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SubjectRe: [PATCH v2] rcu: exp: Protect all sync_rcu_preempt_exp_done() with rcu_node lock
On Thu, Mar 08, 2018 at 04:48:27PM +0800, Boqun Feng wrote:
> 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>

Again, good catch, applied for testing and review. Thank you!

Thanx, Paul

> ---
> v1 --> v2:
> Kill unnecessary blank lines
>
>
> kernel/rcu/tree_exp.h | 28 +++++++++++++++++++++++++---
> 1 file changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 2fd882b08b7c..3f30cc3b7669 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -20,6 +20,8 @@
> * Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> */
>
> +#include <linux/lockdep.h>
> +
> /*
> * Record the start of an expedited grace period.
> */
> @@ -158,10 +160,30 @@ static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp)
> */
> static bool sync_rcu_preempt_exp_done(struct rcu_node *rnp)
> {
> + lockdep_assert_held(&rnp->lock);
> +
> return rnp->exp_tasks == NULL &&
> READ_ONCE(rnp->expmask) == 0;
> }
>
> +/*
> + * Like sync_rcu_preempt_exp_done(), but this function assumes the caller
> + * doesn't hold the rcu_node's ->lock, and will acquire and release the lock
> + * itself
> + */
> +static bool sync_rcu_preempt_exp_done_unlocked(struct rcu_node *rnp)
> +{
> + unsigned long flags;
> + bool ret;
> +
> + raw_spin_lock_irqsave_rcu_node(rnp, flags);
> + ret = sync_rcu_preempt_exp_done(rnp);
> + raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> +
> + return ret;
> +}
> +
> +
> /*
> * Report the exit from RCU read-side critical section for the last task
> * that queued itself during or before the current expedited preemptible-RCU
> @@ -498,9 +520,9 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
> for (;;) {
> ret = swait_event_timeout(
> rsp->expedited_wq,
> - sync_rcu_preempt_exp_done(rnp_root),
> + sync_rcu_preempt_exp_done_unlocked(rnp_root),
> jiffies_stall);
> - if (ret > 0 || sync_rcu_preempt_exp_done(rnp_root))
> + if (ret > 0 || sync_rcu_preempt_exp_done_unlocked(rnp_root))
> return;
> WARN_ON(ret < 0); /* workqueues should not be signaled. */
> if (rcu_cpu_stall_suppress)
> @@ -533,7 +555,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
> rcu_for_each_node_breadth_first(rsp, rnp) {
> if (rnp == rnp_root)
> continue; /* printed unconditionally */
> - if (sync_rcu_preempt_exp_done(rnp))
> + if (sync_rcu_preempt_exp_done_unlocked(rnp))
> continue;
> pr_cont(" l=%u:%d-%d:%#lx/%c",
> rnp->level, rnp->grplo, rnp->grphi,
> --
> 2.16.2
>

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