Messages in this thread | | | Date | Tue, 4 Sep 2012 15:46:59 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 09/15] rcu: Avoid rcu_print_detail_task_stall_rnp() segfault |
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On Fri, Aug 31, 2012 at 11:19:17AM -0700, Josh Triplett wrote: > 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);
You might well be right, but doing that gets me another line longer than 80 characters.
Hey, I have an excuse -- I actually spent a significant fraction of my career using punched cards. ;-)
Thanx, Paul
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