From e424aff5f307227b1c2512bbb8ece891bb895cef Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Thu, 13 Aug 2015 10:41:50 +0200 Subject: [PATCH] mirror: Fix coroutine reentrance This fixes a regression introduced by commit dcfb3beb ("mirror: Do zero write on target if sectors not allocated"), which was reported to cause aborts with the message "Co-routine re-entered recursively". The cause for this bug is the following code in mirror_iteration_done(): if (s->common.busy) { qemu_coroutine_enter(s->common.co, NULL); } This has always been ugly because - unlike most places that reenter - it doesn't have a specific yield that it pairs with, but is more uncontrolled. What we really mean here is "reenter the coroutine if it's in one of the four explicit yields in mirror.c". This used to be equivalent with s->common.busy because neither mirror_run() nor mirror_iteration() call any function that could yield. However since commit dcfb3beb this doesn't hold true any more: bdrv_get_block_status_above() can yield. So what happens is that bdrv_get_block_status_above() wants to take a lock that is already held, so it adds itself to the queue of waiting coroutines and yields. Instead of being woken up by the unlock function, however, it gets woken up by mirror_iteration_done(), which is obviously wrong. In most cases the code actually happens to cope fairly well with such cases, but in this specific case, the unlock must already have scheduled the coroutine for wakeup when mirror_iteration_done() reentered it. And then the coroutine happened to process the scheduled restarts and tried to reenter itself recursively. This patch fixes the problem by pairing the reenter in mirror_iteration_done() with specific yields instead of abusing s->common.busy. Cc: qemu-stable@nongnu.org Signed-off-by: Kevin Wolf Reviewed-by: Paolo Bonzini Reviewed-by: Stefan Hajnoczi Reviewed-by: Jeff Cody Message-id: 1439455310-11263-1-git-send-email-kwolf@redhat.com Signed-off-by: Jeff Cody --- block/mirror.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/block/mirror.c b/block/mirror.c index 0841964e97..94744432eb 100644 --- a/block/mirror.c +++ b/block/mirror.c @@ -60,6 +60,7 @@ typedef struct MirrorBlockJob { int sectors_in_flight; int ret; bool unmap; + bool waiting_for_io; } MirrorBlockJob; typedef struct MirrorOp { @@ -114,11 +115,7 @@ static void mirror_iteration_done(MirrorOp *op, int ret) qemu_iovec_destroy(&op->qiov); g_slice_free(MirrorOp, op); - /* Enter coroutine when it is not sleeping. The coroutine sleeps to - * rate-limit itself. The coroutine will eventually resume since there is - * a sleep timeout so don't wake it early. - */ - if (s->common.busy) { + if (s->waiting_for_io) { qemu_coroutine_enter(s->common.co, NULL); } } @@ -203,7 +200,9 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s) /* Wait for I/O to this cluster (from a previous iteration) to be done. */ while (test_bit(next_chunk, s->in_flight_bitmap)) { trace_mirror_yield_in_flight(s, sector_num, s->in_flight); + s->waiting_for_io = true; qemu_coroutine_yield(); + s->waiting_for_io = false; } do { @@ -239,7 +238,9 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s) */ while (nb_chunks == 0 && s->buf_free_count < added_chunks) { trace_mirror_yield_buf_busy(s, nb_chunks, s->in_flight); + s->waiting_for_io = true; qemu_coroutine_yield(); + s->waiting_for_io = false; } if (s->buf_free_count < nb_chunks + added_chunks) { trace_mirror_break_buf_busy(s, nb_chunks, s->in_flight); @@ -337,7 +338,9 @@ static void mirror_free_init(MirrorBlockJob *s) static void mirror_drain(MirrorBlockJob *s) { while (s->in_flight > 0) { + s->waiting_for_io = true; qemu_coroutine_yield(); + s->waiting_for_io = false; } } @@ -510,7 +513,9 @@ static void coroutine_fn mirror_run(void *opaque) if (s->in_flight == MAX_IN_FLIGHT || s->buf_free_count == 0 || (cnt == 0 && s->in_flight > 0)) { trace_mirror_yield(s, s->in_flight, s->buf_free_count, cnt); + s->waiting_for_io = true; qemu_coroutine_yield(); + s->waiting_for_io = false; continue; } else if (cnt != 0) { delay_ns = mirror_iteration(s); -- 2.11.4.GIT