On Fri, Nov 22, 2013 at 5:25 PM, Linus Torvalds <[email protected]> wrote: > On Fri, Nov 22, 2013 at 4:56 PM, Davidlohr Bueso <[email protected]> wrote: >> In futex_wake() there is clearly no point in taking the hb->lock if >> we know beforehand that there are no tasks to be woken. This comes >> at the smaller cost of doing some atomic operations to keep track of >> the list's size. > > Hmm. Why? Afaik, you only care about "empty or not". And if you don't > need the serialization from locking, then afaik you can just do a > "plist_head_empty()" without holding the lock. > > NOTE! > > The "list_empty()" function is very much designed to work even without > holding a lock (as long as the head itself exists reliably, of course) > BUT you have to then guarantee yourself that your algorithm doesn't > have any races wrt other CPU's adding an entry to the list at the same > time. Not holding a lock obviously means that you are not serialized > against that.. We've had problems with people doing > > if (!list_empty(waiters)) > wake_up_list(..) > > because they wouldn't wake people up who just got added. > > But considering that your atomic counter checking has the same lack of > serialization, at least the plist_head_empty() check shouldn't be any > worse than that counter thing.. And doesn't need any steenking atomic > ops or a new counter field.
Hi Linus, In this patch, since we do the atomic increments before holding the hb->lock during situations where we may potentially add a task to the list, then we would only skip attempting the wake up if no other thread has already held the hb->lock and is in the process of adding a task to the list, in addition to the list being empty. Would this be enough to protect it from any race condition? Thanks, Jason -- 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/
