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Re: [PATCH 1/4] spinlock: Document memory barrier rules

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On 08/29/2016 03:44 PM, Peter Zijlstra wrote:
>
> If you add a barrier, the Changelog had better be clear. And I'm still
> not entirely sure I get what exactly this barrier should do, nor why it
> defaults to a full smp_mb. If what I suspect it should do, only PPC and
> ARM64 need the barrier.
The barrier must ensure that taking the spinlock (as observed by another 
cpu with spin_unlock_wait()) and a following read are ordered.

start condition: sma->complex_mode = false;

CPU 1:
     spin_lock(&sem->lock); /* sem_nsems instances */
     smp_mb__after_spin_lock();
     if (!smp_load_acquire(&sma->complex_mode)) {
         /* fast path successful! */
         return sops->sem_num;
     }
      /* slow path, not relevant */

CPU 2: (holding sma->sem_perm.lock)

         smp_store_mb(sma->complex_mode, true);

         for (i = 0; i < sma->sem_nsems; i++) {
                 spin_unlock_wait(&sma->sem_base[i].lock);
         }

It must not happen that both CPUs proceed:
Either CPU1 proceeds, then CPU2 must spin in spin_unlock_wait()
or CPU2 proceeds, then CPU1 must enter the slow path.

What about this?
/*
  * spin_lock() provides ACQUIRE semantics regarding reading the lock.
  * There are no guarantees that the store of the lock is visible before
  * any read or write operation within the protected area is performed.
  * If the store of the lock must happen first, this function is required.
  */
#define spin_lock_store_acquire()

I would update the patch series.

--
     Manfred

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[PATCH 0/4] Clarify/standardize memory barriers for lock/unlock Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 14:00 +0200
  [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 14:00 +0200
    [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 14:00 +0200
      Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to  barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-28 15:50 +0200
        [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 18:40 +0200
        Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to  barrier.h Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 20:10 +0200
    Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-08-29 12:50 +0200
      Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-29 15:00 +0200
        Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-08-29 15:50 +0200
          Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-31 07:00 +0200
            Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-08-31 17:50 +0200
              Re: [PATCH 1/4] spinlock: Document memory barrier rules Will Deacon <will.deacon@arm.com> - 2016-08-31 18:50 +0200
                Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-31 20:40 +0200
                Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-09-01 10:50 +0200
                Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-09-01 13:10 +0200
                Re: [PATCH 1/4] spinlock: Document memory barrier rules Will Deacon <will.deacon@arm.com> - 2016-09-01 13:20 +0200
                Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-09-01 14:00 +0200
                Re: [PATCH 1/4] spinlock: Document memory barrier rules Boqun Feng <boqun.feng@gmail.com> - 2016-09-01 16:10 +0200
  Re: [PATCH 0/4] Clarify/standardize memory barriers for lock/unlock Peter Zijlstra <peterz@infradead.org> - 2016-08-29 13:00 +0200

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