding switches control back to the caller of qemu_coroutine_enter(). >> This is typically used to switch back to the main thread's event loop >> after issuing an asynchronous I/O request. =A0The request callback will >> then invoke qemu_coroutine_enter() once more to switch back to the >> coroutine. >> >> Note that coroutines never execute concurrently and should only be used >> from threads which hold the global mutex. =A0This restriction makes >> programming with coroutines easier than with threads. =A0Race conditions >> cannot occur since only one coroutine may be active at any time. =A0Othe= r >> coroutines can only run across yield. > > Mmh, is there anything that conceptually prevent fixing this limitation > later on? I would really like to remove such dependency long-term as > well to have VCPUs operate truly independently on independent device mode= ls. The use case that has motivated coroutines is the block layer. It is synchronous in many places and definitely not thread-safe. Coroutines is a step that solves the "synchronous" part of the problem but does not tackle the "not thread-safe" part. It is possible to move from coroutines to threads but we need to remove single-thread assumptions from all the block layer code, which isn't a small task. Coroutines does not prevent us from making the block layer thread-safe! Stefan