[PATCH] block: Prevent hang_check firing during long I/O

[Mark Lord <kernel@teksavvy.com>]

During long I/O operations, the hang_check timer may fire,
trigger stack dumps that unnecessarily alarm the user.

Eg.  hdparm --security-erase NULL /dev/sdb  ## can take *hours* to complete

So, if hang_check is armed, we should wake up periodically
to prevent it from triggering.  This patch uses a wake-up interval
equal to half the hang_check timer period, which keeps overhead low enough.

Signed-off-by: Mark Lord <mlord@pobox.com>

--- old/block/blk-exec.c	2010-09-20 19:56:53.000000000 -0400
+++ linux/block/blk-exec.c	2010-09-24 09:43:32.342604574 -0400
@@ -80,6 +80,7 @@
 	DECLARE_COMPLETION_ONSTACK(wait);
 	char sense[SCSI_SENSE_BUFFERSIZE];
 	int err = 0;
+	unsigned long hang_check;
 
 	/*
 	 * we need an extra reference to the request, so we can look at
@@ -95,7 +96,13 @@
 
 	rq->end_io_data = &wait;
 	blk_execute_rq_nowait(q, bd_disk, rq, at_head, blk_end_sync_rq);
-	wait_for_completion(&wait);
+
+	/* Prevent hang_check timer from firing at us during very long I/O */
+	hang_check = sysctl_hung_task_timeout_secs;
+	if (hang_check)
+		while (!wait_for_completion_timeout(&wait, hang_check * (HZ/2)));
+	else
+		wait_for_completion(&wait);
 
 	if (rq->errors)
 		err = -EIO;