On Sat, Aug 10, 2013 at 11:43:59AM +0800, Lai Jiangshan wrote:
> Hi, Steven
>
> I was considering rtmutex's lock->wait_lock is a scheduler lock,
> But it is not, and it is just a spinlock of process context.
> I hope you change it to a spinlock of irq context.
>
> 1) it causes rcu read site more deadlockable, example:
> x is a spinlock of softirq context.
>
> CPU1 cpu2(rcu boost)
> rcu_read_lock() rt_mutext_lock()
> <preemption and reschedule back> raw_spin_lock(lock->wait_lock)
> spin_lock_bh(x) <interrupt and doing softirq
> after irq>
> rcu_read_unlock() do_softirq()
> rcu_read_unlock_special()
> rt_mutext_unlock()
> raw_spin_lock(lock->wait_lock) spin_lock_bh(x) DEADLOCK
>
> This example can happen on any one of these code:
> without my patchset
> with my patchset
> with my patchset but reverted patch2
>
> 2) why it causes more deadlockable: it extends the range of suspect locks.
> #DEFINE suspect locks: any lock can be (chained) nested in
> rcu_read_unlock_special().
>
> So the suspect locks are: rnp->lock, scheduler locks, rtmutex's
> lock->wait_lock,
> locks in prink()/WARN_ON() and the locks which can be chained/indirectly
> nested
> in the above locks.
>
> If the rtmutex's lock->wait_lock is a spinlock of irq context, all suspect
> locks are
> some spinlocks of irq context.
>
> If the rtmutex's lock->wait_lock is a spinlock of process context, suspect
> locks
> will be extended to, all spinlocks of irq context, all spinlocks of softirq
> context,
> and (all spinlocks of process context but nested in rtmutex's
> lock->wait_lock).
>
> We can see from the definition, if rcu_read_unlock_special() is called from
> any suspect lock, it may be deadlock like the example. the rtmutex's
> lock->wait_lock
> extends the range of suspect locks, it causes more deadlockable.
>
> 3) How my algorithm works, why smaller range of suspect locks help us.
> Since rcu_read_unlock_special() can't be called from suspect locks context,
> we should defer rcu_read_unlock_special() when in these contexts.
> It is hard to find out current context is suspect locks context or not,
> but we can determine it based on irq/softirq/process context.
>
> if all suspect locks are some spinlocks of irq context:
> if (irqs_disabled) /* we may be in suspect locks context */
> defer rcu_read_unlock_special().
>
> if all suspect locks are some spinlocks of irq/softirq/process context:
> if (irqs_disabled || in_atomic()) /* we may be in suspect locks context
> */
> defer rcu_read_unlock_special().
> In this case, the deferring becomes large more, I can't accept it.
> So I have to narrow the range of suspect locks. Two choices:
> A) don't call rt_mutex_unlock() from rcu_read_unlock(), only call it
> from rcu_preempt_not_context_switch(). we need to rework these
> two functions and it will add complexity to RCU, and it also still
> adds some probability of deferring.
One advantage of bh-disable locks is that enabling bh checks
TIF_NEED_RESCHED, so that there is no deferring beyond that
needed by bh disable. The same of course applies to preempt_disable().
So one approach is to defer when rcu_read_unlock_special() is entered
with either preemption or bh disabled. Your current set_need_resched()
trick would work fine in this case. Unfortunately, re-enabling interrupts
does -not- check TIF_NEED_RESCHED, which is why we have latency problems
in that case. (Hence my earlier question about making self-IPI safe
on all arches, which would result in an interrupt as soon as interrupts
were re-enabled.)
Another possibility is to defer only when preemption or bh are disabled
on entry ro rcu_read_unlock_special(), but to retain the current
(admittedly ugly) nesting rules for the scheduler locks.
> B) change rtmutex's lock->wait_lock to irqs-disabled.
I have to defer to Steven on this one.
Thanx, Paul
> 4) In the view of rtmutex, I think it will be better if ->wait_lock is
> irqs-disabled.
> A) like trylock of mutex/rw_sem, we may call rt_mutex_trylock() in irq in
> future.
> B) the critical section of ->wait_lock is short,
> making it irqs-disabled don't hurts responsibility/latency.
> C) almost all time of the critical section of ->wait_lock is irqs-disabled
> (due to task->pi_lock), I think converting whole time of the critical
> section
> of ->wait_lock to irqs-disabled is OK.
>
> So I hope you change rtmutex's lock->wait_lock.
>
> Any feedback from anyone is welcome.
>
> Thanks,
> Lai
>
> On 08/09/2013 04:40 AM, Paul E. McKenney wrote:
> > On Wed, Aug 07, 2013 at 06:25:01PM +0800, Lai Jiangshan wrote:
> >> Background)
> >>
> >> Although all articles declare that rcu read site is deadlock-immunity.
> >> It is not true for rcu-preempt, it will be deadlock if rcu read site
> >> overlaps with scheduler lock.
> >>
> >> ec433f0c, 10f39bb1 and 016a8d5b just partially solve it. But rcu read site
> >> is still not deadlock-immunity. And the problem described in 016a8d5b
> >> is still existed(rcu_read_unlock_special() calls wake_up).
> >>
> >> Aim)
> >>
> >> We want to fix the problem forever, we want to keep rcu read site
> >> is deadlock-immunity as books say.
> >>
> >> How)
> >>
> >> The problem is solved by "if rcu_read_unlock_special() is called inside
> >> any lock which can be (chained) nested in rcu_read_unlock_special(),
> >> we defer rcu_read_unlock_special()".
> >> This kind locks include rnp->lock, scheduler locks, perf ctx->lock, locks
> >> in printk()/WARN_ON() and all locks nested in these locks or chained nested
> >> in these locks.
> >>
> >> The problem is reduced to "how to distinguish all these locks(context)",
> >> We don't distinguish all these locks, we know that all these locks
> >> should be nested in local_irqs_disable().
> >>
> >> we just consider if rcu_read_unlock_special() is called in irqs-disabled
> >> context, it may be called in these suspect locks, we should defer
> >> rcu_read_unlock_special().
> >>
> >> The algorithm enlarges the probability of deferring, but the probability
> >> is still very very low.
> >>
> >> Deferring does add a small overhead, but it offers us:
> >> 1) really deadlock-immunity for rcu read site
> >> 2) remove the overhead of the irq-work(250 times per second in avg.)
> >
> > One problem here -- it may take quite some time for a set_need_resched()
> > to take effect. This is especially a problem for RCU priority boosting,
> > but can also needlessly delay preemptible-RCU grace periods because
> > local_irq_restore() and friends don't check the TIF_NEED_RESCHED bit.
> >
> > OK, alternatives...
> >
> > o Keep the current rule saying that if the scheduler is going
> > to exit an RCU read-side critical section while holding
> > one of its spinlocks, preemption has to have been disabled
> > throughout the full duration of that critical section.
> > Well, we can certainly do this, but it would be nice to get
> > rid of this rule.
> >
> > o Use per-CPU variables, possibly injecting delay. This has ugly
> > disadvantages as noted above.
> >
> > o irq_work_queue() can wait a jiffy (or on some architectures,
> > quite a bit longer) before actually doing anything.
> >
> > o raise_softirq() is more immediate and is an easy change, but
> > adds a softirq vector -- which people are really trying to
> > get rid of. Also, wakeup_softirqd() calls things that acquire
> > the scheduler locks, which is exactly what we were trying to
> > avoid doing.
> >
> > o invoke_rcu_core() can invoke raise_softirq() as above.
> >
> > o IPI to self. From what I can see, not all architectures
> > support this. Easy to fake if you have at least two CPUs,
> > but not so good from an OS jitter viewpoint...
> >
> > o Add a check to local_irq_disable() and friends. I would guess
> > that this suggestion would not make architecture maintainers
> > happy.
> >
> > Other thoughts?
> >
> > Thanx, Paul
> >
> >> Signed-off-by: Lai Jiangshan <la...@cn.fujitsu.com>
> >> ---
> >> include/linux/rcupdate.h | 2 +-
> >> kernel/rcupdate.c | 2 +-
> >> kernel/rcutree_plugin.h | 47
> >> +++++++++++++++++++++++++++++++++++++++++----
> >> 3 files changed, 44 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> >> index 4b14bdc..00b4220 100644
> >> --- a/include/linux/rcupdate.h
> >> +++ b/include/linux/rcupdate.h
> >> @@ -180,7 +180,7 @@ extern void synchronize_sched(void);
> >>
> >> extern void __rcu_read_lock(void);
> >> extern void __rcu_read_unlock(void);
> >> -extern void rcu_read_unlock_special(struct task_struct *t);
> >> +extern void rcu_read_unlock_special(struct task_struct *t, bool unlock);
> >> void synchronize_rcu(void);
> >>
> >> /*
> >> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> >> index cce6ba8..33b89a3 100644
> >> --- a/kernel/rcupdate.c
> >> +++ b/kernel/rcupdate.c
> >> @@ -90,7 +90,7 @@ void __rcu_read_unlock(void)
> >> #endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
> >> barrier(); /* assign before ->rcu_read_unlock_special load */
> >> if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
> >> - rcu_read_unlock_special(t);
> >> + rcu_read_unlock_special(t, true);
> >> barrier(); /* ->rcu_read_unlock_special load before assign */
> >> t->rcu_read_lock_nesting = 0;
> >> }
> >> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> >> index fc8b36f..997b424 100644
> >> --- a/kernel/rcutree_plugin.h
> >> +++ b/kernel/rcutree_plugin.h
> >> @@ -242,15 +242,16 @@ static void rcu_preempt_note_context_switch(int cpu)
> >> ? rnp->gpnum
> >> : rnp->gpnum + 1);
> >> raw_spin_unlock_irqrestore(&rnp->lock, flags);
> >> - } else if (t->rcu_read_lock_nesting < 0 &&
> >> - !WARN_ON_ONCE(t->rcu_read_lock_nesting != INT_MIN) &&
> >> - t->rcu_read_unlock_special) {
> >> + } else if (t->rcu_read_lock_nesting == 0 ||
> >> + (t->rcu_read_lock_nesting < 0 &&
> >> + !WARN_ON_ONCE(t->rcu_read_lock_nesting != INT_MIN))) {
> >>
> >> /*
> >> * Complete exit from RCU read-side critical section on
> >> * behalf of preempted instance of __rcu_read_unlock().
> >> */
> >> - rcu_read_unlock_special(t);
> >> + if (t->rcu_read_unlock_special)
> >> + rcu_read_unlock_special(t, false);
> >> }
> >>
> >> /*
> >> @@ -333,7 +334,7 @@ static struct list_head *rcu_next_node_entry(struct
> >> task_struct *t,
> >> * notify RCU core processing or task having blocked during the RCU
> >> * read-side critical section.
> >> */
> >> -void rcu_read_unlock_special(struct task_struct *t)
> >> +void rcu_read_unlock_special(struct task_struct *t, bool unlock)
> >> {
> >> int empty;
> >> int empty_exp;
> >> @@ -364,6 +365,42 @@ void rcu_read_unlock_special(struct task_struct *t)
> >>
> >> /* Clean up if blocked during RCU read-side critical section. */
> >> if (special & RCU_READ_UNLOCK_BLOCKED) {
> >> + /*
> >> + * If rcu read lock overlaps with scheduler lock,
> >> + * rcu_read_unlock_special() may lead to deadlock:
> >> + *
> >> + * rcu_read_lock();
> >> + * preempt_schedule[_irq]() (when preemption)
> >> + * scheduler lock; (or some other locks can be (chained) nested
> >> + * in rcu_read_unlock_special()/rnp->lock)
> >> + * access and check rcu data
> >> + * rcu_read_unlock();
> >> + * rcu_read_unlock_special();
> >> + * wake_up(); DEAD LOCK
> >> + *
> >> + * To avoid all these kinds of deadlock, we should quit
> >> + * rcu_read_unlock_special() here and defer it to
> >> + * rcu_preempt_note_context_switch() or next outmost
> >> + * rcu_read_unlock() if we consider this case may happen.
> >> + *
> >> + * Although we can't know whether current _special()
> >> + * is nested in scheduler lock or not. But we know that
> >> + * irqs are always disabled in this case. so we just quit
> >> + * and defer it to rcu_preempt_note_context_switch()
> >> + * when irqs are disabled.
> >> + *
> >> + * It means we always defer _special() when it is
> >> + * nested in irqs disabled context, but
> >> + * (special & RCU_READ_UNLOCK_BLOCKED) &&
> >> + * irqs_disabled_flags(flags)
> >> + * is still unlikely to be true.
> >> + */
> >> + if (unlikely(unlock && irqs_disabled_flags(flags))) {
> >> + set_need_resched();
> >> + local_irq_restore(flags);
> >> + return;
> >> + }
> >> +
> >> t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
> >>
> >> /*
> >> --
> >> 1.7.4.4
> >>
> >
> >
>
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