On Tue, 8 Dec 2009, Linus Torvalds wrote: > > The whole readers vs. writers thing is a non-sequitur. > > No it's not. > > It's a 100% equivalent problem. It's purely a change of wording. The end > result is the same. Well, of course the end result is the same (ignoring bugs) -- that was the point. It doesn't follow that the two locking mechanisms are 100% equivalent. > And note how even though you sprinkled random memory barriers around, you > still got it wrong. Yes. That comes of trying to think at the keyboard. > It's certainly not smaller. It's not faster. It doesn't have support for > lockdep. And it's BUGGY. Lockdep will choke on the rwsem approach anyway. It has never been very good at handling tree-structured locking, especially when there are non-parent-child interactions. But never mind. > Really. Tell me why you want to re-implement an existing lock - badly. I didn't want to. The whole exercise was intended to make a point -- that rwsems do more than we really need here. > [ Hint: you need a smp_mb() *before* the atomic_dec() in wait-unlock, so > that anybody else who sees the new value will be guaranteed to have seen > anything else the unlocker did. Yes. > You also need a smp_mb() in the wait_for_lock(), not a smp_rmb(). Can't > allow writes to migrate up either. 'atomic_read()' does not imply any > barriers. No, that's not needed. Unlike reads, writes can't move in front of data or control dependencies. Or so I've been lead to believe... > That "wait_for_lock()" is equivalent to a 'read_lock()+read_unlock()'. Not really. It also corresponds to a 'write_lock()+write_unlock()' (in the suspend routine). Are you claiming these two compound operations are equivalent? > We > _could_ expose such a mechanism for rwsem's too, but why do it? It's > actually nicer to use a real read-lock - and do it _around_ the operation, > because now the locking also automatically gets things like overlapping > suspends and resumes right. > > (Which you'd obviously hope never happens, but it's nice from a conceptual > standpoint to know that the locking is robust). > Take heed. You got it wrong. Admit it. Locking is _hard_. SMP memory > ordering is HARD. Oh, there's no question about that. I never seriously intended this stuff to be adopted. It was just for discussion. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
