[ltt-dev] [RFC PATCH 4/7] urcu:implement URCU_WAIT_READER

[Lai Jiangshan]

On 08/18/2011 01:06 AM, Paul E. McKenney wrote:
> On Wed, Aug 17, 2011 at 09:43:59AM +0800, Lai Jiangshan wrote:
>> On 08/17/2011 03:20 AM, Paul E. McKenney wrote:
>>> On Tue, Aug 16, 2011 at 03:58:07PM +0800, Lai Jiangshan wrote:
>>>> synchronize_rcu() find out ongoing readers and wait for them
>>>
>>> Cute!
>>>
>>> But some questions and spelling nits below.
>>>
>>> 							Thanx, Paul
>>>
>>>> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
>>>> ---
>>>>  urcu.c             |   49 +++++++++++++++++++++++++++++++++++++++++++++++++
>>>>  urcu/static/urcu.h |   42 ++++++++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 91 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/urcu.c b/urcu.c
>>>> index bd54467..c13be67 100644
>>>> --- a/urcu.c
>>>> +++ b/urcu.c
>>>> @@ -419,6 +419,53 @@ static void urcu_sync_unlock_if_not_proxy_unlocked(int32_t *lock, int32_t self)
>>>>  		urcu_sync_slow_unlock(lock);
>>>>  }
>>>>
>>>> +/* also implies mb() */
>>>> +void __urcu_read_unlock_specail(void)
>>>
>>> s/__urcu_read_unlock_specail/__urcu_read_unlock_special/, please.
>>>
>>>> +{
>>>> +	urcu_sync_unlock_if_not_proxy_unlocked(&rcu_reader.sync,
>>>> +			rcu_reader.tid);
>>>> +}
>>>> +
>>>> +/*
>>>> + * synchronize_rcu		rcu_read_unlock(outmost)
>>>> + *
>>>> + *
>>>> + * sync = tid;			ctr = 0;
>>>> + * smp_mb_master();		smp_mb_slave();
>>>
>>
>> Sorry, I use this comments to explain why synchronize_rcu()
>> will not sleep endless when synchronize_rcu() find ctr != 0,
>>
>>> But smp_mb_slave() expands to 'asm volatile("" : : : "memory")'
>>> for RCU_SIGNAL, which would open up a number of vulnerabilities.
>>>
>>> And yes, smp_mb_master() does a memory barrier on all CPUs in
>>> the RCU_SIGNAL case, but that only guarantees that if there was
>>> a reader pre-dating the beginning of the grace period, that if
>>> that reader accessed any pre-grace-period state, we also see
>>> that reader.  Misordering of the rcu_read_unlock() with the
>>> prior critical section, and this is why there is an smp_mb_master()
>>> prior to exit from synchronize_rcu().
>>>
>>> So for this to work, I believe that you need an smp_mb_master()
>>> between checking the index->ctr and doing the urcu_sync_proxy_unlock().
>>
>> Calling urcu_sync_proxy_unlock() earlier is OK.
>> __urcu_read_unlock_special() do nothing when it found the lock is proxy unlocked.
>>
>> If we find ctr == 0:
>> I did add a smp_mb_master() after we find ctr == 0(also after urcu_sync_proxy_unlock()).
> 
> So it is OK to have urcu_sync_proxy_unlock() execute concurrently
> with urcu_sync_unlock_if_not_proxy_unlocked()?  If so, no problem.

Yes. They are OK even they execute concurrently.
I will add some comments for it.

> 
>>> Or am I missing something that restricts URCU priority boosting
>>> to RCU_MB (where it looks like it might work)?  Or missing some
>>> extra synchronization in there somewhere?
>>
>> initial: sync=0,ctr>0
>>
>> sync = tid;			ctr = 0
>> smp_mb_master();		smp_mb_slave();
>> local_ctr = ctr;		local_sync = sync;
>> 	assert(local_ctr==0 || local_sync == tid);
>>
>> So if we find the ctr != 0(local_ctr!=0):
>> the reader will call __urcu_read_unlock_special(),
>> synchronize_rcu() can be woke up and will not sleep endless.
> 
> Yep, I was worried about synchronizing access to the boost mutex rather
> than the effects on the grace period.
> 
>>>> + * test ctr and wait;		test sync and wakeup;
>>>> + */
>>>> +
>>>> +void synchronize_rcu(void)
>>>> +{
>>>> +	struct rcu_reader *index;
>>>> +	int32_t self = syscall(SYS_gettid);
>>>> +
>>>> +	mutex_lock(&rcu_gp_lock);
>>>> +
>>>> +	if (cds_list_empty(&registry))
>>>> +		goto out;
>>>> +
>>>> +	cds_list_for_each_entry(index, &registry, node)
>>>> +		urcu_sync_proxy_lock(&index->sync, index->tid);
>>>> +
>>>> +	smp_mb_master(RCU_MB_GROUP);
>>>> +
>>>> +	cds_list_for_each_entry(index, &registry, node) {
>>>> +		if (_CMM_LOAD_SHARED(index->ctr) == 0) {
>>>> +			urcu_sync_proxy_unlock(&index->sync);
>>>> +		} else {
>>>> +			/* reader still running, we need to wait reader */
>>>> +			urcu_sync_lock(&index->sync, self);
>>>> +			urcu_sync_unlock(&index->sync, self);
>>>> +		}
>>>> +	}
>>>> +
>>>> +	/* ensure rcu_read_unlock() finish when we found the ctr==0 */
>>>> +	smp_mb_master(RCU_MB_GROUP); /* ensure rcu_read_unlock() finish */
>>
>> This smp_mb_master() is needed after we find ctr == 0.
> 
> Again, my concern was the manipulations of index->sync rather than
> the grace period.

index->sync is a userspace lock, it has such values: (the same as pthread mutexes)
	0			UNLOCK
	tid			LOCKED by tid
	tid | FUTEX_WAITERS	LOCKED by tid, some other thread compete for it
(In this patchset "some other thread" is only the thread calling synchronize_rcu())

index->sync belongs to a reader thread,
the reader thread can do 2 things only:
	read value of this lock.
	unlock it when the reader thread find this lock is locked by itself.

If synchronize_rcu() finds the reader is still running, because smp_mb_master()/smp_mb_slave(),
the reader thread must see "index->sync = tid;", it means synchronize_rcu() thread and
the reader thread both agreed that this lock is held by the reader thread, and it will
be unlock by the reader thread, So synchronize_rcu() calls
urcu_sync_lock() compete for it. This competing will make synchronize_rcu() thread
wait for the reader thread. If index->sync is a pi-lock, the synchronize_rcu() thread
will boost the reader thread. When urcu_sync_lock() returns, the C.S. of the reader
thread are totally done.

Any problem is welcome.
Thank you very much.
Lai

> 
>>>> +
>>>> +out:
>>>> +	mutex_unlock(&rcu_gp_lock);
>>>> +}
>>>>  #endif /* #else #ifndef URCU_WAIT_READER */
>>>>
>>>>  /*
>>>> @@ -437,7 +484,9 @@ void rcu_read_unlock(void)
>>>>
>>>>  void rcu_register_thread(void)
>>>>  {
>>>> +	rcu_reader.tid = syscall(SYS_gettid);
>>>>  	rcu_reader.pthread = pthread_self();
>>>> +	rcu_reader.sync = 0;
>>>>  	assert(rcu_reader.need_mb == 0);
>>>>  	assert(!(rcu_reader.ctr & RCU_GP_CTR_NEST_MASK));
>>>>
>>>> diff --git a/urcu/static/urcu.h b/urcu/static/urcu.h
>>>> index 32d1af8..0941fd2 100644
>>>> --- a/urcu/static/urcu.h
>>>> +++ b/urcu/static/urcu.h
>>>> @@ -221,9 +221,11 @@ static inline void smp_mb_slave(int group)
>>>>  struct rcu_reader {
>>>>  	/* Data used by both reader and synchronize_rcu() */
>>>>  	unsigned long ctr;
>>>> +	int32_t sync;
>>>>  	char need_mb;
>>>>  	/* Data used for registry */
>>>>  	struct cds_list_head node __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
>>>> +	pid_t tid;
>>>>  	pthread_t pthread;
>>>>  };
>>>>
>>>> @@ -313,6 +315,46 @@ static inline int32_t urcu_sync_lock_onwer(int32_t *lock)
>>>>  	return _CMM_LOAD_SHARED(*lock) & ~FUTEX_WAITERS;
>>>>  }
>>>>
>>>> +void __urcu_read_unlock_specail(void);
>>>> +
>>>> +static inline void _rcu_read_lock(void)
>>>> +{
>>>> +	unsigned long tmp;
>>>> +
>>>> +	cmm_barrier();
>>>> +	tmp = rcu_reader.ctr;
>>>> +
>>>> +	if (!tmp) {
>>>> +		_CMM_STORE_SHARED(rcu_reader.ctr, 1);
>>>> +		smp_mb_slave(RCU_MB_GROUP);
>>>> +	} else {
>>>> +		_CMM_STORE_SHARED(rcu_reader.ctr, tmp + 1);
>>>> +		cmm_barrier();
>>>> +	}
>>>> +}
>>>> +
>>>> +static inline void _rcu_read_unlock(void)
>>>> +{
>>>> +	unsigned long tmp;
>>>> +
>>>> +	cmm_barrier();
>>>> +	tmp = rcu_reader.ctr;
>>>> +
>>>> +	if (tmp == 1) {
>>>> +		smp_mb_slave(RCU_MB_GROUP);
>>>> +
>>>> +		rcu_reader.ctr = 0;
>>>> +		smp_mb_slave(RCU_MB_GROUP);
>>>> +
>>>> +		if (unlikely(urcu_sync_lock_onwer(&rcu_reader.sync)
>>>> +				== rcu_reader.tid))
>>>> +			__urcu_read_unlock_specail();
>>>> +	} else {
>>>> +		_CMM_STORE_SHARED(rcu_reader.ctr, tmp - 1);
>>>> +		cmm_barrier();
>>>> +	}
>>>> +}
>>>> +
>>>>  #endif /* #else #ifndef URCU_WAIT_READER */
>>>>
>>>>  #ifdef __cplusplus
>>>> -- 
>>>> 1.7.4.4
>>>>
>>>
>>
>