linux-kernel - [PATCH 4.8 02/10] Using BUG_ON() as an assert() is _never

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Message-Id: <20161006074741.004520594@linuxfoundation.org>
Date:   Thu,  6 Oct 2016 10:18:25 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.8 02/10] Using BUG_ON() as an assert() is _never_ acceptable

4.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@...ux-foundation.org>

commit 21f54ddae449f4bdd9f1498124901d67202243d9 upstream.

That just generally kills the machine, and makes debugging only much
harder, since the traces may long be gone.

Debugging by assert() is a disease.  Don't do it.  If you can continue,
you're much better off doing so with a live machine where you have a
much higher chance that the report actually makes it to the system logs,
rather than result in a machine that is just completely dead.

The only valid situation for BUG_ON() is when continuing is not an
option, because there is massive corruption.  But if you are just
verifying that something is true, you warn about your broken assumptions
(preferably just once), and limp on.

Fixes: 22f2ac51b6d6 ("mm: workingset: fix crash in shadow node shrinker caused by replace_page_cache_page()")
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Miklos Szeredi <miklos@...redi.hu>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 include/linux/swap.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -257,7 +257,7 @@ static inline void workingset_node_pages
 
 static inline void workingset_node_pages_dec(struct radix_tree_node *node)
 {
-	VM_BUG_ON(!workingset_node_pages(node));
+	VM_WARN_ON_ONCE(!workingset_node_pages(node));
 	node->count--;
 }
 
@@ -273,7 +273,7 @@ static inline void workingset_node_shado
 
 static inline void workingset_node_shadows_dec(struct radix_tree_node *node)
 {
-	VM_BUG_ON(!workingset_node_shadows(node));
+	VM_WARN_ON_ONCE(!workingset_node_shadows(node));
 	node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
 }