[PATCH for-5.0 2/2] block: Fix blk->in_flight during blk_wait_while_drained()

[Kevin Wolf]

Calling blk_wait_while_drained() while blk->in_flight is increased for
the current request is wrong because it will cause the drain operation
to deadlock.

Many callers of blk_wait_while_drained() have already increased
blk->in_flight when called in a blk_aio_*() path, but can also be called
in synchonous code paths where blk->in_flight isn't increased. This
means that these calls of blk_wait_while_drained() are wrong at least in
some cases.

In order to fix this, increase blk->in_flight even for synchronous
operations and temporarily decrease the counter again in
blk_wait_while_drained().

Fixes: cf3129323f900ef5ddbccbe86e4fa801e88c566e
Signed-off-by: Kevin Wolf <kw...@redhat.com>
---
 block/block-backend.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 3124e367b3..7bd16402b8 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1143,7 +1143,9 @@ static int blk_check_byte_request(BlockBackend *blk, 
int64_t offset,
 static void coroutine_fn blk_wait_while_drained(BlockBackend *blk)
 {
     if (blk->quiesce_counter && !blk->disable_request_queuing) {
+        blk_dec_in_flight(blk);
         qemu_co_queue_wait(&blk->queued_requests, NULL);
+        blk_inc_in_flight(blk);
     }
 }
 
@@ -1260,6 +1262,7 @@ static int blk_prw(BlockBackend *blk, int64_t offset, 
uint8_t *buf,
         .ret    = NOT_DONE,
     };
 
+    blk_inc_in_flight(blk);
     if (qemu_in_coroutine()) {
         /* Fast-path if already in coroutine context */
         co_entry(&rwco);
@@ -1268,6 +1271,7 @@ static int blk_prw(BlockBackend *blk, int64_t offset, 
uint8_t *buf,
         bdrv_coroutine_enter(blk_bs(blk), co);
         BDRV_POLL_WHILE(blk_bs(blk), rwco.ret == NOT_DONE);
     }
+    blk_dec_in_flight(blk);
 
     return rwco.ret;
 }
@@ -1386,9 +1390,7 @@ static void blk_aio_read_entry(void *opaque)
     QEMUIOVector *qiov = rwco->iobuf;
 
     if (rwco->blk->quiesce_counter) {
-        blk_dec_in_flight(rwco->blk);
         blk_wait_while_drained(rwco->blk);
-        blk_inc_in_flight(rwco->blk);
     }
 
     assert(qiov->size == acb->bytes);
@@ -1404,9 +1406,7 @@ static void blk_aio_write_entry(void *opaque)
     QEMUIOVector *qiov = rwco->iobuf;
 
     if (rwco->blk->quiesce_counter) {
-        blk_dec_in_flight(rwco->blk);
         blk_wait_while_drained(rwco->blk);
-        blk_inc_in_flight(rwco->blk);
     }
 
     assert(!qiov || qiov->size == acb->bytes);
-- 
2.20.1