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Date: Mon, 28 Jun 2010 17:32:14 +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?
On Mon 28-06-10 16:42:46, Michal Hocko wrote:
> Hi Darren,
Hmm, I think that I've found the reason. I have used one additional
tracing patch (bellow) and we are getting ESRCH because cread and pcred
don't match:
version = 6
CPU 0 is empty
cpus=2
field->offset = 16 size=4
<...>-22260 [001] 139.669672: bprint: do_futex : futex_lock_pi start
<...>-22260 [001] 139.669678: bprint: do_futex : futex_lock_pi done ret=0
<...>-22281 [001] 139.693690: bprint: do_futex : futex_lock_pi start
<...>-22281 [001] 139.693696: bprint: lookup_pi_state : cred(1004,1004) != pcred(1003,1003)
<...>-22281 [001] 139.693697: bprint: lookup_pi_state : futex_find_get_task failed with -3
<...>-22281 [001] 139.693697: bprint: futex_lock_pi_atomic : lookup_pi_state: -ESRCH for pid=22280
<...>-22281 [001] 139.693698: bprint: futex_lock_pi_atomic : ownerdied not detected, returning -ESRCH
<...>-22281 [001] 139.693698: bprint: futex_lock_pi_atomic : lookup_pi_state: -3
<...>-22281 [001] 139.693699: bprint: futex_lock_pi : returning -ESRCH to userspace
<...>-22281 [001] 139.693700: bprint: do_futex : futex_lock_pi done ret=-3
<...>-22280 [001] 139.694033: bprint: do_futex : futex_unlock_pi start
<...>-22280 [001] 139.694035: bprint: do_futex : futex_unlock_pi: TID->0 transition 2147505928
<...>-22280 [001] 139.694036: bprint: do_futex : futex_unlock_pi: no waiters, unlock the futex ret=0 uval=-2147461368
<...>-22280 [001] 139.694036: bprint: do_futex : futex_unlock_pi done ret=0
<...>-22488 [001] 139.874967: bprint: do_futex : futex_lock_pi start
<...>-22488 [001] 139.874972: bprint: do_futex : futex_lock_pi done ret=0
This would answer why we cannot reproduce with a single user.
Btw. I guess that there is a typo in the condition and it should be
like this:
if (cred->euid != pcred->euid &&
- cred->euid != pcred->uid)
+ cred->uid != pcred->uid)
--
>From f0743d2b2d2f81c40700c4d6816ad3abfd113c1b Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.cz>
Date: Mon, 28 Jun 2010 17:08:56 +0200
Subject: [PATCH] more traces
---
kernel/futex.c | 13 +++++++++++--
1 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index d114fee..d1c2489 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -435,11 +435,16 @@ static struct task_struct * futex_find_get_task(pid_t pid)
p = find_task_by_vpid(pid);
if (!p) {
p = ERR_PTR(-ESRCH);
+ trace_printk("%u pid not found\n", pid);
} else {
pcred = __task_cred(p);
if (cred->euid != pcred->euid &&
- cred->euid != pcred->uid)
+ cred->euid != pcred->uid) {
p = ERR_PTR(-ESRCH);
+ trace_printk("cred(%u,%u) != pcred(%u,%u)\n",
+ cred->euid, cred->uid,
+ pcred->euid, pcred->uid);
+ }
else
get_task_struct(p);
}
@@ -564,8 +569,11 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
if (!pid)
return -ESRCH;
p = futex_find_get_task(pid);
- if (IS_ERR(p))
+ if (IS_ERR(p)) {
+ trace_printk("futex_find_get_task failed with %d\n",
+ PTR_ERR(p));
return PTR_ERR(p);
+ }
/*
* We need to look at the task state flags to figure out,
@@ -581,6 +589,7 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
* cleanup:
*/
int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;
+ trace_printk("pid=%u is exiting ret=%d\n", pid, ret);
raw_spin_unlock_irq(&p->pi_lock);
put_task_struct(p);
--
1.7.1
>
> --
> 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
--
Michal Hocko
L3 team
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
--
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