[PATCH] swap_readpage: avoid blk_wake_io_task() if !synchronous

[Oleg Nesterov <oleg@redhat.com>]

swap_readpage() sets waiter = bio->bi_private even if synchronous = F,
this means that the caller can get the spurious wakeup after return. This
can be fatal if blk_wake_io_task() does set_current_state(TASK_RUNNING)
after the caller does set_special_state(), in the worst case the kernel
can crash in do_task_dead().

Reported-by: Qian Cai <cai@lca.pw>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 mm/page_io.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 2e8019d..3098895 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -140,8 +140,10 @@ static void end_swap_bio_read(struct bio *bio)
 	unlock_page(page);
 	WRITE_ONCE(bio->bi_private, NULL);
 	bio_put(bio);
-	blk_wake_io_task(waiter);
-	put_task_struct(waiter);
+	if (waiter) {
+		blk_wake_io_task(waiter);
+		put_task_struct(waiter);
+	}
 }
 
 int generic_swapfile_activate(struct swap_info_struct *sis,
@@ -398,11 +400,12 @@ int swap_readpage(struct page *page, bool synchronous)
 	 * Keep this task valid during swap readpage because the oom killer may
 	 * attempt to access it in the page fault retry time check.
 	 */
-	get_task_struct(current);
-	bio->bi_private = current;
 	bio_set_op_attrs(bio, REQ_OP_READ, 0);
-	if (synchronous)
+	if (synchronous) {
 		bio->bi_opf |= REQ_HIPRI;
+		get_task_struct(current);
+		bio->bi_private = current;
+	}
 	count_vm_event(PSWPIN);
 	bio_get(bio);
 	qc = submit_bio(bio);
-- 
2.5.0