On Sunday, 24 of February 2008, Pavel Machek wrote: > On Sun 2008-02-24 15:33:01, Alan Stern wrote: > > On Sun, 24 Feb 2008, Pavel Machek wrote: > > > > > > > What locking protects this variable? What happens when suspending_task > > > > > exits? (Hmm, that would probably be bug, anyway?) > > > > > > > > It's protected by whatever existing locking scheme allows only one > > > > task to start a system sleep at a time. For example, the suspending > > > > task has to get a write lock on pm_sleep_rwsem. > > > > > > And readers of suspending_task are protected by? > > > > I added a comment about that too. > > > > > At the very least, you'd need rmb() before reading it and wmb() after > > > writing to it, but I'm not sure if that's enough on every obscure > > > architecture out there. > > > > No, neither one is needed because of the way suspending_task is used. > > > > It's not necessary for a reader R to see the variable's actual value; > > all R needs to know is whether or not suspending_task is equal to R. > > Since the only process which can set suspending_task to R is R itself, > > and since R will set suspending_task back to NULL before releasing the > > write lock on pm_sleep_rwsem, there's never any ambiguity. > > Subtle. > > Very subtly wrong ;-). > > imagine suspending_task == 0xabcdef01. Now task "R" with current == > 0xabcd0000 reads suspending_task while the other cpu is writing to it, > and sees 0xabcd0000 (0xef01 was not yet written) -- and mistakenly > believes that "R" == suspending_task. > > I agree it is very unlikely, and it will not happen on i386. But what > about just using atomic_t suspending_task, and store current->pid into > it?
I'd rather use a lock, frankly. For example, we can require the readers to take pm_sleep_rwsem for reading in order to access that. Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
