Re: [Qemu-devel] [PATCH v2 for-2.5] rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu()

[Paolo Bonzini]

On 28/07/2015 04:26, Wen Congyang wrote:
> If rcu_(un)register_thread() is called together with synchronize_rcu(),
> it will wait for the synchronize_rcu() to finish. But when synchronize_rcu()
> waits for some events, we can modify the list registry.
> We also use the lock rcu_gp_lock to assume that synchronize_rcu() isn't
> executed in more than one thread at the same time. Add a new mutex lock
> rcu_sync_lock to assume it and rename rcu_gp_lock to rcu_registry_lock.
> Release rcu_registry_lock when synchronize_rcu() waits for some events.
> 
> Signed-off-by: Wen Congyang <address@hidden>
> ---
>  util/rcu.c | 48 +++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 35 insertions(+), 13 deletions(-)
> 
> diff --git a/util/rcu.c b/util/rcu.c
> index cdcad67..4dd33df 100644
> --- a/util/rcu.c
> +++ b/util/rcu.c
> @@ -47,7 +47,8 @@
>  unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
>  
>  QemuEvent rcu_gp_event;
> -static QemuMutex rcu_gp_lock;
> +static QemuMutex rcu_registry_lock;
> +static QemuMutex rcu_sync_lock;
>  
>  /*
>   * Check whether a quiescent state was crossed between the beginning of
> @@ -66,7 +67,7 @@ static inline int rcu_gp_ongoing(unsigned long *ctr)
>   */
>  __thread struct rcu_reader_data rcu_reader;
>  
> -/* Protected by rcu_gp_lock.  */
> +/* Protected by rcu_registry_lock.  */
>  typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
>  static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);
>  
> @@ -114,10 +115,26 @@ static void wait_for_readers(void)
>              break;
>          }
>  
> -        /* Wait for one thread to report a quiescent state and
> -         * try again.
> +        /* 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
> +         * move the new thread's rcu_reader from &registry to &qsreaders,
> +         * because rcu_gp_ongoing() will return false.
> +         *
> +         * 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.
>           */
> +        qemu_mutex_unlock(&rcu_registry_lock);
>          qemu_event_wait(&rcu_gp_event);
> +        qemu_mutex_lock(&rcu_registry_lock);
>      }
>  
>      /* put back the reader list in the registry */
> @@ -126,7 +143,8 @@ static void wait_for_readers(void)
>  
>  void synchronize_rcu(void)
>  {
> -    qemu_mutex_lock(&rcu_gp_lock);
> +    qemu_mutex_lock(&rcu_sync_lock);
> +    qemu_mutex_lock(&rcu_registry_lock);
>  
>      if (!QLIST_EMPTY(&registry)) {
>          /* In either case, the atomic_mb_set below blocks stores that free
> @@ -149,7 +167,8 @@ void synchronize_rcu(void)
>          wait_for_readers();
>      }
>  
> -    qemu_mutex_unlock(&rcu_gp_lock);
> +    qemu_mutex_unlock(&rcu_registry_lock);
> +    qemu_mutex_unlock(&rcu_sync_lock);
>  }
>  
>  
> @@ -273,23 +292,24 @@ void call_rcu1(struct rcu_head *node, void 
> (*func)(struct rcu_head *node))
>  void rcu_register_thread(void)
>  {
>      assert(rcu_reader.ctr == 0);
> -    qemu_mutex_lock(&rcu_gp_lock);
> +    qemu_mutex_lock(&rcu_registry_lock);
>      QLIST_INSERT_HEAD(&registry, &rcu_reader, node);
> -    qemu_mutex_unlock(&rcu_gp_lock);
> +    qemu_mutex_unlock(&rcu_registry_lock);
>  }
>  
>  void rcu_unregister_thread(void)
>  {
> -    qemu_mutex_lock(&rcu_gp_lock);
> +    qemu_mutex_lock(&rcu_registry_lock);
>      QLIST_REMOVE(&rcu_reader, node);
> -    qemu_mutex_unlock(&rcu_gp_lock);
> +    qemu_mutex_unlock(&rcu_registry_lock);
>  }
>  
>  static void rcu_init_complete(void)
>  {
>      QemuThread thread;
>  
> -    qemu_mutex_init(&rcu_gp_lock);
> +    qemu_mutex_init(&rcu_registry_lock);
> +    qemu_mutex_init(&rcu_sync_lock);
>      qemu_event_init(&rcu_gp_event, true);
>  
>      qemu_event_init(&rcu_call_ready_event, false);
> @@ -306,12 +326,14 @@ static void rcu_init_complete(void)
>  #ifdef CONFIG_POSIX
>  static void rcu_init_lock(void)
>  {
> -    qemu_mutex_lock(&rcu_gp_lock);
> +    qemu_mutex_lock(&rcu_sync_lock);
> +    qemu_mutex_lock(&rcu_registry_lock);
>  }
>  
>  static void rcu_init_unlock(void)
>  {
> -    qemu_mutex_unlock(&rcu_gp_lock);
> +    qemu_mutex_unlock(&rcu_registry_lock);
> +    qemu_mutex_unlock(&rcu_sync_lock);
>  }
>  #endif
>  
> 

Ok, thanks.

Paolo