Messages in this thread | | | Date | Wed, 9 May 2012 06:57:14 -0700 | From | "Paul E. McKenney" <> | Subject | Re: rcu: BUG on exit_group |
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On Wed, May 09, 2012 at 01:59:59PM +0900, KAMEZAWA Hiroyuki wrote: > (2012/05/04 14:33), Paul E. McKenney wrote: > > > On Fri, May 04, 2012 at 06:08:34AM +0200, Sasha Levin wrote: > >> On Thu, May 3, 2012 at 7:01 PM, Paul E. McKenney > >> <paulmck@linux.vnet.ibm.com> wrote: > >>> On Thu, May 03, 2012 at 05:55:14PM +0200, Sasha Levin wrote: > >>>> On Thu, May 3, 2012 at 5:41 PM, Paul E. McKenney > >>>> <paulmck@linux.vnet.ibm.com> wrote: > >>>>> On Thu, May 03, 2012 at 10:57:19AM +0200, Sasha Levin wrote: > >>>>>> Hi Paul, > >>>>>> > >>>>>> I've hit a BUG similar to the schedule_tail() one when. It happened > >>>>>> when I've started fuzzing exit_group() syscalls, and all of the traces > >>>>>> are starting with exit_group() (there's a flood of them). > >>>>>> > >>>>>> I've verified that it indeed BUGs due to the rcu preempt count. > >>>>> > >>>>> Hello, Sasha, > >>>>> > >>>>> Which version of -next are you using? I did some surgery on this > >>>>> yesterday based on some bugs Hugh Dickins tracked down, so if you > >>>>> are using something older, please move to the current -next. > >>>> > >>>> I'm using -next from today (3.4.0-rc5-next-20120503-sasha-00002-g09f55ae-dirty). > >>> > >>> Hmmm... Looking at this more closely, it looks like there really is > >>> an attempt to acquire a mutex within an RCU read-side critical section, > >>> which is illegal. Could you please bisect this? > >> > >> Right, the issue is as you described, taking a mutex inside rcu_read_lock(). > >> > >> The offending commit is (I've cc'ed all parties from it): > >> > >> commit adf79cc03092ee4aec70da10e91b05fb8116ac7b > >> Author: Ying Han <yinghan@google.com> > >> Date: Thu May 3 15:44:01 2012 +1000 > >> > >> memcg: add mlock statistic in memory.stat > >> > >> With the issue there being is that in munlock_vma_page(), it now does > >> a mem_cgroup_begin_update_page_stat() which takes the rcu_read_lock(), > >> so when the older code that was there previously will try taking a > >> mutex you'll get a BUG. > > > > Hmmm... One approach would be to switch from rcu_read_lock() to > > srcu_read_lock(), though this means carrying the index returned from > > the srcu_read_lock() to the matching srcu_read_unlock() -- and making > > the update side use synchronize_srcu() rather than synchronize_rcu(). > > Alternatively, it might be possible to defer acquiring the lock until > > after exiting the RCU read-side critical section, but I don't know enough > > about mm to even guess whether this might be possible. > > > > There are probably other approaches as well... > > > How about this ?
That looks to me to avoid acquiring the mutex within an RCU read-side critical section, so good. I have to defer to you guys on whether the placement of the mem_cgroup_end_update_page_stat() works.
Thanx, Paul
> == > [PATCH] memcg: fix taking mutex under rcu at munlock > > Following bug was reported because mutex is held under rcu_read_lock(). > > [ 83.820976] BUG: sleeping function called from invalid context at > kernel/mutex.c:269 > [ 83.827870] in_atomic(): 0, irqs_disabled(): 0, pid: 4506, name: trinity > [ 83.832154] 1 lock held by trinity/4506: > [ 83.834224] #0: (rcu_read_lock){.+.+..}, at: [<ffffffff811a7d87>] > munlock_vma_page+0x197/0x200 > [ 83.839310] Pid: 4506, comm: trinity Tainted: G W > 3.4.0-rc5-next-20120503-sasha-00002-g09f55ae-dirty #108 > [ 83.849418] Call Trace: > [ 83.851182] [<ffffffff810e7218>] __might_sleep+0x1f8/0x210 > [ 83.854076] [<ffffffff82d9540a>] mutex_lock_nested+0x2a/0x50 > [ 83.857120] [<ffffffff811b0830>] try_to_unmap_file+0x40/0x2f0 > [ 83.860242] [<ffffffff82d984bb>] ? _raw_spin_unlock_irq+0x2b/0x80 > [ 83.863423] [<ffffffff810e7ffe>] ? sub_preempt_count+0xae/0xf0 > [ 83.866347] [<ffffffff82d984e9>] ? _raw_spin_unlock_irq+0x59/0x80 > [ 83.869570] [<ffffffff811b0caa>] try_to_munlock+0x6a/0x80 > [ 83.872667] [<ffffffff811a7cc6>] munlock_vma_page+0xd6/0x200 > [ 83.875646] [<ffffffff811a7d87>] ? munlock_vma_page+0x197/0x200 > [ 83.878798] [<ffffffff811a7e7f>] munlock_vma_pages_range+0x8f/0xd0 > [ 83.882235] [<ffffffff811a8b8a>] exit_mmap+0x5a/0x160 > > This bug was introduced by mem_cgroup_begin/end_update_page_stat() > which uses rcu_read_lock(). This patch fixes the bug by modifying > the range of rcu_read_lock(). > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> > --- > mm/mlock.c | 5 +++-- > 1 files changed, 3 insertions(+), 2 deletions(-) > > diff --git a/mm/mlock.c b/mm/mlock.c > index 2fd967a..05ac10d1 100644 > --- a/mm/mlock.c > +++ b/mm/mlock.c > @@ -123,6 +123,7 @@ void munlock_vma_page(struct page *page) > if (TestClearPageMlocked(page)) { > dec_zone_page_state(page, NR_MLOCK); > mem_cgroup_dec_page_stat(page, MEMCG_NR_MLOCK); > + mem_cgroup_end_update_page_stat(page, &locked, &flags); > if (!isolate_lru_page(page)) { > int ret = SWAP_AGAIN; > > @@ -154,8 +155,8 @@ void munlock_vma_page(struct page *page) > else > count_vm_event(UNEVICTABLE_PGMUNLOCKED); > } > - } > - mem_cgroup_end_update_page_stat(page, &locked, &flags); > + } else > + mem_cgroup_end_update_page_stat(page, &locked, &flags); > } > > /** > -- > 1.7.4.1 > > >
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