On Wed, 2015-04-29 at 21:44 +0200, Manfred Spraul wrote: > Hi Davidlohr, > > On 04/28/2015 06:59 PM, Davidlohr Bueso wrote: > > On Tue, 2015-04-28 at 18:43 +0200, Peter Zijlstra wrote: > >> Well, if you can 'guarantee' the cmpxchg will not fail, you can then > >> rely on the fact that cmpxchg implies a full barrier, which would > >> obviate the need for the wmb. > > Yes, assuming it implies barriers on both sides. And we could obviously > > remove the need for pairing. With wake_q being local to wq_sleep() I > > cannot see duplicate tasks trying to add themselves in the list. Failed > > cmpxchg should only occur when users start misusing the wake_q. > > > > Manfred, do you have any objections to this? Perhaps I've missed the > > real purpose of the barriers. > I don't remember the details either, so let's check what should happen: > > CPU1: sender copies message to kernel memory > aaaa > CPU1: sender does receiver->msg = message; > ** barrier 1 > CPU1: sender does receiver->state = STATE_READY; > > CPU2: receiver notices receiver->state = STATE_READY; > ** barrier 2 > CPU2: receiver reads receiver->msg > bbbb > CPU2: receiver reads *receiver->msg > > Failures would be: > - write to receiver->state is visible before the write to receiver->msg > or to *receiver->msg > ** barrier 1 needs to be an smp_wmb() > - cpu 2 reads receiver->msg before receiver->state > ** barrier 2 needs to be an smp_rmb(). > > As far as I can see, no barrier is needed in pos aaaa or bbbb.
Thanks for confirming. > > With regards to failed cmpxchg(): > I don't see that mqueue could cause it by itself. Agreed. > > Who is allowed to use wake_q? > If it is permitted to use wake_q for e.g. timeout/signal delivery > wakeup, then that user might have a pending wakeup stored in the task > struct. No, this is not the case. All users are expected to do the wakeup right away. Thanks, Davidlohr -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
