Re: futex: race in lock and unlock&exit for robust futex with PI?

From: Michal Hocko
Date: Mon Jun 28 2010 - 11:58:58 EST

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On Mon 28-06-10 17:32:14, Michal Hocko wrote:
> 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.

I am trying to understand why futex_find_get_task has to check the
credentials at all. I guess that there might be some concerns from
futex_requeue callers but does it make sense from futex_lock_pi_atomic
call path? This would mean that the futex is unusable in multi-user process
synchronization, right? (Is this documented somewhere?)

[...]
>
> --
> From f0743d2b2d2f81c40700c4d6816ad3abfd113c1b Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxx>
> 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@xxxxxxx>
> > 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@xxxxxxxxxx>
> > > 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

--
Michal Hocko
L3 team
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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