ACK/cmnt: [SRU][Artful][v2][PATCH 1/1] UBUNTU: SAUCE: (no-up) s390: fix rwlock implementation

[Stefan Bader]

On 11.05.2018 15:31, 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: Heiko Carstens <heiko.carstens at de.ibm.com>
> Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>

> ---

Just to note that this sending v2 (or any other increment) as replies is causing
very hard to follow threads (at least in thunderbird). It would be much simpler
to nack the old submission and start a new thread...

-Stefan

>  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;
> 


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