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Date: Mon, 28 Jun 2010 16:42:46 +0200
From: Michal Hocko <mhocko@...e.cz>
To: Darren Hart <dvhltc@...ibm.com>
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?
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:
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)
--
1.7.1
>
> --
> Darren Hart
>
> From 92014a07df73489460ff788274506255ff0f775d Mon Sep 17 00:00:00 2001
> From: Darren Hart <dvhltc@...ibm.com>
> Date: Fri, 25 Jun 2010 13:54:25 -0700
> Subject: [PATCH] robust pi futex tracing
>
> ---
> kernel/futex.c | 24 ++++++++++++++++++++----
> 1 files changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index e7a35f1..24ac437 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -683,6 +683,8 @@ retry:
> */
> if (unlikely(ownerdied || !(curval & FUTEX_TID_MASK))) {
> /* Keep the OWNER_DIED bit */
> + if (ownerdied)
> + trace_printk("ownerdied, taking over lock\n");
> newval = (curval & ~FUTEX_TID_MASK) | task_pid_vnr(task);
> ownerdied = 0;
> lock_taken = 1;
> @@ -692,14 +694,18 @@ retry:
>
> if (unlikely(curval == -EFAULT))
> return -EFAULT;
> - if (unlikely(curval != uval))
> + if (unlikely(curval != uval)) {
> + trace_printk("cmpxchg failed, retrying\n");
> goto retry;
> + }
>
> /*
> * We took the lock due to owner died take over.
> */
> - if (unlikely(lock_taken))
> + if (unlikely(lock_taken)) {
> + trace_printk("ownerdied, lock acquired, return 1\n");
> return 1;
> + }
>
> /*
> * We dont have the lock. Look up the PI state (or create it if
> @@ -710,13 +716,16 @@ retry:
> if (unlikely(ret)) {
> switch (ret) {
> case -ESRCH:
> + trace_printk("lookup_pi_state: -ESRCH\n");
> /*
> * No owner found for this futex. Check if the
> * OWNER_DIED bit is set to figure out whether
> * this is a robust futex or not.
> */
> - if (get_futex_value_locked(&curval, uaddr))
> + if (get_futex_value_locked(&curval, uaddr)) {
> + trace_printk("get_futex_value_locked: -EFAULT\n");
> return -EFAULT;
> + }
>
> /*
> * We simply start over in case of a robust
> @@ -724,10 +733,13 @@ retry:
> * and return happy.
> */
> if (curval & FUTEX_OWNER_DIED) {
> + trace_printk("ownerdied, goto retry\n");
> ownerdied = 1;
> goto retry;
> }
> + trace_printk("ownerdied not detected, returning -ESRCH\n");
> default:
> + trace_printk("lookup_pi_state: %d\n", ret);
> break;
> }
> }
> @@ -1950,6 +1962,8 @@ retry_private:
> put_futex_key(fshared, &q.key);
> cond_resched();
> goto retry;
> + case -ESRCH:
> + trace_printk("returning -ESRCH to userspace\n");
> default:
> goto out_unlock_put_key;
> }
> @@ -2537,8 +2551,10 @@ void exit_robust_list(struct task_struct *curr)
> /*
> * Avoid excessively long or circular lists:
> */
> - if (!--limit)
> + if (!--limit) {
> + trace_printk("excessively long list, aborting\n");
> break;
> + }
>
> cond_resched();
> }
> --
> 1.7.0.4
>
> --
> Darren Hart
> IBM Linux Technology Center
> Real-Time Linux Team
--
Michal Hocko
L3 team
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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