Re: [Qemu-devel] [PATCH v2 2/4] block: add BlockBackend->in_flight counter

[Eric Blake]

On 02/13/2018 08:21 AM, Stefan Hajnoczi wrote:

BlockBackend currently relies on BlockDriverState->in_flight to track
requests for blk_drain().  There is a corner case where
BlockDriverState->in_flight cannot be used though: blk->root can be NULL
when there is no medium.  This results in a segfault when the NULL
pointer is dereferenced.

Introduce a BlockBackend->in_flight counter for aio requests so it works
even when blk->root == NULL.

Based on a patch by Kevin Wolf .

Signed-off-by: Kevin Wolf 
Signed-off-by: Stefan Hajnoczi 
---
  block.c   |  2 +-
  block/block-backend.c | 60 +--
  2 files changed, 54 insertions(+), 8 deletions(-)



Reviewed-by: Eric Blake 

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

[Qemu-devel] [PATCH v2 2/4] block: add BlockBackend->in_flight counter

[Stefan Hajnoczi]

BlockBackend currently relies on BlockDriverState->in_flight to track
requests for blk_drain().  There is a corner case where
BlockDriverState->in_flight cannot be used though: blk->root can be NULL
when there is no medium.  This results in a segfault when the NULL
pointer is dereferenced.

Introduce a BlockBackend->in_flight counter for aio requests so it works
even when blk->root == NULL.

Based on a patch by Kevin Wolf .

Signed-off-by: Kevin Wolf 
Signed-off-by: Stefan Hajnoczi 
---
 block.c   |  2 +-
 block/block-backend.c | 60 +--
 2 files changed, 54 insertions(+), 8 deletions(-)

diff --git a/block.c b/block.c
index 9e4da81213..a83037c2a5 100644
--- a/block.c
+++ b/block.c
@@ -4713,7 +4713,7 @@ out:
 
 AioContext *bdrv_get_aio_context(BlockDriverState *bs)
 {
-return bs->aio_context;
+return bs ? bs->aio_context : qemu_get_aio_context();
 }
 
 AioWait *bdrv_get_aio_wait(BlockDriverState *bs)
diff --git a/block/block-backend.c b/block/block-backend.c
index 0266ac990b..a775a3dd2f 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -73,6 +73,14 @@ struct BlockBackend {
 int quiesce_counter;
 VMChangeStateEntry *vmsh;
 bool force_allow_inactivate;
+
+/* Number of in-flight aio requests.  BlockDriverState also counts
+ * in-flight requests but aio requests can exist even when blk->root is
+ * NULL, so we cannot rely on its counter for that case.
+ * Accessed with atomic ops.
+ */
+unsigned int in_flight;
+AioWait wait;
 };
 
 typedef struct BlockBackendAIOCB {
@@ -1225,11 +1233,22 @@ int blk_make_zero(BlockBackend *blk, BdrvRequestFlags 
flags)
 return bdrv_make_zero(blk->root, flags);
 }
 
+static void blk_inc_in_flight(BlockBackend *blk)
+{
+atomic_inc(>in_flight);
+}
+
+static void blk_dec_in_flight(BlockBackend *blk)
+{
+atomic_dec(>in_flight);
+aio_wait_kick(>wait);
+}
+
 static void error_callback_bh(void *opaque)
 {
 struct BlockBackendAIOCB *acb = opaque;
 
-bdrv_dec_in_flight(acb->common.bs);
+blk_dec_in_flight(acb->blk);
 acb->common.cb(acb->common.opaque, acb->ret);
 qemu_aio_unref(acb);
 }
@@ -1240,7 +1259,7 @@ BlockAIOCB *blk_abort_aio_request(BlockBackend *blk,
 {
 struct BlockBackendAIOCB *acb;
 
-bdrv_inc_in_flight(blk_bs(blk));
+blk_inc_in_flight(blk);
 acb = blk_aio_get(_backend_aiocb_info, blk, cb, opaque);
 acb->blk = blk;
 acb->ret = ret;
@@ -1263,7 +1282,7 @@ static const AIOCBInfo blk_aio_em_aiocb_info = {
 static void blk_aio_complete(BlkAioEmAIOCB *acb)
 {
 if (acb->has_returned) {
-bdrv_dec_in_flight(acb->common.bs);
+blk_dec_in_flight(acb->rwco.blk);
 acb->common.cb(acb->common.opaque, acb->rwco.ret);
 qemu_aio_unref(acb);
 }
@@ -1284,7 +1303,7 @@ static BlockAIOCB *blk_aio_prwv(BlockBackend *blk, 
int64_t offset, int bytes,
 BlkAioEmAIOCB *acb;
 Coroutine *co;
 
-bdrv_inc_in_flight(blk_bs(blk));
+blk_inc_in_flight(blk);
 acb = blk_aio_get(_aio_em_aiocb_info, blk, cb, opaque);
 acb->rwco = (BlkRwCo) {
 .blk= blk,
@@ -1521,14 +1540,41 @@ int blk_flush(BlockBackend *blk)
 
 void blk_drain(BlockBackend *blk)
 {
-if (blk_bs(blk)) {
-bdrv_drain(blk_bs(blk));
+BlockDriverState *bs = blk_bs(blk);
+
+if (bs) {
+bdrv_drained_begin(bs);
+}
+
+/* We may have -ENOMEDIUM completions in flight */
+AIO_WAIT_WHILE(>wait,
+blk_get_aio_context(blk),
+atomic_mb_read(>in_flight) > 0);
+
+if (bs) {
+bdrv_drained_end(bs);
 }
 }
 
 void blk_drain_all(void)
 {
-bdrv_drain_all();
+BlockBackend *blk = NULL;
+
+bdrv_drain_all_begin();
+
+while ((blk = blk_all_next(blk)) != NULL) {
+AioContext *ctx = blk_get_aio_context(blk);
+
+aio_context_acquire(ctx);
+
+/* We may have -ENOMEDIUM completions in flight */
+AIO_WAIT_WHILE(>wait, ctx,
+atomic_mb_read(>in_flight) > 0);
+
+aio_context_release(ctx);
+}
+
+bdrv_drain_all_end();
 }
 
 void blk_set_on_error(BlockBackend *blk, BlockdevOnError on_read_error,
-- 
2.14.3