Messages in this thread | | | Date | Wed, 8 Jun 2011 08:58:55 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] sched: remove unreliable pointer in mutex_spin_on_owner() |
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On Wed, Jun 08, 2011 at 08:49:53PM +0800, Hillf Danton wrote: > The dereference of unreliable owner pointer is unnecessary in owner_running(), > though under RCU protection, because the true result is only determined by > checking the validity of lock owner, as the comment says, due to likely heavy > lock contention, which has little to do with whether owner->on_cpu is false. > > If owner->on_cpu is really false, only the owner_running loop is shortened, > but also returns incorrect result, since the lock owner is not changed, though > maybe changed soon. >
Hillf, have you read anything that I posted before?
We don't want to spin if the owner of the lock sleeps. If it sleeps, then the owner's on_cpu will be zero. That's the point of checking it.
The patch you just added would devastate the performance of the system. Now if we have contention on a lock, and the owner sleeps, we continue to spin. If the spinner is an RT task, this could also cause a deadlock, especially if the owner is bound to the same CPU that the RT task is on.
-- Steve
> Signed-off-by: Hillf Danton <dhillf@gmail.com> > --- > kernel/sched.c | 36 +----------------------------------- > 1 files changed, 1 insertions(+), 35 deletions(-) > > diff --git a/kernel/sched.c b/kernel/sched.c > index fd18f39..2c32616 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -4293,52 +4293,18 @@ EXPORT_SYMBOL(schedule); > > #ifdef CONFIG_MUTEX_SPIN_ON_OWNER > > -static inline bool owner_running(struct mutex *lock, struct task_struct *owner) > -{ > - bool ret = false; > - > - rcu_read_lock(); > - if (lock->owner != owner) > - goto fail; > - > - /* > - * Ensure we emit the owner->on_cpu, dereference _after_ checking > - * lock->owner still matches owner, if that fails, owner might > - * point to free()d memory, if it still matches, the rcu_read_lock() > - * ensures the memory stays valid. > - */ > - barrier(); > - > - ret = owner->on_cpu; > -fail: > - rcu_read_unlock(); > - > - return ret; > -} > - > -/* > - * Look out! "owner" is an entirely speculative pointer > - * access and not reliable. > - */ > int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) > { > if (!sched_feat(OWNER_SPIN)) > return 0; > > - while (owner_running(lock, owner)) { > + while (lock->owner != NULL) { > if (need_resched()) > return 0; > > arch_mutex_cpu_relax(); > } > > - /* > - * If the owner changed to another task there is likely > - * heavy contention, stop spinning. > - */ > - if (lock->owner) > - return 0; > - > return 1; > } > #endif > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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