Messages in this thread | | | Date | Mon, 5 Aug 2019 17:43:28 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/percpu_rwsem: Rewrite to not use rwsem |
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On Mon, Aug 05, 2019 at 10:58:13PM +0800, Boqun Feng wrote: > On Mon, Aug 05, 2019 at 10:43:18PM +0800, Boqun Feng wrote: > > On Mon, Aug 05, 2019 at 04:02:41PM +0200, Peter Zijlstra wrote: > > [...] > > > > > > static inline void percpu_up_read(struct percpu_rw_semaphore *sem) > > > { > > > + rwsem_release(&sem->dep_map, 1, _RET_IP_); > > > + > > > preempt_disable(); > > > /* > > > * Same as in percpu_down_read(). > > > */ > > > - if (likely(rcu_sync_is_idle(&sem->rss))) > > > + if (likely(rcu_sync_is_idle(&sem->rss))) { > > > __this_cpu_dec(*sem->read_count); > > > - else > > > - __percpu_up_read(sem); /* Unconditional memory barrier */ > > > - preempt_enable(); > > > + preempt_enable(); > > > + return; > > > + } > > > > > > - rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_); > > > > Missing a preempt_enable() here? > > > > Ah.. you modified the semantics of __percpu_up_read() to imply a > preempt_enable(), sorry for the noise...
Yes indeed; I suppose I should've noted that in the Changlog. The reason is that waitqueues use spin_lock() which change into a sleepable lock on RT and thus cannot be used with preeption disabled. We also cannot (easily) switch to swait because we use both exclusive and !exclusive waits.
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