Re: [PATCHv7 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@...>

> ---
>  arch/x86/include/asm/spinlock.h |    4 ++++
>  include/linux/spinlock.h        |    5 +++++
>  include/net/sock.h              |    5 ++++-
>  3 files changed, 13 insertions(+), 1 deletions(-)
>
> 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..2c0da92 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_sleeper 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);
>  }
>