linux-kernel - Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock

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Message-ID: <ce8d6522-355b-a093-fa6a-f527f01186dc@colorfullife.com>
Date:   Sun, 28 Aug 2016 20:00:43 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     benh@...nel.crashing.org, Ingo Molnar <mingo@...e.hu>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to
 barrier.h

On 08/28/2016 03:43 PM, Paul E. McKenney wrote:
>
>> Without the smp_mb__after_unlock_lock(), other CPUs can observe the
>> write to d without seeing the write to a.
>>
>> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
> With the upgraded commit log, I am OK with the patch below.
Done.
> However, others will probably want to see at least one use of
> smp_mb__after_unlock_lock() outside of RCU.
I would look at it from the other side:
There are at least half a dozen hardware/spinlock implementations that 
must support rcu.
And for half a dozen implementations, a global header file makes sense, 
regardless of the number of users.

With this in the global header file
 > #ifndef complex_memory_barrier()
 > #define complex_memory_barrier()    always_safe_fallback()
 > #endif

it is easier for the architectures to support rcu, ipc/sem and nf_conntrack.
Especially if everything is (as it is now) in <linux/spinlock.h>

--
     Manfred