Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

Jiri Olsa
2009-07-03T11:19:22Z

On Fri, Jul 03, 2009 at 12:25:30PM +0200, Ingo Molnar wrote:
>
> * Jiri Olsa <jolsa@redhat.com> wrote:
>
> > On Fri, Jul 03, 2009 at 11:24:38AM +0200, Ingo Molnar wrote:
> > >
> > > * Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > >
> > > > Ingo Molnar a écrit :
> > > > > * Jiri Olsa <jolsa@redhat.com> wrote:
> > > > >
> > > > >> +++ b/arch/x86/include/asm/spinlock.h
> > > > >> @@ -302,4 +302,7 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
> > > > >> #define _raw_read_relax(lock) cpu_relax()
> > > > >> #define _raw_write_relax(lock) cpu_relax()
> > > > >>
> > > > >> +/* The {read|write|spin}_lock() on x86 are full memory barriers. */
> > > > >> +#define smp_mb__after_lock() do { } while (0)
> > > > >
> > > > > Two small stylistic comments, please make this an inline function:
> > > > >
> > > > > static inline void smp_mb__after_lock(void) { }
> > > > > #define smp_mb__after_lock
> > > > >
> > > > > (untested)
> > > > >
> > > > >> +/* The lock does not imply full memory barrier. */
> > > > >> +#ifndef smp_mb__after_lock
> > > > >> +#define smp_mb__after_lock() smp_mb()
> > > > >> +#endif
> > > > >
> > > > > ditto.
> > > > >
> > > > > Ingo
> > > >
> > > > This was following existing implementations of various smp_mb__??? helpers :
> > > >
> > > > # grep -4 smp_mb__before_clear_bit include/asm-generic/bitops.h
> > > >
> > > > /*
> > > > * clear_bit may not imply a memory barrier
> > > > */
> > > > #ifndef smp_mb__before_clear_bit
> > > > #define smp_mb__before_clear_bit() smp_mb()
> > > > #define smp_mb__after_clear_bit() smp_mb()
> > > > #endif
> > >
> > > Did i mention that those should be fixed too? :-)
> > >
> > > Ingo
> >
> > ok, could I include it in the 2/2 or you prefer separate patch?
>
> depends on whether it will regress ;-)
>
> If it regresses, it's better to have it separate. If it wont, it can
> be included. If unsure, default to the more conservative option.
>
> Ingo

how about this..
and similar change for smp_mb__before_clear_bit in a separate patch

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index b7e5db8..4e77853 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -302,4 +302,8 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
#define _raw_read_relax(lock) cpu_relax()
#define _raw_write_relax(lock) cpu_relax()

+/* The {read|write|spin}_lock() on x86 are full memory barriers. */
+static inline void smp_mb__after_lock(void) { }
+#define ARCH_HAS_SMP_MB_AFTER_LOCK
+
#endif /* _ASM_X86_SPINLOCK_H */
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 252b245..4be57ab 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -132,6 +132,11 @@ do { \
#endif /*__raw_spin_is_contended*/
#endif

+/* The lock does not imply full memory barrier. */
+#ifndef ARCH_HAS_SMP_MB_AFTER_LOCK
+static inline void smp_mb__after_lock(void) { smp_mb(); }
+#endif
+
/**
* spin_unlock_wait - wait until the spinlock gets unlocked
* @lock: the spinlock in question.
diff --git a/include/net/sock.h b/include/net/sock.h
index 4eb8409..98afcd9 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1271,6 +1271,9 @@ static inline int sk_has_allocations(const struct sock *sk)
* in its cache, and so does the tp->rcv_nxt update on CPU2 side. The CPU1
* could then endup calling schedule and sleep forever if there are no more
* data on the socket.
+ *
+ * The sk_has_helper is always called right after a call to read_lock, so we
+ * can use smp_mb__after_lock barrier.
*/
static inline int sk_has_sleeper(struct sock *sk)
{
@@ -1280,7 +1283,7 @@ static inline int sk_has_sleeper(struct sock *sk)
*
* This memory barrier is paired in the sock_poll_wait.
*/
- smp_mb();
+ smp_mb__after_lock();
return sk->sk_sleep && waitqueue_active(sk->sk_sleep);
}