Messages in this thread | | | Date | Sun, 11 Nov 2012 16:45:09 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily |
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On 11/09, Paul E. McKenney wrote: > > On Fri, Nov 09, 2012 at 07:10:48PM +0100, Oleg Nesterov wrote: > > > > static bool xxx(brw) > > { > > down_write(&brw->rw_sem); > > down_write_trylock() > > As you noted in your later email. Presumably you return false if > the attempt to acquire it fails.
Yes, yes, thanks.
> > But first we should do other changes, I think. IMHO we should not do > > synchronize_sched() under mutex_lock() and this will add (a bit) more > > complications. We will see. > > Indeed, that does put considerable delay on the writers. There is always > synchronize_sched_expedited(), I suppose.
I am not sure about synchronize_sched_expedited() (at least unconditionally), but: only the 1st down_write() needs synchronize_, and up_write() do not need to sleep in synchronize_ at all.
To simplify, lets ignore the fact that the writers need to serialize with each other. IOW, the pseudo-code below is obviously deadly wrong and racy, just to illustrate the idea.
1. We remove brw->writer_mutex and add "atomic_t writers_ctr".
update_fast_ctr() uses atomic_read(brw->writers_ctr) == 0 instead of !mutex_is_locked().
2. down_write() does
if (atomic_add_return(brw->writers_ctr) == 1) { // first writer synchronize_sched(); ... } else { ... XXX: wait for percpu_up_write() from the first writer ... }
3. up_write() does
if (atomic_dec_unless_one(brw->writers_ctr)) { ... wake up XXX writers above ... return; } else { // the last writer call_rcu_sched( func => { atomic_dec(brw->writers_ctr) } ); }
Once again, this all is racy, but hopefully the idea is clear:
- down_write(brw) sleeps in synchronize_sched() only if brw has already switched back to fast-path-mode
- up_write() never sleeps in synchronize_sched(), it uses call_rcu_sched() or wakes up the next writer.
Of course I am not sure this all worth the trouble, this should be discussed. (and, cough, I'd like to add the multi-writers mode which I'm afraid nobody will like) But I am not going to even try to do this until the current patch is applied, I need it to fix the bug in uprobes and I think the current code is "good enough". These changes can't help to speedup the readers, and the writers are slow/rare anyway.
Thanks!
Oleg.
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