Patch "kernel/debug/debug_core.c: more properly delay for secondary CPUs" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

kernel/debug/debug_core.c: more properly delay for secondary CPUs

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kernel-debug-debug_core.c-more-properly-delay-for-secondary-cpus.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Douglas Anderson <[email protected]>
Date: Wed, 14 Dec 2016 15:05:49 -0800
Subject: kernel/debug/debug_core.c: more properly delay for secondary CPUs

From: Douglas Anderson <[email protected]>

commit 2d13bb6494c807bcf3f78af0e96c0b8615a94385 upstream.

We've got a delay loop waiting for secondary CPUs. That loop uses loops_per_jiffy. However, loops_per_jiffy doesn't actually mean how many tight loops make up a jiffy on all architectures. It is quite common to see things like this in the boot log:

Calibrating delay loop (skipped), value calculated using timer frequency.. 48.00 BogoMIPS (lpj=24000)

In my case I was seeing lots of cases where other CPUs timed out entering the debugger only to print their stack crawls shortly after the kdb> prompt was written.

Elsewhere in kgdb we already use udelay(), so that should be safe enough to use to implement our timeout. We'll delay 1 ms for 1000 times, which should give us a full second of delay (just like the old code wanted) but allow us to notice that we're done every 1 ms.

---
 kernel/debug/debug_core.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -598,11 +598,11 @@ return_normal:
 	/*
 	 * Wait for the other CPUs to be notified and be waiting for us:
 	 */
-	time_left = loops_per_jiffy * HZ;
+	time_left = MSEC_PER_SEC;
 	while (kgdb_do_roundup && --time_left &&
 	       (atomic_read(&masters_in_kgdb) + atomic_read(&slaves_in_kgdb)) !=
 		   online_cpus)
-		cpu_relax();
+		udelay(1000);
 	if (!time_left)
 		pr_crit("Timed out waiting for secondary CPUs.\n");
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/kernel-debug-debug_core.c-more-properly-delay-for-secondary-cpus.patch