spin_lock implicit/explicit memory barrier

From	Manfred Spraul <manfred@colorfullife.com>
Newsgroups	linux.kernel
Subject	spin_lock implicit/explicit memory barrier
Date	2016-08-09 21:00 +0200
Message-ID	<s4kpP-2Ta-7@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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Hi Benjamin, Hi Michael,

regarding commit 51d7d5205d33 ("powerpc: Add smp_mb() to 
arch_spin_is_locked()"):

For the ipc/sem code, I would like to replace the spin_is_locked() with 
a smp_load_acquire(), see:

http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n367

http://www.ozlabs.org/~akpm/mmots/broken-out/ipc-semc-fix-complex_count-vs-simple-op-race.patch

To my understanding, I must now add a smp_mb(), otherwise it would be 
broken on PowerPC:

The approach that the memory barrier is added into spin_is_locked() 
doesn't work because the code doesn't use spin_is_locked().

Correct?

--

     Manfred

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spin_lock implicit/explicit memory barrier Manfred Spraul <manfred@colorfullife.com> - 2016-08-09 21:00 +0200
  Re: spin_lock implicit/explicit memory barrier Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2016-08-10 02:10 +0200
    Re: spin_lock implicit/explicit memory barrier Manfred Spraul <manfred@colorfullife.com> - 2016-08-10 20:30 +0200
      Re: spin_lock implicit/explicit memory barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-08-10 21:30 +0200
        Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-10 23:10 +0200
      Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-10 23:00 +0200
    Re: spin_lock implicit/explicit memory barrier "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-10 22:00 +0200

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