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Date: Mon, 28 Jun 2010 07:56:37 -0700
From: Darren Hart <dvhltc@...ibm.com>
To: Michal Hocko <mhocko@...e.cz>
CC: Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
LKML <linux-kernel@...r.kernel.org>,
Nick Piggin <npiggin@...e.de>,
Alexey Kuznetsov <kuznet@....inr.ac.ru>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: futex: race in lock and unlock&exit for robust futex with PI?
On 06/28/2010 07:42 AM, Michal Hocko wrote:
> Hi Darren,
>
> On Fri 25-06-10 16:35:14, Darren Hart wrote:
> [...]
>> # trace-cmd record -p nop ./runSimple.sh
>> <snip>
>>
>> # ps -eLo pid,comm,wchan | grep "simple "
>> 20636 simple pause
>> 20876 simple pause
>>
>> # trace-cmd report
>> version = 6
>> CPU 0 is empty
>> cpus=4
>> field->offset = 24 size=8
>> <...>-20636 [003] 1778.965860: bprint: futex_lock_pi_atomic : lookup_pi_state: -ESRCH
>> <...>-20636 [003] 1778.965865: bprint: futex_lock_pi_atomic : ownerdied not detected, returning -ESRCH
>> <...>-20636 [003] 1778.965866: bprint: futex_lock_pi_atomic : lookup_pi_state: -3
>>>> ---> <...>-20636 [003] 1778.965867: bprint: futex_lock_pi : returning -ESRCH to userspace
>> <...>-20876 [001] 1780.199394: bprint: futex_lock_pi_atomic : cmpxchg failed, retrying
>> <...>-20876 [001] 1780.199400: bprint: futex_lock_pi_atomic : lookup_pi_state: -ESRCH
>> <...>-20876 [001] 1780.199401: bprint: futex_lock_pi_atomic : ownerdied not detected, returning -ESRCH
>> <...>-20876 [001] 1780.199402: bprint: futex_lock_pi_atomic : lookup_pi_state: -3
>>>> ---> <...>-20876 [001] 1780.199403: bprint: futex_lock_pi : returning -ESRCH to userspace
>> <...>-21316 [002] 1782.300695: bprint: futex_lock_pi_atomic : cmpxchg failed, retrying
>> <...>-21316 [002] 1782.300698: bprint: futex_lock_pi_atomic : cmpxchg failed, retrying
>>
> [...]
>
> I have updated the test case slightly (reduced the number of lock/unlock
> cycles to 1).
>
> Then, I have used the additional patch (see bellow) on top of the one
> you have posted and here is the log I am getting:
Interesting. I'm going to start pouring over lookup_pi_state() and
futex_lock_pi() to see if I can spot any races. I'm also wondering about
glibc's usage of the FUTEX_WAITERS bit.
While I investigate that, could you do a git bisect from 2.6.31 (after
which a lot of the most recent futex changes hit) and see if you can
identify a particular patch, or even kernel version, where this broke?
Thanks,
Darren
>
> version = 6
> cpus=2
> field->offset = 16 size=4
> <...>-13232 [001] 226.693880: bprint: do_futex : futex_lock_pi start
> <...>-13232 [001] 226.693886: bprint: do_futex : futex_lock_pi done ret=0
> <...>-13235 [001] 226.700204: bprint: do_futex : futex_lock_pi start
> <...>-13235 [001] 226.700210: bprint: futex_lock_pi_atomic : lookup_pi_state: -ESRCH for pid=13242
> <...>-13235 [001] 226.700211: bprint: futex_lock_pi_atomic : ownerdied not detected, returning -ESRCH
> <...>-13235 [001] 226.700211: bprint: futex_lock_pi_atomic : lookup_pi_state: -3
> <...>-13235 [001] 226.700212: bprint: futex_lock_pi : returning -ESRCH to userspace
> <...>-13235 [001] 226.700212: bprint: do_futex : futex_lock_pi done ret=-3
> <...>-13240 [000] 226.705574: bprint: do_futex : futex_lock_pi start
> <...>-13240 [000] 226.705580: bprint: futex_lock_pi_atomic : lookup_pi_state: -ESRCH for pid=13242
> <...>-13240 [000] 226.705581: bprint: futex_lock_pi_atomic : ownerdied not detected, returning -ESRCH
> <...>-13240 [000] 226.705582: bprint: futex_lock_pi_atomic : lookup_pi_state: -3
> <...>-13240 [000] 226.705582: bprint: futex_lock_pi : returning -ESRCH to userspace
> <...>-13240 [000] 226.705583: bprint: do_futex : futex_lock_pi done ret=-3
> <...>-13231 [000] 226.708095: bprint: do_futex : futex_lock_pi start
> <...>-13231 [000] 226.708101: bprint: futex_lock_pi_atomic : lookup_pi_state: -ESRCH for pid=13242
> <...>-13231 [000] 226.708102: bprint: futex_lock_pi_atomic : ownerdied not detected, returning -ESRCH
> <...>-13231 [000] 226.708102: bprint: futex_lock_pi_atomic : lookup_pi_state: -3
> <...>-13231 [000] 226.708103: bprint: futex_lock_pi : returning -ESRCH to userspace
> <...>-13231 [000] 226.708103: bprint: do_futex : futex_lock_pi done ret=-3
> <...>-13242 [001] 226.709246: bprint: do_futex : futex_unlock_pi start
> <...>-13242 [001] 226.709249: bprint: do_futex : futex_unlock_pi: TID->0 transition 2147496890
> <...>-13242 [001] 226.709250: bprint: do_futex : futex_unlock_pi: no waiters, unlock the futex ret=0 uval=-2147470406
> <...>-13242 [001] 226.709250: bprint: do_futex : futex_unlock_pi done ret=0
>
> As you can see lookup_pi_state fails for the pid (13242) which is at the very
> bottom and that is unlocking the futex. This smells fishy to me. I can
> see this pattern consistently for all failures. Maybe I am doing
> something wrong or the timestamps are not precise enough but from what I
> can see this looks like a bug in lookup_pi_state which doesn't find an
> existing PID.
>
> --
> From 733816347db91670f27d206382b8c2e57e5ef125 Mon Sep 17 00:00:00 2001
> From: Michal Hocko<mhocko@...e.cz>
> Date: Mon, 28 Jun 2010 13:42:29 +0200
> Subject: [PATCH] futex pi unlock tracing added
>
> ---
> kernel/futex.c | 24 +++++++++++++++++++-----
> 1 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 24ac437..d114fee 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -716,7 +716,8 @@ retry:
> if (unlikely(ret)) {
> switch (ret) {
> case -ESRCH:
> - trace_printk("lookup_pi_state: -ESRCH\n");
> + trace_printk("lookup_pi_state: -ESRCH for pid=%u\n",
> + uval& FUTEX_TID_MASK);
> /*
> * No owner found for this futex. Check if the
> * OWNER_DIED bit is set to figure out whether
> @@ -2070,8 +2071,10 @@ retry:
> * again. If it succeeds then we can return without waking
> * anyone else up:
> */
> - if (!(uval& FUTEX_OWNER_DIED))
> + if (!(uval& FUTEX_OWNER_DIED)) {
> uval = cmpxchg_futex_value_locked(uaddr, task_pid_vnr(current), 0);
> + trace_printk("futex_unlock_pi: TID->0 transition %u\n", uval);
> + }
>
>
> if (unlikely(uval == -EFAULT))
> @@ -2080,8 +2083,10 @@ retry:
> * Rare case: we managed to release the lock atomically,
> * no need to wake anyone else up:
> */
> - if (unlikely(uval == task_pid_vnr(current)))
> + if (unlikely(uval == task_pid_vnr(current))) {
> + trace_printk("futex_unlock_pi: release without wakeup\n");
> goto out_unlock;
> + }
>
> /*
> * Ok, other tasks may need to be woken up - check waiters
> @@ -2093,6 +2098,7 @@ retry:
> if (!match_futex (&this->key,&key))
> continue;
> ret = wake_futex_pi(uaddr, uval, this);
> + trace_printk("futex_unlock_pi: wake ret=%d uval=%u this=%p\n", ret, uval, this);
> /*
> * The atomic access to the futex value
> * generated a pagefault, so retry the
> @@ -2107,6 +2113,8 @@ retry:
> */
> if (!(uval& FUTEX_OWNER_DIED)) {
> ret = unlock_futex_pi(uaddr, uval);
> + trace_printk("futex_unlock_pi: no waiters, unlock the futex ret=%d uval=%d\n",
> + ret, uval);
> if (ret == -EFAULT)
> goto pi_faulted;
> }
> @@ -2600,12 +2608,18 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
> ret = futex_wake_op(uaddr, fshared, uaddr2, val, val2, val3);
> break;
> case FUTEX_LOCK_PI:
> - if (futex_cmpxchg_enabled)
> + if (futex_cmpxchg_enabled) {
> + trace_printk("futex_lock_pi start\n");
> ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
> + trace_printk("futex_lock_pi done ret=%d\n", ret);
> + }
> break;
> case FUTEX_UNLOCK_PI:
> - if (futex_cmpxchg_enabled)
> + if (futex_cmpxchg_enabled) {
> + trace_printk("futex_unlock_pi start\n");
> ret = futex_unlock_pi(uaddr, fshared);
> + trace_printk("futex_unlock_pi done ret=%d\n", ret);
> + }
> break;
> case FUTEX_TRYLOCK_PI:
> if (futex_cmpxchg_enabled)
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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