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Date: Sun, 1 Nov 2015 18:26:02 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Boqun Feng <boqun.feng@...il.com> Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Michael Ellerman <mpe@...erman.id.au>, Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will.deacon@....com>, stable@...r.kernel.org Subject: Re: [PATCH powerpc/next 1/2] powerpc: Make value-returning atomics fully ordered On Mon, Nov 02, 2015 at 09:30:31AM +0800, Boqun Feng wrote: > According to memory-barriers.txt: > > > Any atomic operation that modifies some state in memory and returns > > information about the state (old or new) implies an SMP-conditional > > general memory barrier (smp_mb()) on each side of the actual > > operation ... > > Which mean these operations should be fully ordered. However on PPC, > PPC_ATOMIC_ENTRY_BARRIER is the barrier before the actual operation, > which is currently "lwsync" if SMP=y. The leading "lwsync" can not > guarantee fully ordered atomics, according to Paul Mckenney: > > https://lkml.org/lkml/2015/10/14/970 > > To fix this, we define PPC_ATOMIC_ENTRY_BARRIER as "sync" to guarantee > the fully-ordered semantics. > > This also makes futex atomics fully ordered, which can avoid possible > memory ordering problems if userspace code relies on futex system call > for fully ordered semantics. > > Cc: <stable@...r.kernel.org> # 3.4+ > Signed-off-by: Boqun Feng <boqun.feng@...il.com> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> > --- > These two are separated and splited from the patchset of powerpc atomic > variants implementation, whose link is: > > https://lkml.org/lkml/2015/10/26/141 > > Based on next branch of powerpc tree, tested by 0day. > > arch/powerpc/include/asm/synch.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/powerpc/include/asm/synch.h b/arch/powerpc/include/asm/synch.h > index e682a71..c508686 100644 > --- a/arch/powerpc/include/asm/synch.h > +++ b/arch/powerpc/include/asm/synch.h > @@ -44,7 +44,7 @@ static inline void isync(void) > MAKE_LWSYNC_SECTION_ENTRY(97, __lwsync_fixup); > #define PPC_ACQUIRE_BARRIER "\n" stringify_in_c(__PPC_ACQUIRE_BARRIER) > #define PPC_RELEASE_BARRIER stringify_in_c(LWSYNC) "\n" > -#define PPC_ATOMIC_ENTRY_BARRIER "\n" stringify_in_c(LWSYNC) "\n" > +#define PPC_ATOMIC_ENTRY_BARRIER "\n" stringify_in_c(sync) "\n" > #define PPC_ATOMIC_EXIT_BARRIER "\n" stringify_in_c(sync) "\n" > #else > #define PPC_ACQUIRE_BARRIER > -- > 2.6.2 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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