Messages in this thread | | | Date | Mon, 28 Sep 2009 15:02:51 +0530 | Subject | Re: [BUGFIX][PATCH][rc1] memcg: fix refcnt goes to minus | From | Balbir Singh <> |
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> __mem_cgroup_largest_soft_limit_node() returns a mem_cgroup_per_zone "mz" > with incremnted mz->mem->css's refcnt. > Then, the caller of this function has to call css_put(mz->mem->css). > > But, mz can be !NULL even if "not found" i.e. without css_get(). > By this, css->refcnt will go down to minus. > > This may cause various things...one of results will be > initite-loop in css_tryget() as this. > > INFO: RCU detected CPU 0 stall (t=10000 jiffies) > sending NMI to all CPUs: > NMI backtrace for cpu 0 > CPU 0: > <snip> > > <<EOE>> <IRQ> [<ffffffff810884bd>] trace_hardirqs_off+0xd/0x10 > [<ffffffff8102a940>] flat_send_IPI_mask+0x90/0xb0 > [<ffffffff8102a9c9>] flat_send_IPI_all+0x69/0x70 > [<ffffffff81027372>] arch_trigger_all_cpu_backtrace+0x62/0xa0 > [<ffffffff810bff8e>] __rcu_pending+0x7e/0x370 > [<ffffffff810c02c7>] rcu_check_callbacks+0x47/0x130 > [<ffffffff81063a26>] update_process_times+0x46/0x70 > [<ffffffff81085930>] tick_sched_timer+0x60/0x160 > [<ffffffff810858d0>] ? tick_sched_timer+0x0/0x160 > [<ffffffff8107a03a>] __run_hrtimer+0xba/0x150 > [<ffffffff8107a325>] hrtimer_interrupt+0xd5/0x1b0 > [<ffffffff81426dfe>] ? trace_hardirqs_off_thunk+0x3a/0x3c > [<ffffffff8142cacd>] smp_apic_timer_interrupt+0x6d/0x9b > [<ffffffff8100cb33>] apic_timer_interrupt+0x13/0x20 > <EOI> [<ffffffff811317b6>] ? mem_cgroup_walk_tree+0x156/0x180 > [<ffffffff811316d3>] ? mem_cgroup_walk_tree+0x73/0x180 > [<ffffffff81131692>] ? mem_cgroup_walk_tree+0x32/0x180 > [<ffffffff81131a00>] ? mem_cgroup_get_local_stat+0x0/0x110 > [<ffffffff81131d5b>] ? mem_control_stat_show+0x14b/0x330 > [<ffffffff810a57fd>] ? cgroup_seqfile_show+0x3d/0x60 > > Above shows CPU0 caught in css_tryget()'s inifinite loop because > of bad refcnt. > > This is a fix to set mz=NULL at the top of retry path. > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> > > --- > mm/memcontrol.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > Index: linux-2.6.32-rc1/mm/memcontrol.c > =================================================================== > --- linux-2.6.32-rc1.orig/mm/memcontrol.c > +++ linux-2.6.32-rc1/mm/memcontrol.c > @@ -447,9 +447,10 @@ static struct mem_cgroup_per_zone * > __mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_zone *mctz) > { > struct rb_node *rightmost = NULL; > - struct mem_cgroup_per_zone *mz = NULL; > + struct mem_cgroup_per_zone *mz; > > retry: > + mz = NULL; > rightmost = rb_last(&mctz->rb_root); > if (!rightmost) > goto done; /* Nothing to reclaim from */ >
Good catch! So we fail at css_tryget() once, but mz is valid, we return a non NULL mz and we do a css_put() causing ref count to go bad. The next iteration that uses this mz will hang? I've not been able to hit, may be I should test in a really small cgroup under stress.
Balbir Singh. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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