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    Subject[PATCH v2 tip/core/rcu 04/14] rcu: Remove lock-acquisition loop from rcu_read_unlock_special()
    Date
    Several releases have come and gone without the warning triggering,
    so remove the lock-acquisition loop. Retain the WARN_ON_ONCE()
    out of sheer paranoia.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    kernel/rcu/tree_plugin.h | 18 ++++++------------
    1 file changed, 6 insertions(+), 12 deletions(-)

    diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
    index 57ba873d2f18..ae4ce2b665f8 100644
    --- a/kernel/rcu/tree_plugin.h
    +++ b/kernel/rcu/tree_plugin.h
    @@ -449,19 +449,13 @@ void rcu_read_unlock_special(struct task_struct *t)

    /*
    * Remove this task from the list it blocked on. The task
    - * now remains queued on the rcu_node corresponding to
    - * the CPU it first blocked on, so the first attempt to
    - * acquire the task's rcu_node's ->lock will succeed.
    - * Keep the loop and add a WARN_ON() out of sheer paranoia.
    + * now remains queued on the rcu_node corresponding to the
    + * CPU it first blocked on, so there is no longer any need
    + * to loop. Retain a WARN_ON_ONCE() out of sheer paranoia.
    */
    - for (;;) {
    - rnp = t->rcu_blocked_node;
    - raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
    - if (rnp == t->rcu_blocked_node)
    - break;
    - WARN_ON_ONCE(1);
    - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
    - }
    + rnp = t->rcu_blocked_node;
    + raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
    + WARN_ON_ONCE(rnp != t->rcu_blocked_node);
    empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
    empty_exp = sync_rcu_preempt_exp_done(rnp);
    smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
    --
    2.5.2


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