Messages in this thread | | | Date | Wed, 11 May 2016 11:31:27 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable |
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On Wed 11-05-16 11:17:33, Peter Zijlstra wrote: > On Wed, May 11, 2016 at 11:04:42AM +0200, Michal Hocko wrote: > > On Wed 11-05-16 10:44:01, Peter Zijlstra wrote: > > [...] > > > @@ -504,6 +502,18 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) > > > raw_spin_unlock_irq(&sem->wait_lock); > > > > > > return ret; > > > + > > > +out_nolock: > > > + __set_current_state(TASK_RUNNING); > > > + raw_spin_lock_irq(&sem->wait_lock); > > > + list_del(&waiter.list); > > > + if (list_empty(&sem->wait_list)) > > > + rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem); > > > + else > > > + __rwsem_do_wake(sem, RWSEM_WAKE_READERS); > > > + raw_spin_unlock_irq(&sem->wait_lock); > > > + > > > + return ERR_PTR(-EINTR); > > > } > > > > Looks much better but don't we have to check the count for potentially > > pending writers? > > Ah, so I was thinking that if we get here, there must still be an owner, > otherwise we'd have acquired the lock. And if there is an owner, we > cannot go wake writers. Hence the WAKE_READERS thing.
I was worried about the case where the owner is writer and we would wake readers but I have missed that this wouldn't happen because of
if (wake_type != RWSEM_WAKE_READ_OWNED) { adjustment = RWSEM_ACTIVE_READ_BIAS; try_reader_grant: oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { /* A writer stole the lock. Undo our reader grant. */ if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) goto out; /* Last active locker left. Retry waking readers. */ goto try_reader_grant; } }
> Then again, WAKE_ANY would not harm, in that if we do wake a pending > writer it will not proceed if it cannot and it'll go back to sleep > again.
true
> So yeah, maybe WAKE_ANY is the prudent thing to do.
I guess so.
Care to cook up a full patch?
Thanks! -- Michal Hocko SUSE Labs
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