linux-kernel - [PATCH] slub: avoid false-postive warning

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Date:   Mon, 24 Oct 2016 17:56:13 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Laura Abbott <labbott@...oraproject.org>,
        Alexander Potapenko <glider@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] slub: avoid false-postive warning

The slub allocator gives us some incorrect warnings when
CONFIG_PROFILE_ANNOTATED_BRANCHES is set, as the unlikely()
macro prevents it from seeing that the return code matches
what it was before:

mm/slub.c: In function ‘kmem_cache_free_bulk’:
mm/slub.c:262:23: error: ‘df.s’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
mm/slub.c:2943:3: error: ‘df.cnt’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
mm/slub.c:2933:4470: error: ‘df.freelist’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
mm/slub.c:2943:3: error: ‘df.tail’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

I have not been able to come up with a perfect way for dealing with
this, the three options I see are:

- add a bogus initialization, which would increase the runtime overhead
- replace unlikely() with unlikely_notrace()
- remove the unlikely() annotation completely

I checked the object code for a typical x86 configuration and the
last two cases produce the same result, so I went for the last
one, which is the simplest.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 mm/slub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 2b3e740609e9..68b84f93d38d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3076,7 +3076,7 @@ void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
 		struct detached_freelist df;
 
 		size = build_detached_freelist(s, size, p, &df);
-		if (unlikely(!df.page))
+		if (!df.page)
 			continue;
 
 		slab_free(df.s, df.page, df.freelist, df.tail, df.cnt,_RET_IP_);
-- 
2.9.0