[SRU][Artful][PATCH 1/1] UBUNTU: SAUCE: (no-up) s390: fix rwlock implementation

[Kleber Souza]

On 05/11/18 14:04, Joseph Salisbury wrote:
> On 05/11/2018 06:54 AM, Kleber Souza wrote:
>> On 05/07/18 23:59, Joseph Salisbury wrote:
>>> From: Heiko Carstens <heiko.carstens at de.ibm.com>
>>>
>>> BugLink: http://bugs.launchpad.net/bugs/1761674
>>>
>>> Description:  kernel: fix rwlock implementation
>>> Symptom:      Kernel hangs, due to deadlock on an rwlock.
>>> Problem:      With upstream commit 94232a4332de ("s390/rwlock: improve writer
>>>               fairness") rwlock writer fairness was supposed to be
>>>               implemented. If a writer tries to take an rwlock it sets
>>>               unconditionally the writer bit within the lock word and waits
>>>               until all readers have released the lock. This however can lead
>>>               to a deadlock since rwlocks can be taken recursively by readers.
>>>               If e.g. CPU 0 holds the lock as a reader, and CPU 1 wants to
>>>               write-lock the lock, then CPU 1 sets the writer bit and
>>>               afterwards busy waits for CPU 0 to release the lock. If now CPU 0
>>>               tries to read-lock the lock again (recursively) it will also busy
>>>               wait until CPU 1 removes the writer bit, which will never happen,
>>>               since it waits for the first reader on CPU 0 to release the lock.
>>> Solution:     Revert the rwlock writer fairness semantics again.
>>>
>>> Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
>> Shouldn't the patch have a sign-off from the original author as well?th 
> Whoops, yes it should.  Want me to NAK this one and send a v2 with the SOB?

Hi Joe,

Could you please send a v2?

Thanks,
Kleber

>>
>>
>> Thanks,
>> Kleber
>>
>>> ---
>>>  arch/s390/lib/spinlock.c | 46 +++++++++-------------------------------------
>>>  1 file changed, 9 insertions(+), 37 deletions(-)
>>>
>>> diff --git a/arch/s390/lib/spinlock.c b/arch/s390/lib/spinlock.c
>>> index ffb15bd..b3d5e7a 100644
>>> --- a/arch/s390/lib/spinlock.c
>>> +++ b/arch/s390/lib/spinlock.c
>>> @@ -174,40 +174,18 @@ int _raw_read_trylock_retry(arch_rwlock_t *rw)
>>>  EXPORT_SYMBOL(_raw_read_trylock_retry);
>>>  
>>>  #ifdef CONFIG_HAVE_MARCH_Z196_FEATURES
>>> -
>>>  void _raw_write_lock_wait(arch_rwlock_t *rw, int prev)
>>> -{
>>> -	int count = spin_retry;
>>> -	int owner, old;
>>> -
>>> -	owner = 0;
>>> -	while (1) {
>>> -		if (count-- <= 0) {
>>> -			if (owner && arch_vcpu_is_preempted(~owner))
>>> -				smp_yield_cpu(~owner);
>>> -			count = spin_retry;
>>> -		}
>>> -		old = ACCESS_ONCE(rw->lock);
>>> -		owner = ACCESS_ONCE(rw->owner);
>>> -		smp_mb();
>>> -		if (old >= 0) {
>>> -			prev = __RAW_LOCK(&rw->lock, 0x80000000, __RAW_OP_OR);
>>> -			old = prev;
>>> -		}
>>> -		if ((old & 0x7fffffff) == 0 && prev >= 0)
>>> -			break;
>>> -	}
>>> -}
>>> -EXPORT_SYMBOL(_raw_write_lock_wait);
>>> -
>>> -#else /* CONFIG_HAVE_MARCH_Z196_FEATURES */
>>> -
>>> +#else
>>>  void _raw_write_lock_wait(arch_rwlock_t *rw)
>>> +#endif
>>>  {
>>>  	int count = spin_retry;
>>> -	int owner, old, prev;
>>> +	int owner, old;
>>>  
>>> -	prev = 0x80000000;
>>> +#ifdef CONFIG_HAVE_MARCH_Z196_FEATURES
>>> +	if ((int) prev > 0)
>>> +		__RAW_UNLOCK(&rw->lock, 0x7fffffff, __RAW_OP_AND);
>>> +#endif
>>>  	owner = 0;
>>>  	while (1) {
>>>  		if (count-- <= 0) {
>>> @@ -217,19 +195,13 @@ void _raw_write_lock_wait(arch_rwlock_t *rw)
>>>  		}
>>>  		old = ACCESS_ONCE(rw->lock);
>>>  		owner = ACCESS_ONCE(rw->owner);
>>> -		if (old >= 0 &&
>>> -		    __atomic_cmpxchg_bool(&rw->lock, old, old | 0x80000000))
>>> -			prev = old;
>>> -		else
>>> -			smp_mb();
>>> -		if ((old & 0x7fffffff) == 0 && prev >= 0)
>>> +		if (old == 0 && __atomic_cmpxchg_bool(&rw->lock, old, old | 0x80000000))
>>>  			break;
>>> +		smp_mb();
>>>  	}
>>>  }
>>>  EXPORT_SYMBOL(_raw_write_lock_wait);
>>>  
>>> -#endif /* CONFIG_HAVE_MARCH_Z196_FEATURES */
>>> -
>>>  int _raw_write_trylock_retry(arch_rwlock_t *rw)
>>>  {
>>>  	int count = spin_retry;
>>>
>