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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jan 23, 2020 at 11:06:35AM +0100, Peter Zijlstra wrote: > On Thu, Jan 23, 2020 at 10:26:58AM +0100, Peter Zijlstra wrote: > > On Tue, Jan 14, 2020 at 10:59:18PM -0500, Alex Kogan wrote: > > > +/* this function is called only when the primary queue is empty */ > > > +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val, > > > + struct mcs_spinlock *node) > > > +{ > > > + struct mcs_spinlock *head_2nd, *tail_2nd; > > > + u32 new; > > > + > > > + /* If the secondary queue is empty, do what MCS does. */ > > > + if (node->locked <= 1) > > > + return __try_clear_tail(lock, val, node); > > > + > > > + /* > > > + * Try to update the tail value to the last node in the secondary queue. > > > + * If successful, pass the lock to the first thread in the secondary > > > + * queue. Doing those two actions effectively moves all nodes from the > > > + * secondary queue into the main one. > > > + */ > > > + tail_2nd = decode_tail(node->locked); > > > + head_2nd = tail_2nd->next; > > > + new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL; > > > + > > > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) { > > > + /* > > > + * Try to reset @next in tail_2nd to NULL, but no need to check > > > + * the result - if failed, a new successor has updated it. > > > + */ > > > > I think you actually have an ordering bug here; the load of head_2nd > > *must* happen before the atomic_try_cmpxchg(), otherwise it might > > observe the new next and clear a valid next pointer. > > > > What would be the best fix for that; I'm thinking: > > > > head_2nd = smp_load_acquire(&tail_2nd->next); > > > > Will? > > Hmm, given we've not passed the lock around yet; why wouldn't something > like this work: > > smp_store_release(&tail_2nd->next, NULL); Argh, make that: tail_2nd->next = NULL; smp_wmb(); > if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) { > tail_2nd->next = head_2nd; > return false; > } > > The whole second queue is only ever modified by the lock owner, and that > is us, so we can pre-terminate the secondary queue (break the circular > link), try the cmpxchg and fix it back up when it fails. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel