Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering

On Mon, Jan 23, 2017 at 07:12:03PM +1100, Michael Ellerman wrote:

"Paul E. McKenney" [off-list ref] writes:

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On Sat, Jan 14, 2017 at 11:54:17AM -0800, Paul E. McKenney wrote:

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On Sat, Jan 14, 2017 at 10:35:50AM +0100, Ingo Molnar wrote:

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* Paul E. McKenney [off-list ref] wrote:

[ . . . ]

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+ */
+#ifdef CONFIG_PPC
+#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
+#else /* #ifdef CONFIG_PPC */
+#define smp_mb__after_unlock_lock()	do { } while (0)
+#endif /* #else #ifdef CONFIG_PPC */

Yeah, so I realize that this was pre-existing code, but putting CONFIG_$ARCH
#ifdefs into generic headers is generally frowned upon.

The canonical approach would be either to define a helper Kconfig variable that 
can be set by PPC (but other architectures don't need to set it), or to expose a 
suitable macro (function) for architectures to define in their barrier.h arch 
header file.

Very well, I will add a separate commit for this.  4.11 OK?

Does the patch below seem reasonable?

							Thanx, Paul

------------------------------------------------------------------------

commit 271c0601237c41a279f975563e13837bace0df03
Author: Paul E. McKenney [off-list ref]
Date:   Sat Jan 14 13:32:50 2017 -0800

    rcu: Make arch select smp_mb__after_unlock_lock() strength
    
    The definition of smp_mb__after_unlock_lock() is currently smp_mb()
    for CONFIG_PPC and a no-op otherwise.  It would be better to instead
    provide an architecture-selectable Kconfig option, and select the
    strength of smp_mb__after_unlock_lock() based on that option.  This
    commit therefore creates CONFIG_ARCH_WEAK_RELACQ, has PPC select it,
    and bases the definition of smp_mb__after_unlock_lock() on this new
    CONFIG_ARCH_WEAK_RELACQ Kconfig option.
    
    Reported-by: Ingo Molnar [off-list ref]
    Signed-off-by: Paul E. McKenney [off-list ref]
    Cc: Peter Zijlstra [off-list ref]
    Cc: Will Deacon [off-list ref]
    Cc: Boqun Feng [off-list ref]
    Cc: Benjamin Herrenschmidt [off-list ref]
    Cc: Paul Mackerras [off-list ref]
    Cc: Michael Ellerman [off-list ref]
    Cc: [off-list ref]

Personally I'd call it ARCH_WEAK_RELEASE_ACQUIRE, which is longer but
clearer I think. But it's not a big deal, so which ever you prefer.

ARCH_WEAK_RELEASE_ACQUIRE it is!

Acked-by: Michael Ellerman <mpe@ellerman.id.au>

Applied, thank you!

							Thanx, Paul