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Subject[PATCHv5 2/2] memory barrier: adding smp_mb__after_lock
Adding smp_mb__after_lock define to be used as a smp_mb call after
a lock.

Making it nop for x86, since {read|write|spin}_lock() on x86 are
full memory barriers.
wbr,
jirka


Signed-off-by: Jiri Olsa <jolsa@redhat.com>

---
arch/x86/include/asm/spinlock.h | 3 +++
include/linux/spinlock.h | 5 +++++
include/net/sock.h | 5 ++++-
3 files changed, 12 insertions(+), 1 deletions(-)
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index b7e5db8..39ecc5f 100644
--- a/arch/x86/include/asm/spinlock.h
+++ 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)
+
#endif /* _ASM_X86_SPINLOCK_H */
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 252b245..ae053bd 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 smp_mb__after_lock
+#define smp_mb__after_lock() 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);
}


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