Messages in this thread Patch in this message |  | | Date | Tue, 17 Dec 2019 11:28:08 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem |
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On Tue, Dec 17, 2019 at 11:26:54AM +0100, Peter Zijlstra wrote: > 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.
Does this clarify?
--- a/kernel/locking/percpu-rwsem.c +++ b/kernel/locking/percpu-rwsem.c @@ -101,6 +101,19 @@ static bool __percpu_rwsem_trylock(struc return __percpu_down_write_trylock(sem); } +/* + * The return value of wait_queue_entry::func means: + * + * <0 - error, wakeup is terminated and the error is returned + * 0 - no wakeup, a next waiter is tried + * >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive. + * + * We use EXCLUSIVE for both readers and writers to preserve FIFO order, + * and play games with the return value to allow waking multiple readers. + * + * Specifically, we wake readers until we've woken a single writer, or until a + * trylock fails. + */ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, int wake_flags, void *key) @@ -119,7 +132,7 @@ static int percpu_rwsem_wake_function(st wake_up_process(p); put_task_struct(p); - return !reader; /* wake 'all' readers and 1 writer */ + return !reader; /* wake (readers until) 1 writer */ } static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
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