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Date: Fri, 17 Jul 2015 10:32:21 +0100
From: Will Deacon <will.deacon@....com>
To: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: "paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
Michael Ellerman <mpe@...erman.id.au>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
smp_mb__after_unlock_lock()
On Thu, Jul 16, 2015 at 11:54:25PM +0100, Benjamin Herrenschmidt wrote:
> On Thu, 2015-07-16 at 08:11 -0700, Paul E. McKenney wrote:
> > > So isync in lock in architecturally incorrect, despite being what
> > the
> > > architecture recommends using, yay !
> >
> > Well, the architecture isn't expecting that crazies like myself would
> > want to have an unlock-lock provide ordering to some CPU not holding
> > the lock. :-/
>
> Yes, isync in lock effectively allows any load or store before the lock
> to leak into the lock and get re-ordered with things in there.
>
> lwsync leaves us exposed to the re-order inside the LL/SC of a
> subsequent load.
>
> So to get the full barrier semantic, the only option is a full sync,
> either in lock or unlock. Instinctively I prefer in lock but there's
> an argument to have it in unlock so we can get rid of the iosync
> business.
Yeah, removing the iosync logic kills mmiowb() as well (totally untested
diff below and I was too scared to touch that paca thing!). It also sticks
the full barrier in one place, since there's no try_unlock to worry about.
Will
--->8
diff --git a/arch/powerpc/include/asm/io.h b/arch/powerpc/include/asm/io.h
index a8d2ef30d473..bb34f3bb66dc 100644
--- a/arch/powerpc/include/asm/io.h
+++ b/arch/powerpc/include/asm/io.h
@@ -105,12 +105,6 @@ extern bool isa_io_special;
*
*/
-#ifdef CONFIG_PPC64
-#define IO_SET_SYNC_FLAG() do { local_paca->io_sync = 1; } while(0)
-#else
-#define IO_SET_SYNC_FLAG()
-#endif
-
/* gcc 4.0 and older doesn't have 'Z' constraint */
#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ == 0)
#define DEF_MMIO_IN_X(name, size, insn) \
@@ -127,7 +121,6 @@ static inline void name(volatile u##size __iomem *addr, u##size val) \
{ \
__asm__ __volatile__("sync;"#insn" %1,0,%2" \
: "=m" (*addr) : "r" (val), "r" (addr) : "memory"); \
- IO_SET_SYNC_FLAG(); \
}
#else /* newer gcc */
#define DEF_MMIO_IN_X(name, size, insn) \
@@ -144,7 +137,6 @@ static inline void name(volatile u##size __iomem *addr, u##size val) \
{ \
__asm__ __volatile__("sync;"#insn" %1,%y0" \
: "=Z" (*addr) : "r" (val) : "memory"); \
- IO_SET_SYNC_FLAG(); \
}
#endif
@@ -162,7 +154,6 @@ static inline void name(volatile u##size __iomem *addr, u##size val) \
{ \
__asm__ __volatile__("sync;"#insn"%U0%X0 %1,%0" \
: "=m" (*addr) : "r" (val) : "memory"); \
- IO_SET_SYNC_FLAG(); \
}
DEF_MMIO_IN_D(in_8, 8, lbz);
@@ -626,23 +617,7 @@ static inline void name at \
#define writel_relaxed(v, addr) writel(v, addr)
#define writeq_relaxed(v, addr) writeq(v, addr)
-#ifdef CONFIG_PPC32
#define mmiowb()
-#else
-/*
- * Enforce synchronisation of stores vs. spin_unlock
- * (this does it explicitly, though our implementation of spin_unlock
- * does it implicitely too)
- */
-static inline void mmiowb(void)
-{
- unsigned long tmp;
-
- __asm__ __volatile__("sync; li %0,0; stb %0,%1(13)"
- : "=&r" (tmp) : "i" (offsetof(struct paca_struct, io_sync))
- : "memory");
-}
-#endif /* !CONFIG_PPC32 */
static inline void iosync(void)
{
diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
index 4dbe072eecbe..9da89ea4ff31 100644
--- a/arch/powerpc/include/asm/spinlock.h
+++ b/arch/powerpc/include/asm/spinlock.h
@@ -28,8 +28,6 @@
#include <asm/synch.h>
#include <asm/ppc-opcode.h>
-#define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
-
#ifdef CONFIG_PPC64
/* use 0x800000yy when locked, where yy == CPU number */
#ifdef __BIG_ENDIAN__
@@ -41,19 +39,6 @@
#define LOCK_TOKEN 1
#endif
-#if defined(CONFIG_PPC64) && defined(CONFIG_SMP)
-#define CLEAR_IO_SYNC (get_paca()->io_sync = 0)
-#define SYNC_IO do { \
- if (unlikely(get_paca()->io_sync)) { \
- mb(); \
- get_paca()->io_sync = 0; \
- } \
- } while (0)
-#else
-#define CLEAR_IO_SYNC
-#define SYNC_IO
-#endif
-
static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
{
return lock.slock == 0;
@@ -91,7 +76,6 @@ static inline unsigned long __arch_spin_trylock(arch_spinlock_t *lock)
static inline int arch_spin_trylock(arch_spinlock_t *lock)
{
- CLEAR_IO_SYNC;
return __arch_spin_trylock(lock) == 0;
}
@@ -122,7 +106,6 @@ extern void __rw_yield(arch_rwlock_t *lock);
static inline void arch_spin_lock(arch_spinlock_t *lock)
{
- CLEAR_IO_SYNC;
while (1) {
if (likely(__arch_spin_trylock(lock) == 0))
break;
@@ -140,7 +123,6 @@ void arch_spin_lock_flags(arch_spinlock_t *lock, unsigned long flags)
{
unsigned long flags_dis;
- CLEAR_IO_SYNC;
while (1) {
if (likely(__arch_spin_trylock(lock) == 0))
break;
@@ -158,9 +140,7 @@ void arch_spin_lock_flags(arch_spinlock_t *lock, unsigned long flags)
static inline void arch_spin_unlock(arch_spinlock_t *lock)
{
- SYNC_IO;
- __asm__ __volatile__("# arch_spin_unlock\n\t"
- PPC_RELEASE_BARRIER: : :"memory");
+ smp_mb();
lock->slock = 0;
}
--
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