[BUGFIX][PATCH] memcg: avoid use cmpxchg in swap cgroup maintainance (Was Re: 34-rc1-git3 build failure with CGROUP_MEM_RES_CTLR_SWAP=y

[KAMEZAWA Hiroyuki]

On Sun, 14 Mar 2010 16:18:06 +0530
Sachin Sant <sachinp at in.ibm.com> wrote:

> On a PowerPC box, latest 34-rc1 git(d89b218b8...) fails to build
> with CGROUPS_MEM_RES_CTRL_SWAP=y. 
> 
> LD      init/built-in.o
> LD      .tmp_vmlinux1
> mm/built-in.o: In function __xchg:
> arch/powerpc/include/asm/system.h:331: undefined reference to .__xchg_called_with_bad_pointer
> mm/built-in.o: In function __cmpxchg:
> arch/powerpc/include/asm/system.h:474: undefined reference to .__cmpxchg_called_with_bad_pointer
> make: *** [.tmp_vmlinux1] Error 1
> 
> The code in question was added via commit 024914477e...
> 
> memcg: move charges of anonymous swap
> 
Oh..ok, powerpc (and other archs?) can't do 2byte cmpxchg and xchg.
Then, we should use spinlock rather than that.

How about this ? Nishimura-san, could you consider something better ?
We need a quick fix.

==
swap_cgroup uses 2bytes data and uses cmpxchg in a new operation.
2byte cmpxchg/xchg is not available on some archs. This patch replaces
cmpxchg/xchg with operations under lock.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
---
 mm/page_cgroup.c |   20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

Index: mmotm-2.6.34-Mar11/mm/page_cgroup.c
===================================================================
--- mmotm-2.6.34-Mar11.orig/mm/page_cgroup.c
+++ mmotm-2.6.34-Mar11/mm/page_cgroup.c
@@ -284,6 +284,7 @@ static DEFINE_MUTEX(swap_cgroup_mutex);
 struct swap_cgroup_ctrl {
 	struct page **map;
 	unsigned long length;
+	spinlock_t	lock;
 };
 
 struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
@@ -353,16 +354,22 @@ unsigned short swap_cgroup_cmpxchg(swp_e
 	struct swap_cgroup_ctrl *ctrl;
 	struct page *mappage;
 	struct swap_cgroup *sc;
+	unsigned long flags;
+	unsigned short retval;
 
 	ctrl = &swap_cgroup_ctrl[type];
 
 	mappage = ctrl->map[idx];
 	sc = page_address(mappage);
 	sc += pos;
-	if (cmpxchg(&sc->id, old, new) == old)
-		return old;
+	spin_lock_irqsave(&ctrl->lock, flags);
+	retval = sc->id;
+	if (retval == old)
+		sc->id = new;
 	else
-		return 0;
+		retval = 0;
+	spin_unlock_irqrestore(&ctrl->lock, flags);
+	return retval;
 }
 
 /**
@@ -383,13 +390,17 @@ unsigned short swap_cgroup_record(swp_en
 	struct page *mappage;
 	struct swap_cgroup *sc;
 	unsigned short old;
+	unsigned long flags;
 
 	ctrl = &swap_cgroup_ctrl[type];
 
 	mappage = ctrl->map[idx];
 	sc = page_address(mappage);
 	sc += pos;
-	old = xchg(&sc->id, id);
+	spin_lock_irqsave(&ctrl->lock, flags);
+	old = sc->id;
+	sc->id = id;
+	spin_unlock_irqrestore(&ctrl->lock, flags);
 
 	return old;
 }
@@ -441,6 +452,7 @@ int swap_cgroup_swapon(int type, unsigne
 	mutex_lock(&swap_cgroup_mutex);
 	ctrl->length = length;
 	ctrl->map = array;
+	spin_lock_init(&ctrl->lock);
 	if (swap_cgroup_prepare(type)) {
 		/* memory shortage */
 		ctrl->map = NULL;