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Currently some callsites of sync_rcu_preempt_exp_done() are not called
with the corresponding rcu_node's ->lock held, which could introduces
bugs as per Paul:
o CPU 0 in sync_rcu_preempt_exp_done() reads ->exp_tasks and
sees that it is NULL.
o CPU 1 blocks within an RCU read-side critical section, so
it enqueues the task and points ->exp_tasks at it and
clears CPU 1's bit in ->expmask.
o All other CPUs clear their bits in ->expmask.
o CPU 0 reads ->expmask, sees that it is zero, so incorrectly
concludes that all quiescent states have completed, despite
the fact that ->exp_tasks is non-NULL.
To fix this, sync_rcu_preempt_exp_unlocked() is introduced to replace
lockless callsites of sync_rcu_preempt_exp_done().
Further, a lockdep annotation is added into sync_rcu_preempt_exp_done()
to prevent mis-use in the future.
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
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