Ack: [PATCH Trusty SRU] net: avoid dependency of net_get_random_once on nop patching

[Brad Figg]

On 06/16/2014 02:37 PM, Tim Gardner wrote:
> From: Hannes Frederic Sowa <hannes at stressinduktion.org>
> 
> BugLink: http://bugs.launchpad.net/bugs/1330671
> 
> net_get_random_once depends on the static keys infrastructure to patch up
> the branch to the slow path during boot. This was realized by abusing the
> static keys api and defining a new initializer to not enable the call
> site while still indicating that the branch point should get patched
> up. This was needed to have the fast path considered likely by gcc.
> 
> The static key initialization during boot up normally walks through all
> the registered keys and either patches in ideal nops or enables the jump
> site but omitted that step on x86 if ideal nops where already placed at
> static_key branch points. Thus net_get_random_once branches not always
> became active.
> 
> This patch switches net_get_random_once to the ordinary static_key
> api and thus places the kernel fast path in the - by gcc considered -
> unlikely path.  Microbenchmarks on Intel and AMD x86-64 showed that
> the unlikely path actually beats the likely path in terms of cycle cost
> and that different nop patterns did not make much difference, thus this
> switch should not be noticeable.
> 
> Fixes: a48e42920ff38b ("net: introduce new macro net_get_random_once")
> Reported-by: Tuomas Räsänen <tuomasjjrasanen at tjjr.fi>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Signed-off-by: Hannes Frederic Sowa <hannes at stressinduktion.org>
> Signed-off-by: David S. Miller <davem at davemloft.net>
> (cherry picked from commit 3d4405226d27b3a215e4d03cfa51f536244e5de7)
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> ---
>  include/linux/net.h | 15 ++++-----------
>  net/core/utils.c    |  8 ++++----
>  2 files changed, 8 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/net.h b/include/linux/net.h
> index 69be3e6..4b425b3 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -251,24 +251,17 @@ do {								\
>  bool __net_get_random_once(void *buf, int nbytes, bool *done,
>  			   struct static_key *done_key);
>  
> -#ifdef HAVE_JUMP_LABEL
> -#define ___NET_RANDOM_STATIC_KEY_INIT ((struct static_key) \
> -		{ .enabled = ATOMIC_INIT(0), .entries = (void *)1 })
> -#else /* !HAVE_JUMP_LABEL */
> -#define ___NET_RANDOM_STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
> -#endif /* HAVE_JUMP_LABEL */
> -
>  #define net_get_random_once(buf, nbytes)				\
>  	({								\
>  		bool ___ret = false;					\
>  		static bool ___done = false;				\
> -		static struct static_key ___done_key =			\
> -			___NET_RANDOM_STATIC_KEY_INIT;			\
> -		if (!static_key_true(&___done_key))			\
> +		static struct static_key ___once_key =			\
> +			STATIC_KEY_INIT_TRUE;				\
> +		if (static_key_true(&___once_key))			\
>  			___ret = __net_get_random_once(buf,		\
>  						       nbytes,		\
>  						       &___done,	\
> -						       &___done_key);	\
> +						       &___once_key);	\
>  		___ret;							\
>  	})
>  
> diff --git a/net/core/utils.c b/net/core/utils.c
> index 2f737bf..eed3433 100644
> --- a/net/core/utils.c
> +++ b/net/core/utils.c
> @@ -348,8 +348,8 @@ static void __net_random_once_deferred(struct work_struct *w)
>  {
>  	struct __net_random_once_work *work =
>  		container_of(w, struct __net_random_once_work, work);
> -	if (!static_key_enabled(work->key))
> -		static_key_slow_inc(work->key);
> +	BUG_ON(!static_key_enabled(work->key));
> +	static_key_slow_dec(work->key);
>  	kfree(work);
>  }
>  
> @@ -367,7 +367,7 @@ static void __net_random_once_disable_jump(struct static_key *key)
>  }
>  
>  bool __net_get_random_once(void *buf, int nbytes, bool *done,
> -			   struct static_key *done_key)
> +			   struct static_key *once_key)
>  {
>  	static DEFINE_SPINLOCK(lock);
>  	unsigned long flags;
> @@ -382,7 +382,7 @@ bool __net_get_random_once(void *buf, int nbytes, bool *done,
>  	*done = true;
>  	spin_unlock_irqrestore(&lock, flags);
>  
> -	__net_random_once_disable_jump(done_key);
> +	__net_random_once_disable_jump(once_key);
>  
>  	return true;
>  }
> 


-- 
Brad Figg brad.figg at canonical.com http://www.canonical.com