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Tue, 21 Jul 2020 16:37:24 +0000 (UTC) Subject: Re: [RFC PATCH-for-5.1] hw/ide: Do not block for AIO while resetting a drive To: Stefan Hajnoczi , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= References: <20200717171938.1249-1-f4bug@amsat.org> <20200720100246.GA24159@stefanha-x1.localdomain> From: John Snow Message-ID: Date: Tue, 21 Jul 2020 12:37:24 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200720100246.GA24159@stefanha-x1.localdomain> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=WINDOWS-1252; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=205.139.110.120; envelope-from=jsnow@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/21 10:58:38 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action 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: Alexander Bulekov , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 7/20/20 6:02 AM, Stefan Hajnoczi wrote: > On Fri, Jul 17, 2020 at 07:19:38PM +0200, Philippe Mathieu-Daudé wrote: >> Last minute chat: >> 19:01 f4bug: use bdrv_aio_cancel_async() if possible because it won't block the current thread. >> 19:02 f4bug: For example, in device emulation code where the guest has requested to cancel an I/O request it's often possible to use the async version. >> 19:02 f4bug: But in synchronous code like device reset it may be necessary to use the synchronous (blocking) bdrv_aio_cancel() API instead. :( >> 19:14 f4bug: The way to decide is: will the current function return to the event loop and is there someone who will handle the request completion callback when cancel finishes? >> 19:14 f4bug: If the next line of code requires the request to finished then async cancel cannot be used. >> 19:15 f4bug: On the other hand, if the function can return and it's okay for the request to cancel at some future time then you can use async. >> >> So I'll revisit this patch :) >> --- >> hw/ide/core.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/hw/ide/core.c b/hw/ide/core.c >> index d997a78e47..e3a9ce7d25 100644 >> --- a/hw/ide/core.c >> +++ b/hw/ide/core.c >> @@ -1315,7 +1315,8 @@ static void ide_reset(IDEState *s) >> trace_ide_reset(s); >> >> if (s->pio_aiocb) { >> - blk_aio_cancel(s->pio_aiocb); >> + /* If there is a pending AIO callback, invoke it now. */ >> + blk_aio_cancel_async(s->pio_aiocb); > > This is a place where an async call is not allowed. The completion > function must be called right away (synchronously) before we can > continue resetting the device. > > I sent a patch that allows bdrv_aio_cancel() to find the AioContext so > it can call aio_poll(). > > Stefan > OK, dropping Phil's patch here. --js