On Sat, Mar 2, 2013 at 2:28 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
