[PATCH] dm crypt: fix sleep-in-atomic-context bug in kcryptd_crypt_tasklet

[Duoming Zhou]

In order to improve the IO performance of the dm-crypt
implementation, the commit 39d42fa96ba1 ("dm crypt:
add flags to optionally bypass kcryptd workqueues")
adds tasklet to do the crypto operations.

The tasklet callback function kcryptd_crypt_tasklet()
calls kcryptd_crypt() which is an original workqueue
callback function that may sleep. As a result, the
sleep-in-atomic-context bug may happen. The process
is shown below.

   (atomic context)
kcryptd_crypt_tasklet()
  kcryptd_crypt()
    kcryptd_crypt_write_convert()
      wait_for_completion() //may sleep

The wait_for_completion() is a function that may sleep.
In order to mitigate the bug, this patch changes
wait_for_completion() to try_wait_for_completion().

Fixes: 39d42fa96ba1 ("dm crypt: add flags to optionally bypass kcryptd workqueues")
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
 drivers/md/dm-crypt.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 8b47b913ee8..5e2b2463d87 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -2085,7 +2085,8 @@ static void kcryptd_crypt_write_convert(struct dm_crypt_io *io)
 	crypt_finished = atomic_dec_and_test(&ctx->cc_pending);
 	if (!crypt_finished && kcryptd_crypt_write_inline(cc, ctx)) {
 		/* Wait for completion signaled by kcryptd_async_done() */
-		wait_for_completion(&ctx->restart);
+		while (!try_wait_for_completion(&ctx->restart))
+			;
 		crypt_finished = 1;
 	}
 
-- 
2.17.1