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: 099ada2c8726 ("io_uring/rw: add write support for IOCB_DIO_CALLER_COMP")
Reported-by: Andres Freund <andres@xxxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
diff --git a/io_uring/rw.c b/io_uring/rw.c
index c8c822fa7980..807d83ab756e 100644
--- a/io_uring/rw.c
+++ b/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)
--
Jens Axboe
