Re: [PATCH -mm] memrlimit: fix task_lock() recursive locking
From: Andrea Righi
Date: Thu Sep 18 2008 - 16:14:37 EST
Hi Balbir,
Balbir Singh wrote:
>> static void memrlimit_cgroup_mm_owner_changed(struct cgroup_subsys *ss,
>> struct cgroup *old_cgrp,
>> @@ -246,7 +246,7 @@ static void memrlimit_cgroup_mm_owner_changed(struct cgroup_subsys *ss,
>> struct task_struct *p)
>> {
>> struct memrlimit_cgroup *memrcg, *old_memrcg;
>> - struct mm_struct *mm = get_task_mm(p);
>> + struct mm_struct *mm = get_task_mm_task_locked(p);
>>
>
> Since we hold task_lock(), we know that p->mm cannot change and we don't have to
> worry about incrementing mm_users. I think using just p->mm will work, we do
> have checks to make sure we don't pick a kernel thread. I vote for going down
> that road.
Sounds good. What about this?
---
cgroup_mm_owner_callbacks() can be called with task_lock() held in
mm_update_next_owner(), and all the .mm_owner_changed callbacks seem to
be *always* called with task_lock() held.
Actually, memrlimit is using task_lock() via get_task_mm() in
memrlimit_cgroup_mm_owner_changed(), raising the following recursive locking
trace:
[ 5346.421365] =============================================
[ 5346.421374] [ INFO: possible recursive locking detected ]
[ 5346.421381] 2.6.27-rc5-mm1 #20
[ 5346.421385] ---------------------------------------------
[ 5346.421391] interbench/10530 is trying to acquire lock:
[ 5346.421396] (&p->alloc_lock){--..}, at: [<ffffffff8023b034>] get_task_mm+0x24/0x70
[ 5346.421417]
[ 5346.421418] but task is already holding lock:
[ 5346.421423] (&p->alloc_lock){--..}, at: [<ffffffff8023db98>] mm_update_next_owner+0x148/0x230
[ 5346.421438]
[ 5346.421440] other info that might help us debug this:
[ 5346.421446] 2 locks held by interbench/10530:
[ 5346.421450] #0: (&mm->mmap_sem){----}, at: [<ffffffff8023db90>] mm_update_next_owner+0x140/0x230
[ 5346.421467] #1: (&p->alloc_lock){--..}, at: [<ffffffff8023db98>] mm_update_next_owner+0x148/0x230
[ 5346.421483]
[ 5346.421485] stack backtrace:
[ 5346.421491] Pid: 10530, comm: interbench Not tainted 2.6.27-rc5-mm1 #20
[ 5346.421496] Call Trace:
[ 5346.421507] [<ffffffff80263383>] validate_chain+0xb03/0x10d0
[ 5346.421515] [<ffffffff80263c05>] __lock_acquire+0x2b5/0x9c0
[ 5346.421522] [<ffffffff80262cc2>] validate_chain+0x442/0x10d0
[ 5346.421530] [<ffffffff802643aa>] lock_acquire+0x9a/0xe0
[ 5346.421537] [<ffffffff8023b034>] get_task_mm+0x24/0x70
[ 5346.421546] [<ffffffff804757c7>] _spin_lock+0x37/0x70
[ 5346.421553] [<ffffffff8023b034>] get_task_mm+0x24/0x70
[ 5346.421560] [<ffffffff8023b034>] get_task_mm+0x24/0x70
[ 5346.421569] [<ffffffff802b91f8>] memrlimit_cgroup_mm_owner_changed+0x18/0x90
[ 5346.421579] [<ffffffff80278b03>] cgroup_mm_owner_callbacks+0x93/0xc0
[ 5346.421587] [<ffffffff8023dc36>] mm_update_next_owner+0x1e6/0x230
[ 5346.421595] [<ffffffff8023dd72>] exit_mm+0xf2/0x120
[ 5346.421602] [<ffffffff8023f547>] do_exit+0x167/0x930
[ 5346.421610] [<ffffffff8047604a>] _spin_unlock_irq+0x2a/0x50
[ 5346.421618] [<ffffffff8023fd46>] do_group_exit+0x36/0xa0
[ 5346.421626] [<ffffffff8020b7cb>] system_call_fastpath+0x16/0x1b
Since we hold task_lock(), we know that p->mm cannot change and we don't have
to worry about incrementing mm_users. So, just use p->mm directly and check
that we've not picked a kernel thread.
Signed-off-by: Andrea Righi <righi.andrea@xxxxxxxxx>
---
kernel/cgroup.c | 3 ++-
mm/memrlimitcgroup.c | 6 +++---
2 files changed, 5 insertions(+), 4 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 678a680..03cc925 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2757,7 +2757,8 @@ void cgroup_fork_callbacks(struct task_struct *child)
* invoke this routine, since it assigns the mm->owner the first time
* and does not change it.
*
- * The callbacks are invoked with mmap_sem held in read mode.
+ * The callbacks are invoked with task_lock held and mmap_sem held in read
+ * mode.
*/
void cgroup_mm_owner_callbacks(struct task_struct *old, struct task_struct *new)
{
diff --git a/mm/memrlimitcgroup.c b/mm/memrlimitcgroup.c
index 8ee74f6..0e30465 100644
--- a/mm/memrlimitcgroup.c
+++ b/mm/memrlimitcgroup.c
@@ -238,7 +238,7 @@ out:
}
/*
- * This callback is called with mmap_sem held
+ * This callback is called with mmap_sem and task_lock held
*/
static void memrlimit_cgroup_mm_owner_changed(struct cgroup_subsys *ss,
struct cgroup *old_cgrp,
@@ -246,9 +246,9 @@ static void memrlimit_cgroup_mm_owner_changed(struct cgroup_subsys *ss,
struct task_struct *p)
{
struct memrlimit_cgroup *memrcg, *old_memrcg;
- struct mm_struct *mm = get_task_mm(p);
+ struct mm_struct *mm = p->mm;
- BUG_ON(!mm);
+ BUG_ON(!mm || (p->flags & PF_KTHREAD));
/*
* If we don't have a new cgroup, we just uncharge from the old one.
--
1.5.4.3
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