On 02/03/13 02:28, Oleg Nesterov 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 Michel changed my old draft version a little, his version is good enough for me. My new version tries to add a little better nestable support with only adding single __this_cpu_op() in _read_[un]lock(). > > And I can't understand FALLBACK_BASE... > > OK, suppose that CPU_0 does _write_unlock() and releases ->fallback_rwlock. > > CPU_1 does _read_lock(), and ... > >> +void lg_rwlock_local_read_lock(struct lgrwlock *lgrw) >> +{ >> + struct lglock *lg = &lgrw->lglock; >> + >> + preempt_disable(); >> + rwlock_acquire_read(&lg->lock_dep_map, 0, 0, _RET_IP_); >> + if (likely(!__this_cpu_read(*lgrw->reader_refcnt))) { >> + if (!arch_spin_trylock(this_cpu_ptr(lg->lock))) { > > _trylock() fails, > >> + read_lock(&lgrw->fallback_rwlock); >> + __this_cpu_add(*lgrw->reader_refcnt, FALLBACK_BASE); > > so we take ->fallback_rwlock and ->reader_refcnt == FALLBACK_BASE. > > CPU_0 does lg_global_unlock(lgrw->lglock) and finishes _write_unlock(). > > 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. Your are right, I just realized that I had spit a code which should be atomic. I hope this patch(V2) can get more reviews. My first and many locking knowledge is learned from Paul. Paul, would you also review it? Thanks, Lai > > Oleg. > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- 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
