Messages in this thread | | | Date | Tue, 17 Dec 2019 11:26:54 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem |
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On Tue, Nov 19, 2019 at 04:58:26PM +0100, Oleg Nesterov wrote: > On 11/19, Waiman Long wrote: > > > > On 11/13/19 5:21 AM, Peter Zijlstra wrote: > > > +static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, > > > + unsigned int mode, int wake_flags, > > > + void *key) > > > +{ > > > + struct task_struct *p = get_task_struct(wq_entry->private); > > > + bool reader = wq_entry->flags & WQ_FLAG_CUSTOM; > > > + struct percpu_rw_semaphore *sem = key; > > > + > > > + /* concurrent against percpu_down_write(), can get stolen */ > > > + if (!__percpu_rwsem_trylock(sem, reader)) > > > + return 1; > > > + > > > + list_del_init(&wq_entry->entry); > > > + smp_store_release(&wq_entry->private, NULL); > > > + > > > + wake_up_process(p); > > > + put_task_struct(p); > > > + > > > + return !reader; /* wake 'all' readers and 1 writer */ > > > +} > > > + > > > > If I read the function correctly, you are setting the WQ_FLAG_EXCLUSIVE > > for both readers and writers and __wake_up() is called with an exclusive > > count of one. So only one reader or writer is woken up each time. > > This depends on what percpu_rwsem_wake_function() returns. If it returns 1, > __wake_up_common() stops, exactly because all waiters have WQ_FLAG_EXCLUSIVE.
Indeed, let me see if I can clarify that somehow.
> > However, the comment above said we wake 'all' readers and 1 writer. That > > doesn't match the actual code, IMO. > > Well, "'all' readers" probably means "all readers before writer",
Correct.
> > To match the comments, you should > > have set WQ_FLAG_EXCLUSIVE flag only on writer. In this case, you > > probably don't need WQ_FLAG_CUSTOM to differentiate between readers and > > writers. > > See above... > > note also the > > if (!__percpu_rwsem_trylock(sem, reader)) > return 1; > > at the start of percpu_rwsem_wake_function(). We want to stop wake_up_common() > as soon as percpu_rwsem_trylock() fails. Because we know that if it fails once > it can't succeed later. Although iiuc this can only happen if another (new) > writer races with __wake_up(&sem->waiters).
Yes, writer-writer stealing can cause that. I even put a comment in there :-)
> I guess WQ_FLAG_CUSTOM can be avoided, percpu_rwsem_wait() could do > > if (read) > __add_wait_queue_entry_tail(...); > else { > wq_entry.flags |= WQ_FLAG_EXCLUSIVE; > __add_wait_queue(...); > } > > but this is "unfair".
Yes, I could not make it fair without that extra bit, and I figured we have plenty bits there to play with so why not.
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