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

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Date:   Wed, 31 Aug 2016 06:59:07 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     benh@...nel.crashing.org, paulmck@...ux.vnet.ibm.com,
        Ingo Molnar <mingo@...e.hu>, Boqun Feng <boqun.feng@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 1/4] spinlock: Document memory barrier rules

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