[PATCH 10/15] block-backend: always wait for drain before starting operation

[Paolo Bonzini <pbonzini@redhat.com>]

All I/O operations call blk_wait_while_drained() immediately after
blk_inc_in_flight(), except for blk_abort_aio_request() where it
does not hurt to add such a call.  Merge the two functions into one,
and add a note about a disturbing lack of thread-safety that will
be fixed shortly.

While at it, make the quiesce_counter check a loop.  While the check
does not have to be "perfect", i.e. it only matters that an endless
stream of I/Os is stopped sooner or later, it is more logical to check
the counter repeatedly until it is zero.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/block-backend.c | 27 ++++++++-------------------
 1 file changed, 8 insertions(+), 19 deletions(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index fe42d53d655d..627d491d4155 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1270,18 +1270,6 @@ static int blk_check_byte_request(BlockBackend *blk, int64_t offset,
     return 0;
 }
 
-/* To be called between exactly one pair of blk_inc/dec_in_flight() */
-static void coroutine_fn blk_wait_while_drained(BlockBackend *blk)
-{
-    assert(blk->in_flight > 0);
-
-    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);
-    }
-}
-
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static int coroutine_fn
 blk_co_do_preadv_part(BlockBackend *blk, int64_t offset, int64_t bytes,
@@ -1334,7 +1322,6 @@ int coroutine_fn blk_co_preadv(BlockBackend *blk, int64_t offset,
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
-    blk_wait_while_drained(blk);
     ret = blk_co_do_preadv_part(blk, offset, bytes, qiov, 0, flags);
     blk_dec_in_flight(blk);
 
@@ -1349,7 +1336,6 @@ int coroutine_fn blk_co_preadv_part(BlockBackend *blk, int64_t offset,
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
-    blk_wait_while_drained(blk);
     ret = blk_co_do_preadv_part(blk, offset, bytes, qiov, qiov_offset, flags);
     blk_dec_in_flight(blk);
 
@@ -1401,7 +1387,6 @@ int coroutine_fn blk_co_pwritev_part(BlockBackend *blk, int64_t offset,
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
-    blk_wait_while_drained(blk);
     ret = blk_co_do_pwritev_part(blk, offset, bytes, qiov, qiov_offset, flags);
     blk_dec_in_flight(blk);
 
@@ -1466,6 +1451,14 @@ void blk_inc_in_flight(BlockBackend *blk)
 {
     IO_CODE();
     qatomic_inc(&blk->in_flight);
+    if (!blk->disable_request_queuing) {
+        /* TODO: this is not thread-safe! */
+        while (blk->quiesce_counter) {
+            qatomic_dec(&blk->in_flight);
+            qemu_co_queue_wait(&blk->queued_requests, NULL);
+            qatomic_inc(&blk->in_flight);
+        }
+    }
 }
 
 void blk_dec_in_flight(BlockBackend *blk)
@@ -1546,7 +1539,6 @@ static BlockAIOCB *blk_aio_prwv(BlockBackend *blk, int64_t offset,
     Coroutine *co;
 
     blk_inc_in_flight(blk);
-    blk_wait_while_drained(blk);
     acb = blk_aio_get(&blk_aio_em_aiocb_info, blk, cb, opaque);
     acb->rwco = (BlkRwCo) {
         .blk    = blk,
@@ -1685,7 +1677,6 @@ int coroutine_fn blk_co_ioctl(BlockBackend *blk, unsigned long int req,
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
-    blk_wait_while_drained(blk);
     ret = blk_co_do_ioctl(blk, req, buf);
     blk_dec_in_flight(blk);
 
@@ -1749,7 +1740,6 @@ int coroutine_fn blk_co_pdiscard(BlockBackend *blk, int64_t offset,
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
-    blk_wait_while_drained(blk);
     ret = blk_co_do_pdiscard(blk, offset, bytes);
     blk_dec_in_flight(blk);
 
@@ -1790,7 +1780,6 @@ int coroutine_fn blk_co_flush(BlockBackend *blk)
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
-    blk_wait_while_drained(blk);
     ret = blk_co_do_flush(blk);
     blk_dec_in_flight(blk);
 
-- 
2.38.1