From a77fd4bb2988c05953fdc9f1524085870ec1c939 Mon Sep 17 00:00:00 2001 From: Fam Zheng Date: Tue, 5 Apr 2016 19:20:52 +0800 Subject: block: Fix bdrv_drain in coroutine Using the nested aio_poll() in coroutine is a bad idea. This patch replaces the aio_poll loop in bdrv_drain with a BH, if called in coroutine. For example, the bdrv_drain() in mirror.c can hang when a guest issued request is pending on it in qemu_co_mutex_lock(). Mirror coroutine in this case has just finished a request, and the block job is about to complete. It calls bdrv_drain() which waits for the other coroutine to complete. The other coroutine is a scsi-disk request. The deadlock happens when the latter is in turn pending on the former to yield/terminate, in qemu_co_mutex_lock(). The state flow is as below (assuming a qcow2 image): mirror coroutine scsi-disk coroutine ------------------------------------------------------------- do last write qcow2:qemu_co_mutex_lock() ... scsi disk read tracked request begin qcow2:qemu_co_mutex_lock.enter qcow2:qemu_co_mutex_unlock() bdrv_drain while (has tracked request) aio_poll() In the scsi-disk coroutine, the qemu_co_mutex_lock() will never return because the mirror coroutine is blocked in the aio_poll(blocking=true). With this patch, the added qemu_coroutine_yield() allows the scsi-disk coroutine to make progress as expected: mirror coroutine scsi-disk coroutine ------------------------------------------------------------- do last write qcow2:qemu_co_mutex_lock() ... scsi disk read tracked request begin qcow2:qemu_co_mutex_lock.enter qcow2:qemu_co_mutex_unlock() bdrv_drain.enter > schedule BH > qemu_coroutine_yield() > qcow2:qemu_co_mutex_lock.return > ... tracked request end ... (resumed from BH callback) bdrv_drain.return ... Reported-by: Laurent Vivier Signed-off-by: Fam Zheng Message-id: 1459855253-5378-2-git-send-email-famz@redhat.com Suggested-by: Paolo Bonzini Signed-off-by: Fam Zheng Signed-off-by: Stefan Hajnoczi --- block/io.c | 45 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 45 insertions(+) (limited to 'block') diff --git a/block/io.c b/block/io.c index c4869b96c5..a7dbf85b19 100644 --- a/block/io.c +++ b/block/io.c @@ -253,6 +253,47 @@ static void bdrv_drain_recurse(BlockDriverState *bs) } } +typedef struct { + Coroutine *co; + BlockDriverState *bs; + QEMUBH *bh; + bool done; +} BdrvCoDrainData; + +static void bdrv_co_drain_bh_cb(void *opaque) +{ + BdrvCoDrainData *data = opaque; + Coroutine *co = data->co; + + qemu_bh_delete(data->bh); + bdrv_drain(data->bs); + data->done = true; + qemu_coroutine_enter(co, NULL); +} + +void coroutine_fn bdrv_co_drain(BlockDriverState *bs) +{ + BdrvCoDrainData data; + + /* Calling bdrv_drain() from a BH ensures the current coroutine yields and + * other coroutines run if they were queued from + * qemu_co_queue_run_restart(). */ + + assert(qemu_in_coroutine()); + data = (BdrvCoDrainData) { + .co = qemu_coroutine_self(), + .bs = bs, + .done = false, + .bh = aio_bh_new(bdrv_get_aio_context(bs), bdrv_co_drain_bh_cb, &data), + }; + qemu_bh_schedule(data.bh); + + qemu_coroutine_yield(); + /* If we are resumed from some other event (such as an aio completion or a + * timer callback), it is a bug in the caller that should be fixed. */ + assert(data.done); +} + /* * Wait for pending requests to complete on a single BlockDriverState subtree, * and suspend block driver's internal I/O until next request arrives. @@ -269,6 +310,10 @@ void bdrv_drain(BlockDriverState *bs) bool busy = true; bdrv_drain_recurse(bs); + if (qemu_in_coroutine()) { + bdrv_co_drain(bs); + return; + } while (busy) { /* Keep iterating */ bdrv_flush_io_queue(bs); -- cgit v1.2.1