svn commit: r313996 - in head/sys: kern sys

[Gleb Smirnoff]

  Mateusz,

  why do you __predict_false() the recursion scenario? I'm afraid
that performance loss for mispredictions could outweight the
gain due to predictions. AFAIK, mutex recursion is still a pretty
common event in the kernel.

On Mon, Feb 20, 2017 at 07:08:36PM +0000, Mateusz Guzik wrote:
M> Author: mjg
M> Date: Mon Feb 20 19:08:36 2017
M> New Revision: 313996
M> URL: https://svnweb.freebsd.org/changeset/base/313996
M> 
M> Log:
M>   mtx: fix spin mutexes interaction with failed fcmpset
M>   
M>   While doing so move recursion support down to the fallback routine.
M> 
M> Modified:
M>   head/sys/kern/kern_mutex.c
M>   head/sys/sys/mutex.h
M> 
M> Modified: head/sys/kern/kern_mutex.c
M> ==============================================================================
M> --- head/sys/kern/kern_mutex.c	Mon Feb 20 17:33:25 2017	(r313995)
M> +++ head/sys/kern/kern_mutex.c	Mon Feb 20 19:08:36 2017	(r313996)
M> @@ -696,6 +696,14 @@ _mtx_lock_spin_cookie(volatile uintptr_t
M>  	lock_delay_arg_init(&lda, &mtx_spin_delay);
M>  	m = mtxlock2mtx(c);
M>  
M> +	if (__predict_false(v == MTX_UNOWNED))
M> +		v = MTX_READ_VALUE(m);
M> +
M> +	if (__predict_false(v == tid)) {
M> +		m->mtx_recurse++;
M> +		return;
M> +	}
M> +
M>  	if (LOCK_LOG_TEST(&m->lock_object, opts))
M>  		CTR1(KTR_LOCK, "_mtx_lock_spin: %p spinning", m);
M>  	KTR_STATE1(KTR_SCHED, "thread", sched_tdname((struct thread *)tid),
M> 
M> Modified: head/sys/sys/mutex.h
M> ==============================================================================
M> --- head/sys/sys/mutex.h	Mon Feb 20 17:33:25 2017	(r313995)
M> +++ head/sys/sys/mutex.h	Mon Feb 20 19:08:36 2017	(r313996)
M> @@ -223,12 +223,9 @@ void	thread_lock_flags_(struct thread *,
M>  	uintptr_t _v = MTX_UNOWNED;					\
M>  									\
M>  	spinlock_enter();						\
M> -	if (!_mtx_obtain_lock_fetch((mp), &_v, _tid)) {			\
M> -		if (_v == _tid)						\
M> -			(mp)->mtx_recurse++;				\
M> -		else							\
M> -			_mtx_lock_spin((mp), _v, _tid, (opts), (file), (line));\
M> -	} else 								\
M> +	if (!_mtx_obtain_lock_fetch((mp), &_v, _tid)) 			\
M> +		_mtx_lock_spin((mp), _v, _tid, (opts), (file), (line)); \
M> +	else 								\
M>  		LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(spin__acquire,	\
M>  		    mp, 0, 0, file, line);				\
M>  } while (0)
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-- 
Totus tuus, Glebius.