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Paul E. McKenney authored
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>
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