nfs: Run co BH CB in the coroutine’s AioContext

Like in “rbd: Run co BH CB in the coroutine’s AioContext”, drop the
completion flag, yield exactly once, and run the BH in the coroutine’s
AioContext.

(Can be reproduced with multiqueue by adding a usleep(100000) before the
`while (!task.complete)` loops.)

Like in “iscsi: Run co BH CB in the coroutine’s AioContext”, this makes
nfs_co_generic_bh_cb() trivial, so we can drop it in favor of just
calling aio_co_wake() directly.

Cc: qemu-stable@nongnu.org
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20251110154854.151484-5-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/nfs.c

@@ -69,7 +69,6 @@ typedef struct NFSClient {
 typedef struct NFSRPC {
     BlockDriverState *bs;
     int ret;
-    int complete;
     QEMUIOVector *iov;
     struct stat *st;
     Coroutine *co;
@@ -230,14 +229,6 @@ static void coroutine_fn nfs_co_init_task(BlockDriverState *bs, NFSRPC *task)
     };
 }
 
-static void nfs_co_generic_bh_cb(void *opaque)
-{
-    NFSRPC *task = opaque;
-
-    task->complete = 1;
-    aio_co_wake(task->co);
-}
-
 /* Called (via nfs_service) with QemuMutex held.  */
 static void
 nfs_co_generic_cb(int ret, struct nfs_context *nfs, void *data,
@@ -256,8 +247,16 @@ nfs_co_generic_cb(int ret, struct nfs_context *nfs, void *data,
     if (task->ret < 0) {
         error_report("NFS Error: %s", nfs_get_error(nfs));
     }
-    replay_bh_schedule_oneshot_event(task->client->aio_context,
-                                     nfs_co_generic_bh_cb, task);
+
+    /*
+     * Safe to call: nfs_service(), which called us, is only run from the FD
+     * handlers, never from the request coroutine.  The request coroutine in
+     * turn will yield unconditionally.
+     * No need to release the lock, even if we directly enter the coroutine, as
+     * the lock is never re-taken after yielding.  (Note: If we do enter the
+     * coroutine, @task will probably be dangling once aio_co_wake() returns.)
+     */
+    aio_co_wake(task->co);
 }
 
 static int coroutine_fn nfs_co_preadv(BlockDriverState *bs, int64_t offset,
@@ -278,9 +277,7 @@ static int coroutine_fn nfs_co_preadv(BlockDriverState *bs, int64_t offset,
 
         nfs_set_events(client);
     }
-    while (!task.complete) {
-        qemu_coroutine_yield();
-    }
+    qemu_coroutine_yield();
 
     if (task.ret < 0) {
         return task.ret;
@@ -328,9 +325,7 @@ static int coroutine_fn nfs_co_pwritev(BlockDriverState *bs, int64_t offset,
 
         nfs_set_events(client);
     }
-    while (!task.complete) {
-        qemu_coroutine_yield();
-    }
+    qemu_coroutine_yield();
 
     if (my_buffer) {
         g_free(buf);
@@ -358,9 +353,7 @@ static int coroutine_fn nfs_co_flush(BlockDriverState *bs)
 
         nfs_set_events(client);
     }
-    while (!task.complete) {
-        qemu_coroutine_yield();
-    }
+    qemu_coroutine_yield();
 
     return task.ret;
 }
@@ -723,8 +716,8 @@ nfs_get_allocated_file_size_cb(int ret, struct nfs_context *nfs, void *data,
     if (task->ret < 0) {
         error_report("NFS Error: %s", nfs_get_error(nfs));
     }
-    replay_bh_schedule_oneshot_event(task->client->aio_context,
-                                     nfs_co_generic_bh_cb, task);
+    /* Safe to call, see nfs_co_generic_cb() */
+    aio_co_wake(task->co);
 }
 
 static int64_t coroutine_fn nfs_co_get_allocated_file_size(BlockDriverState *bs)
@@ -748,9 +741,7 @@ static int64_t coroutine_fn nfs_co_get_allocated_file_size(BlockDriverState *bs)
 
         nfs_set_events(client);
     }
-    while (!task.complete) {
-        qemu_coroutine_yield();
-    }
+    qemu_coroutine_yield();
 
     return (task.ret < 0 ? task.ret : st.st_blocks * 512);
 }