[RFC] Check if file_data is initialized

[Dmitrii Dolgov <9erthalion6@gmail.com>]

With combination of --fixedbufs and an old version of fio I've managed
to get a strange situation, when doing io_iopoll_complete NULL pointer
dereference on file_data was caused in io_free_req_many. Interesting
enough, the very same configuration doesn't fail on a newest version of
fio (the old one is fc220349e4514, the new one is 2198a6b5a9f4), but I
guess it still makes sense to have this check if it's possible to craft
such request to io_uring.

More details about configuration:

[global]
filename=/dev/vda
rw=randread
bs=256k
direct=1
time_based=1
randrepeat=1
gtod_reduce=1

[fiotest]

fio test.fio \
    --readonly \
    --ioengine=io_uring \
    --iodepth 1024 \
    --fixedbufs \
    --hipri \
    --numjobs=1 \
    --runtime=10

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
---
 fs/io_uring.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

I'm not entirely sure if my analysis is correct, but since this change
fixes the issue for me, I've decided to post it.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index c770c2c0eb52..c5e69dfc0221 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1232,7 +1232,8 @@ static void io_free_req_many(struct io_ring_ctx *ctx, struct req_batch *rb)
 do_free:
 	kmem_cache_free_bulk(req_cachep, rb->to_free, rb->reqs);
 	percpu_ref_put_many(&ctx->refs, rb->to_free);
-	percpu_ref_put_many(&ctx->file_data->refs, rb->to_free);
+	if (ctx->file_data)
+		percpu_ref_put_many(&ctx->file_data->refs, rb->to_free);
 	rb->to_free = rb->need_iter = 0;
 }
 
-- 
2.21.0