From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56907) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1W4ADw-0007R4-Ln for qemu-devel@nongnu.org; Fri, 17 Jan 2014 09:16:46 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1W4ADq-0006S3-Mv for qemu-devel@nongnu.org; Fri, 17 Jan 2014 09:16:40 -0500 Received: from mx1.redhat.com ([209.132.183.28]:16231) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1W4ADq-0006Rl-FV for qemu-devel@nongnu.org; Fri, 17 Jan 2014 09:16:34 -0500 From: Kevin Wolf Date: Fri, 17 Jan 2014 15:15:11 +0100 Message-Id: <1389968119-24771-22-git-send-email-kwolf@redhat.com> In-Reply-To: <1389968119-24771-1-git-send-email-kwolf@redhat.com> References: <1389968119-24771-1-git-send-email-kwolf@redhat.com> Subject: [Qemu-devel] [PATCH v3 21/29] block: Assert serialisation assumptions in pwritev List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: kwolf@redhat.com, pl@kamp.de, mreitz@redhat.com, stefanha@redhat.com, pbonzini@redhat.com, xiawenc@linux.vnet.ibm.com If a request calls wait_serialising_requests() and actually has to wait in this function (i.e. a coroutine yield), other requests can run and previously read data (like the head or tail buffer) could become outdated. In this case, we would have to restart from the beginning to read in the updated data. However, we're lucky and don't actually need to do that: A request can only wait in the first call of wait_serialising_requests() because we mark it as serialising before that call, so any later requests would wait. So as we don't wait in practice, we don't have to reload the data. This is an important assumption that may not be broken or data corruption will happen. Document it with some assertions. Signed-off-by: Kevin Wolf --- block.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/block.c b/block.c index 859e1aa..53d9bd5 100644 --- a/block.c +++ b/block.c @@ -2123,14 +2123,15 @@ static bool tracked_request_overlaps(BdrvTrackedRequest *req, return true; } -static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self) +static bool coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self) { BlockDriverState *bs = self->bs; BdrvTrackedRequest *req; bool retry; + bool waited = false; if (!bs->serialising_in_flight) { - return; + return false; } do { @@ -2156,11 +2157,14 @@ static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self) qemu_co_queue_wait(&req->wait_queue); self->waiting_for = NULL; retry = true; + waited = true; break; } } } } while (retry); + + return waited; } /* @@ -3011,6 +3015,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs, QEMUIOVector *qiov, int flags) { BlockDriver *drv = bs->drv; + bool waited; int ret; int64_t sector_num = offset >> BDRV_SECTOR_BITS; @@ -3019,7 +3024,8 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs, assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0); assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0); - wait_serialising_requests(req); + waited = wait_serialising_requests(req); + assert(!waited || !req->serialising); ret = notifier_with_return_list_notify(&bs->before_write_notifiers, req); @@ -3119,9 +3125,11 @@ static int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs, QEMUIOVector tail_qiov; struct iovec tail_iov; size_t tail_bytes; + bool waited; mark_request_serialising(&req, align); - wait_serialising_requests(&req); + waited = wait_serialising_requests(&req); + assert(!waited || !use_local_qiov); tail_buf = qemu_blockalign(bs, align); tail_iov = (struct iovec) { -- 1.8.1.4