Messages in this thread Patch in this message | | | Date | Tue, 19 Jul 2011 21:54:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/urgent 1/7] rcu: decrease rcu_report_exp_rnp coupling with scheduler |
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On Wed, Jul 20, 2011 at 04:40:18AM +0200, Peter Zijlstra wrote: > On Tue, 2011-07-19 at 17:18 -0700, Paul E. McKenney wrote: > > +++ b/kernel/rcutree_plugin.h > > @@ -696,8 +696,10 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp) > > raw_spin_lock_irqsave(&rnp->lock, flags); > > for (;;) { > > if (!sync_rcu_preempt_exp_done(rnp)) > > + raw_spin_unlock_irqrestore(&rnp->lock, flags); > > break; > > I bet that'll all work much better if you wrap it in curly braces like: > > if (!sync_rcu_preempt_exp_done(rnp)) { > raw_spin_unlock_irqrestore(&rnp->lock, flags); > break; > } > > That might also explain those explosions Ed and Ben have been seeing.
Indeed. Must be the call of the snake. :-(
Thank you for catching this!
> > if (rnp->parent == NULL) { > > + raw_spin_unlock_irqrestore(&rnp->lock, flags); > > wake_up(&sync_rcu_preempt_exp_wq); > > break; > > } > > @@ -707,7 +709,6 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp) > > raw_spin_lock(&rnp->lock); /* irqs already disabled */ > > rnp->expmask &= ~mask; > > } > > - raw_spin_unlock_irqrestore(&rnp->lock, flags); > > }
So this time I am testing the exact patch series before resending. In the meantime, here is the updated version of this patch.
Thanx, Paul
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rcu: decrease rcu_report_exp_rnp coupling with scheduler
PREEMPT_RCU read-side critical sections blocking an expedited grace period invoke rcu_report_exp_rnp(). When the last such critical section has completed, rcu_report_exp_rnp() invokes the scheduler to wake up the task that invoked synchronize_rcu_expedited() -- needlessly holding the root rcu_node structure's lock while doing so, thus needlessly providing a way for RCU and the scheduler to deadlock.
This commit therefore releases the root rcu_node structure's lock before calling wake_up().
Reported-by: Ed Tomlinson <edt@aei.ca> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index 75113cb..6abef3c 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -695,9 +695,12 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp) raw_spin_lock_irqsave(&rnp->lock, flags); for (;;) { - if (!sync_rcu_preempt_exp_done(rnp)) + if (!sync_rcu_preempt_exp_done(rnp)) { + raw_spin_unlock_irqrestore(&rnp->lock, flags); break; + } if (rnp->parent == NULL) { + raw_spin_unlock_irqrestore(&rnp->lock, flags); wake_up(&sync_rcu_preempt_exp_wq); break; } @@ -707,7 +710,6 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp) raw_spin_lock(&rnp->lock); /* irqs already disabled */ rnp->expmask &= ~mask; } - raw_spin_unlock_irqrestore(&rnp->lock, flags); } /*
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