[PATCH RFC] pnfs/blocklayout: fix lost wakeup in bl_resolve_deviceid()

bl_resolve_deviceid() queues itself on nn->bl_wq and calls
rpc_queue_upcall(), but sets TASK_UNINTERRUPTIBLE only after that call
returns.  If blkmapd answers on another CPU in between, the wake_up()
from bl_pipe_downcall() finds the task runnable and the assignment that
follows overwrites it, so schedule() never returns.  The caller holds
nn->bl_mutex, so every later blocklayout device resolution blocks behind
it.

Set the state before queueing the upcall and restore TASK_RUNNING on the
error path.  Of the four rpc_queue_upcall() callers this is the only
open-coded waiter; __cld_pipe_upcall() uses wait_for_completion(), which
cannot lose a wakeup this way.

Fixes: fe0a9b740881 ("pnfsblock: add device operations")
Assisted-by: Claude:claude-opus-5
Signed-off-by: FAN YE <[email protected]>
---
Not seen on a real pNFS mount.  In a VM I reached bl_resolve_deviceid()
through a debugfs hook with a fake blkmapd and an mdelay() widening the
window: the caller sticks in D state at schedule(), and the next caller
then blocks on nn->bl_mutex.  Both go away with this patch.

Two questions:
- Would you rather have bl_wq converted to a completion, like
  __cld_pipe_upcall() in fs/nfsd/nfs4recover.c?
- rpc_queue_upcall() calls dput().  If the pipe were unlinked meanwhile,
  that could sleep with the task already in TASK_UNINTERRUPTIBLE.  Is that
  acceptable here?
---
 fs/nfs/blocklayout/rpc_pipefs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/blocklayout/rpc_pipefs.c b/fs/nfs/blocklayout/rpc_pipefs.c
index d526f5ba7887..15972ca0daf1 100644
--- a/fs/nfs/blocklayout/rpc_pipefs.c
+++ b/fs/nfs/blocklayout/rpc_pipefs.c
@@ -84,14 +84,16 @@ bl_resolve_deviceid(struct nfs_server *server, struct pnfs_block_volume *b,
 
 	dprintk("%s CALLING USERSPACE DAEMON\n", __func__);
 	add_wait_queue(&nn->bl_wq, &wq);
+	set_current_state(TASK_UNINTERRUPTIBLE);
 	rc = rpc_queue_upcall(nn->bl_device_pipe, msg);
 	if (rc < 0) {
+		__set_current_state(TASK_RUNNING);
 		remove_wait_queue(&nn->bl_wq, &wq);
 		goto out_free_data;
 	}
 
-	set_current_state(TASK_UNINTERRUPTIBLE);
 	schedule();
+	__set_current_state(TASK_RUNNING);
 	remove_wait_queue(&nn->bl_wq, &wq);
 
 	if (reply->status != BL_DEVICE_REQUEST_PROC) {

---
base-commit: 818bebeb63dd6bf5f4e07e145f6cdbace520a34c
change-id: 20260821-rpc-pipefs-blwake-b57669bfd26c

Best regards,
--  
FAN YE <[email protected]>