Messages in this thread | | | Date | Sat, 2 Mar 2013 20:13:17 +0800 | Subject | Re: [PATCH] lglock: add read-preference local-global rwlock | From | Michel Lespinasse <> |
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On Sat, Mar 2, 2013 at 2:28 AM, Oleg Nesterov <oleg@redhat.com> wrote: > Lai, I didn't read this discussion except the code posted by Michel. > I'll try to read this patch carefully later, but I'd like to ask > a couple of questions. > > This version looks more complex than Michel's, why? Just curious, I > am trying to understand what I missed. See > http://marc.info/?l=linux-kernel&m=136196350213593
From what I can see, my version used local_refcnt to count how many reentrant locks are represented by the fastpath lglock spinlock; Lai's version uses it to count how many reentrant locks are represented by either the fastpath lglock spinlock or the global rwlock, with FALLBACK_BASE being a bit thrown in so we can remember which of these locks was acquired. My version would be slower if it needs to take the slow path in a reentrant way, but I'm not sure it matters either :)
> Interrupt handler on CPU_1 does _read_lock() notices ->reader_refcnt != 0 > and simply does this_cpu_inc(), so reader_refcnt == FALLBACK_BASE + 1. > > Then irq does _read_unlock(), and > >> +void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw) >> +{ >> + switch (__this_cpu_dec_return(*lgrw->reader_refcnt)) { >> + case 0: >> + lg_local_unlock(&lgrw->lglock); >> + return; >> + case FALLBACK_BASE: >> + __this_cpu_sub(*lgrw->reader_refcnt, FALLBACK_BASE); >> + read_unlock(&lgrw->fallback_rwlock); > > hits this case? > > Doesn't look right, but most probably I missed something.
Good catch. I think this is easily fixed by setting reader_refcn directly to FALLBACK_BASE+1, instead of setting it to FALLBACK_BASE and then incrementing it to FALLBACK_BASE+1.
-- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.
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