[PATCH v8 13/14] block: Add bdrv_lock()/unlock()

[Kevin Wolf <kwolf@redhat.com>]

Inside of coroutine context, we can't directly use aio_context_acquire()
for the AioContext of a block node because we already own the lock of
the current AioContext and we need to avoid double locking to prevent
deadlocks.

This provides helper functions to lock the AioContext of a node only if
it's not the same as the current AioContext.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 include/block/block.h | 14 ++++++++++++++
 block.c               | 27 +++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/block/block.h b/include/block/block.h
index b5fa7b2229..65d79270e6 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -643,6 +643,20 @@ AioContext *coroutine_fn bdrv_co_enter(BlockDriverState *bs);
  */
 void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx);
 
+/**
+ * Locks the AioContext of @bs if it's not the current AioContext. This avoids
+ * double locking which could lead to deadlocks: This is a coroutine_fn, so we
+ * know we already own the lock of the current AioContext.
+ *
+ * May only be called in the main thread.
+ */
+void coroutine_fn bdrv_co_lock(BlockDriverState *bs);
+
+/**
+ * Unlocks the AioContext of @bs if it's not the current AioContext.
+ */
+void coroutine_fn bdrv_co_unlock(BlockDriverState *bs);
+
 /**
  * Transfer control to @co in the aio context of @bs
  */
diff --git a/block.c b/block.c
index 5eac2683b2..e39d5fa3db 100644
--- a/block.c
+++ b/block.c
@@ -6395,6 +6395,33 @@ void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx)
     bdrv_dec_in_flight(bs);
 }
 
+void coroutine_fn bdrv_co_lock(BlockDriverState *bs)
+{
+    AioContext *ctx = bdrv_get_aio_context(bs);
+
+    /* In the main thread, bs->aio_context won't change concurrently */
+    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+
+    /*
+     * We're in coroutine context, so we already hold the lock of the main
+     * loop AioContext. Don't lock it twice to avoid deadlocks.
+     */
+    assert(qemu_in_coroutine());
+    if (ctx != qemu_get_aio_context()) {
+        aio_context_acquire(ctx);
+    }
+}
+
+void coroutine_fn bdrv_co_unlock(BlockDriverState *bs)
+{
+    AioContext *ctx = bdrv_get_aio_context(bs);
+
+    assert(qemu_in_coroutine());
+    if (ctx != qemu_get_aio_context()) {
+        aio_context_release(ctx);
+    }
+}
+
 void bdrv_coroutine_enter(BlockDriverState *bs, Coroutine *co)
 {
     aio_co_enter(bdrv_get_aio_context(bs), co);
-- 
2.25.4