Messages in this thread | | | Date | Fri, 31 Aug 2012 11:19:17 -0700 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 09/15] rcu: Avoid rcu_print_detail_task_stall_rnp() segfault |
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On Thu, Aug 30, 2012 at 11:56:22AM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > The rcu_print_detail_task_stall_rnp() function invokes > rcu_preempt_blocked_readers_cgp() to verify that there are some preempted > RCU readers blocking the current grace period outside of the protection > of the rcu_node structure's ->lock. This means that the last blocked > reader might exit its RCU read-side critical section and remove itself > from the ->blkd_tasks list before the ->lock is acquired, resulting in > a segmentation fault when the subsequent code attempts to dereference > the now-NULL gp_tasks pointer. > > This commit therefore moves the test under the lock. This will not > have measurable effect on lock contention because this code is invoked > only when printing RCU CPU stall warnings, in other words, in the common > case, never. > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > --- > kernel/rcutree_plugin.h | 6 ++++-- > 1 files changed, 4 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > index 139a803..c02dc1d 100644 > --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -422,9 +422,11 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp) > unsigned long flags; > struct task_struct *t; > > - if (!rcu_preempt_blocked_readers_cgp(rnp)) > - return; > raw_spin_lock_irqsave(&rnp->lock, flags); > + if (!rcu_preempt_blocked_readers_cgp(rnp)) { > + raw_spin_unlock_irqrestore(&rnp->lock, flags); > + return; > + } > t = list_entry(rnp->gp_tasks, > struct task_struct, rcu_node_entry); > list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
Given the small number of lines of code inside the critical section here, I think this would look clearer without the early return and duplicate lock release:
raw_spin_lock_irqsave(&rnp->lock, flags); if (rcu_preempt_blocked_readers_cgp(rnp)) { ... } raw_spin_unlock_irqrestore(&rnp->lock, flags);
- Josh Triplett
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