block: Convert bdrv_refresh_total_sectors() to co_wrapper_mixed

BlockDriver->bdrv_getlength is categorized as IO callback, and it
currently doesn't run in a coroutine. We should let it take a graph
rdlock since the callback traverses the block nodes graph, which however
is only possible in a coroutine.

Therefore turn it into a co_wrapper to move the actual function into a
coroutine where the lock can be taken.

Because now this function creates a new coroutine and polls, we need to
take the AioContext lock where it is missing, for the only reason that
internally co_wrapper calls AIO_WAIT_WHILE and it expects to release the
AioContext lock.

This is especially messy when a co_wrapper creates a coroutine and polls
in bdrv_open_driver, because this function has so many callers in so
many context that it can easily lead to deadlocks. Therefore the new
rule for bdrv_open_driver is that the caller must always hold the
AioContext lock of the given bs (except if it is a coroutine), because
the function calls bdrv_refresh_total_sectors() which is now a
co_wrapper.

Once the rwlock is ultimated and placed in every place it needs to be,
we will poll using AIO_WAIT_WHILE_UNLOCKED and remove the AioContext
lock.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230113204212.359076-7-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index 7ba436811b..d698cc3f33 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1599,14 +1599,15 @@ BlockAIOCB *blk_aio_pwrite_zeroes(BlockBackend *blk, int64_t offset,
                         flags | BDRV_REQ_ZERO_WRITE, cb, opaque);
 }
 
-int64_t blk_getlength(BlockBackend *blk)
+int64_t coroutine_fn blk_co_getlength(BlockBackend *blk)
 {
     IO_CODE();
+
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
-    return bdrv_getlength(blk_bs(blk));
+    return bdrv_co_getlength(blk_bs(blk));
 }
 
 void blk_get_geometry(BlockBackend *blk, uint64_t *nb_sectors_ptr)
@@ -1619,14 +1620,15 @@ void blk_get_geometry(BlockBackend *blk, uint64_t *nb_sectors_ptr)
     }
 }
 
-int64_t blk_nb_sectors(BlockBackend *blk)
+int64_t coroutine_fn blk_co_nb_sectors(BlockBackend *blk)
 {
     IO_CODE();
+
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
-    return bdrv_nb_sectors(blk_bs(blk));
+    return bdrv_co_nb_sectors(blk_bs(blk));
 }
 
 BlockAIOCB *blk_aio_preadv(BlockBackend *blk, int64_t offset,