On 02/09/2013 04:17 AM, Paul E. McKenney wrote:
> On Tue, Jan 29, 2013 at 08:12:37PM +0900, Namhyung Kim wrote:
>> On Thu, 24 Jan 2013 10:00:04 +0530, Srivatsa S. Bhat wrote:
>>> On 01/24/2013 01:27 AM, Tejun Heo wrote:
>>>> On Thu, Jan 24, 2013 at 01:03:52AM +0530, Srivatsa S. Bhat wrote:
>>>>> CPU 0 CPU 1
>>>>>
>>>>> read_lock(&rwlock)
>>>>>
>>>>> write_lock(&rwlock) //spins, because CPU 0
>>>>> //has acquired the lock for read
>>>>>
>>>>> read_lock(&rwlock)
>>>>> ^^^^^
>>>>> What happens here? Does CPU 0 start spinning (and hence deadlock) or will
>>>>> it continue realizing that it already holds the rwlock for read?
>>>>
>>>> I don't think rwlock allows nesting write lock inside read lock.
>>>> read_lock(); write_lock() will always deadlock.
>>>>
>>>
>>> Sure, I understand that :-) My question was, what happens when *two* CPUs
>>> are involved, as in, the read_lock() is invoked only on CPU 0 whereas the
>>> write_lock() is invoked on CPU 1.
>>>
>>> For example, the same scenario shown above, but with slightly different
>>> timing, will NOT result in a deadlock:
>>>
>>> Scenario 2:
>>> CPU 0 CPU 1
>>>
>>> read_lock(&rwlock)
>>>
>>>
>>> read_lock(&rwlock) //doesn't spin
>>>
>>> write_lock(&rwlock) //spins, because CPU 0
>>> //has acquired the lock for read
>>>
>>>
>>> So I was wondering whether the "fairness" logic of rwlocks would cause
>>> the second read_lock() to spin (in the first scenario shown above) because
>>> a writer is already waiting (and hence new readers should spin) and thus
>>> cause a deadlock.
>>
>> In my understanding, current x86 rwlock does basically this (of course,
>> in an atomic fashion):
>>
>>
>> #define RW_LOCK_BIAS 0x10000
>>
>> rwlock_init(rwlock)
>> {
>> rwlock->lock = RW_LOCK_BIAS;
>> }
>>
>> arch_read_lock(rwlock)
>> {
>> retry:
>> if (--rwlock->lock >= 0)
>> return;
>>
>> rwlock->lock++;
>> while (rwlock->lock < 1)
>> continue;
>>
>> goto retry;
>> }
>>
>> arch_write_lock(rwlock)
>> {
>> retry:
>> if ((rwlock->lock -= RW_LOCK_BIAS) == 0)
>> return;
>>
>> rwlock->lock += RW_LOCK_BIAS;
>> while (rwlock->lock != RW_LOCK_BIAS)
>> continue;
>>
>> goto retry;
>> }
>>
>>
>> So I can't find where the 'fairness' logic comes from..
>
> I looked through several of the rwlock implementations, and in all of
> them the writer backs off if it sees readers -- or refrains from asserting
> ownership of the lock to begin with.
>
> So it should be OK to use rwlock as shown in the underlying patch.
>
Thanks a lot for confirming that Paul! So I guess we can use rwlocks
as it is, since its behaviour suits our needs perfectly. So I won't tinker
with atomic counters for a while, atleast not until someone starts making
rwlocks fair.. ;-)
Regards,
Srivatsa S. Bhat
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