linux-kernel - [PATCH 1/6] ipc/util.c, ipc_rcu_alloc: cacheline align allocation

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Date:	Mon, 10 Jun 2013 19:16:46 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>, hhuang@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 1/6] ipc/util.c, ipc_rcu_alloc: cacheline align allocation

Enforce that ipc_rcu_alloc returns a cacheline aligned pointer on SMP.

Rational:
The SysV sem code tries to move the main spinlock into a seperate cacheline
(____cacheline_aligned_in_smp). This works only if ipc_rcu_alloc returns
cacheline aligned pointers.
vmalloc and kmalloc return cacheline algined pointers, the implementation
of ipc_rcu_alloc breaks that.

Andrew: Could you merge it into -akpm and then forward it towards Linus' tree?

Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
---
 ipc/util.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/ipc/util.c b/ipc/util.c
index 809ec5e..9623c8e 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -469,9 +469,7 @@ void ipc_free(void* ptr, int size)
 struct ipc_rcu {
 	struct rcu_head rcu;
 	atomic_t refcount;
-	/* "void *" makes sure alignment of following data is sane. */
-	void *data[0];
-};
+} ____cacheline_aligned_in_smp;
 
 /**
  *	ipc_rcu_alloc	-	allocate ipc and rcu space 
@@ -489,12 +487,14 @@ void *ipc_rcu_alloc(int size)
 	if (unlikely(!out))
 		return NULL;
 	atomic_set(&out->refcount, 1);
-	return out->data;
+	return out+1;
 }
 
 int ipc_rcu_getref(void *ptr)
 {
-	return atomic_inc_not_zero(&container_of(ptr, struct ipc_rcu, data)->refcount);
+	struct ipc_rcu *p = ((struct ipc_rcu*)ptr)-1;
+
+	return atomic_inc_not_zero(&p->refcount);
 }
 
 /**
@@ -508,7 +508,7 @@ static void ipc_schedule_free(struct rcu_head *head)
 
 void ipc_rcu_putref(void *ptr)
 {
-	struct ipc_rcu *p = container_of(ptr, struct ipc_rcu, data);
+	struct ipc_rcu *p = ((struct ipc_rcu*)ptr)-1;
 
 	if (!atomic_dec_and_test(&p->refcount))
 		return;
-- 
1.8.1.4

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