Patch "powerpc/mm: Fixup preempt underflow with huge pages" has been added to the 4.4-stable tree

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

powerpc/mm: Fixup preempt underflow with huge pages

to the 4.4-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:
powerpc-mm-fixup-preempt-underflow-with-huge-pages.patch
and it can be found in the queue-4.4 subdirectory.

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

From: Sebastian Siewior <[email protected]>
Date: Tue, 8 Mar 2016 10:03:56 +0100
Subject: powerpc/mm: Fixup preempt underflow with huge pages

From: Sebastian Siewior <[email protected]>

commit 08a5bb2921e490939f78f38fd0d02858bb709942 upstream.

hugepd_free() used __get_cpu_var() once. Nothing ensured that the code accessing the variable did not migrate from one CPU to another and soon this was noticed by Tiejun Chen in 94b09d755462 ("powerpc/hugetlb:
Replace __get_cpu_var with get_cpu_var"). So we had it fixed.

Christoph Lameter was doing his __get_cpu_var() replaces and forgot PowerPC. Then he noticed this and sent his fixed up batch again which got applied as 69111bac42f5 ("powerpc: Replace __get_cpu_var uses").

The careful reader will noticed one little detail: get_cpu_var() got replaced with this_cpu_ptr(). So now we have a put_cpu_var() which does a preempt_enable() and nothing that does preempt_disable() so we underflow the preempt counter.

---
 arch/powerpc/mm/hugetlbpage.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -486,13 +486,13 @@ static void hugepd_free(struct mmu_gathe
 {
 	struct hugepd_freelist **batchp;
 
-	batchp = this_cpu_ptr(&hugepd_freelist_cur);
+	batchp = &get_cpu_var(hugepd_freelist_cur);
 
 	if (atomic_read(&tlb->mm->mm_users) < 2 ||
 	    cpumask_equal(mm_cpumask(tlb->mm),
 			  cpumask_of(smp_processor_id()))) {
 		kmem_cache_free(hugepte_cache, hugepte);
-        put_cpu_var(hugepd_freelist_cur);
+		put_cpu_var(hugepd_freelist_cur);
 		return;
 	}
 

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

queue-4.4/powerpc-mm-fixup-preempt-underflow-with-huge-pages.patch