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

[Wen Congyang]

On 07/28/2015 05:59 PM, Paolo Bonzini wrote:
> 
> 
> 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.

I have a question about rcu: while do we call wait_for_readers() twice for 
32-bit host?

Thanks
Wen Congyang

> 
> Paolo
> .
>