Re: [Qemu-devel] [PATCH for-2.5] rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu()

[Paolo Bonzini]

On 27/07/2015 04:24, Wen Congyang wrote:
> +        /* Wait for one thread to report a quiescent state and try again.
> +         * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
> +         * wait too much time. Note: rcu_unregister_thread() may remove
> +         * the node from qsreaders. That's a bit tricky, but it should work.

"It should work" is a bit optimistic. :D

Does this description look okay?

        /* Wait for one thread to report a quiescent state and try again.
         * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
         * wait too much time.
         *
         * rcu_register_thread() may add nodes to &registry; it will not
         * wake up synchronize_rcu, but that is okay because at least another
         * thread must exit its RCU read-side critical section before
         * synchronize_rcu is done.  The next iteration of the loop will
         * process the new thread or set ->waiting for it.  Hence, this can
         * at worst cause synchronize_rcu() to wait for longer.
         *
         * rcu_unregister_thread() may remove nodes from &qsreaders instead
         * of &registry if it runs during qemu_event_wait.  That's okay;
         * the node then will not be added back to &registry by QLIST_SWAP
         * below.  The invariant is that the node is part of one list when
         * rcu_registry_lock is released.
         */

Paolo