On Thu, Aug 30, 2012 at 11:56:22AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul...@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 <paul...@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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