io_uring/rw: disable IOCB_DIO_CALLER_COMP

If an application does O_DIRECT writes with io_uring and the file system
supports IOCB_DIO_CALLER_COMP, then completions of the dio write side is
done from the task_work that will post the completion event for said
write as well.

Whenever a dio write is done against a file, the inode i_dio_count is
elevated. This enables other callers to use inode_dio_wait() to wait for
previous writes to complete. If we defer the full dio completion to
task_work, we are dependent on that task_work being run before the
inode i_dio_count can be decremented.

If the same task that issues io_uring dio writes with
IOCB_DIO_CALLER_COMP performs a synchronous system call that calls
inode_dio_wait(), then we can deadlock as we're blocked sleeping on
the event to become true, but not processing the completions that will
result in the inode i_dio_count being decremented.

Until we can guarantee that this is the case, then disable the deferred
caller completions.

Fixes: 099ada2 ("io_uring/rw: add write support for IOCB_DIO_CALLER_COMP")
Reported-by: Andres Freund <andres@anarazel.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rw.c

@@ -913,15 +913,6 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags)
 		kiocb_start_write(kiocb);
 	kiocb->ki_flags |= IOCB_WRITE;
 
-	/*
-	 * For non-polled IO, set IOCB_DIO_CALLER_COMP, stating that our handler
-	 * groks deferring the completion to task context. This isn't
-	 * necessary and useful for polled IO as that can always complete
-	 * directly.
-	 */
-	if (!(kiocb->ki_flags & IOCB_HIPRI))
-		kiocb->ki_flags |= IOCB_DIO_CALLER_COMP;
-
 	if (likely(req->file->f_op->write_iter))
 		ret2 = call_write_iter(req->file, kiocb, &s->iter);
 	else if (req->file->f_op->write)