| Message ID | 20160531094844.037298593@infradead.org |
|---|---|
| State | Awaiting Upstream |
| Delegated to: | Pablo Neira |
| Headers | show |
On Tue, May 31, 2016 at 11:41:37AM +0200, Peter Zijlstra wrote: [snip] > @@ -260,16 +260,6 @@ static void sem_rcu_free(struct rcu_head > } > > /* > - * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they > - * are only control barriers. > - * The code must pair with spin_unlock(&sem->lock) or > - * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient. > - * > - * smp_rmb() is sufficient, as writes cannot pass the control barrier. > - */ > -#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb() > - > -/* > * Wait until all currently ongoing simple ops have completed. > * Caller must own sem_perm.lock. > * New simple ops cannot start, because simple ops first check > @@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_ar > sem = sma->sem_base + i; > spin_unlock_wait(&sem->lock); > } > - ipc_smp_acquire__after_spin_is_unlocked(); > + smp_acquire__after_ctrl_dep(); I wonder whether we can kill this barrier after updating spin_unlock_wait() to ACQUIRE? Regards, Boqun > } > > /*
On Wed, Jun 01, 2016 at 09:52:14PM +0800, Boqun Feng wrote: > On Tue, May 31, 2016 at 11:41:37AM +0200, Peter Zijlstra wrote: > > @@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_ar > > sem = sma->sem_base + i; > > spin_unlock_wait(&sem->lock); > > } > > - ipc_smp_acquire__after_spin_is_unlocked(); > > + smp_acquire__after_ctrl_dep(); > > I wonder whether we can kill this barrier after updating > spin_unlock_wait() to ACQUIRE? See patch 5 doing that :-) -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Jun 01, 2016 at 06:22:55PM +0200, Peter Zijlstra wrote: > On Wed, Jun 01, 2016 at 09:52:14PM +0800, Boqun Feng wrote: > > On Tue, May 31, 2016 at 11:41:37AM +0200, Peter Zijlstra wrote: > > > > @@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_ar > > > sem = sma->sem_base + i; > > > spin_unlock_wait(&sem->lock); > > > } > > > - ipc_smp_acquire__after_spin_is_unlocked(); > > > + smp_acquire__after_ctrl_dep(); > > > > I wonder whether we can kill this barrier after updating > > spin_unlock_wait() to ACQUIRE? > > See patch 5 doing that :-) Oops, right ;-) Regards, Boqun
--- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -305,6 +305,18 @@ static __always_inline void __write_once }) /** + * smp_acquire__after_ctrl_dep() - Provide ACQUIRE ordering after a control dependency + * + * A control dependency provides a LOAD->STORE order, the additional RMB + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, + * aka. (load)-ACQUIRE. + * + * Architectures that do not do load speculation can have this be barrier(). + * XXX move into asm/barrier.h + */ +#define smp_acquire__after_ctrl_dep() smp_rmb() + +/** * cmpwait - compare and wait for a variable to change * @ptr: pointer to the variable to wait on * @val: the value it should change from @@ -331,10 +343,6 @@ static __always_inline void __write_once * * Due to C lacking lambda expressions we load the value of *ptr into a * pre-named variable @VAL to be used in @cond. - * - * The control dependency provides a LOAD->STORE order, the additional RMB - * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, - * aka. ACQUIRE. */ #ifndef smp_cond_load_acquire #define smp_cond_load_acquire(ptr, cond_expr) ({ \ @@ -346,7 +354,7 @@ static __always_inline void __write_once break; \ cmpwait(__PTR, VAL); \ } \ - smp_rmb(); /* ctrl + rmb := acquire */ \ + smp_acquire__after_ctrl_dep(); \ VAL; \ }) #endif --- a/ipc/sem.c +++ b/ipc/sem.c @@ -260,16 +260,6 @@ static void sem_rcu_free(struct rcu_head } /* - * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they - * are only control barriers. - * The code must pair with spin_unlock(&sem->lock) or - * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient. - * - * smp_rmb() is sufficient, as writes cannot pass the control barrier. - */ -#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb() - -/* * Wait until all currently ongoing simple ops have completed. * Caller must own sem_perm.lock. * New simple ops cannot start, because simple ops first check @@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_ar sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); } - ipc_smp_acquire__after_spin_is_unlocked(); + smp_acquire__after_ctrl_dep(); } /* @@ -350,7 +340,7 @@ static inline int sem_lock(struct sem_ar * complex_count++; * spin_unlock(sem_perm.lock); */ - ipc_smp_acquire__after_spin_is_unlocked(); + smp_acquire__after_ctrl_dep(); /* * Now repeat the test of complex_count:
Introduce smp_acquire__after_ctrl_dep(), this construct is not uncommen, but the lack of this barrier is. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- include/linux/compiler.h | 18 +++++++++++++----- ipc/sem.c | 14 ++------------ 2 files changed, 15 insertions(+), 17 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html