On Fri, Sep 27, 2013 at 7:19 PM, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Fri, Sep 27, 2013 at 04:54:06PM -0700, Jason Low wrote: >> On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney >> <paulmck@xxxxxxxxxxxxxxxxxx> wrote: >> > Yep. The previous lock holder's smp_wmb() won't keep either the compiler >> > or the CPU from reordering things for the new lock holder. They could for >> > example reorder the critical section to precede the node->locked check, >> > which would be very bad. >> >> Paul, Tim, Longman, >> >> How would you like the proposed changes below? > > Could you point me at what this applies to? I can find flaws looking > at random pieces, given a little luck, but at some point I need to look > at the whole thing. ;-) Sure. Here is a link to the patch we are trying to modify: https://lkml.org/lkml/2013/9/25/532 Also, below is what the mcs_spin_lock() and mcs_spin_unlock() functions would look like after applying the proposed changes. static noinline void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node) { struct mcs_spin_node *prev; /* Init node */ node->locked = 0; node->next = NULL; prev = xchg(lock, node); if (likely(prev == NULL)) { /* Lock acquired. No need to set node->locked since it won't be used */ return; } ACCESS_ONCE(prev->next) = node; /* Wait until the lock holder passes the lock down */ while (!ACCESS_ONCE(node->locked)) arch_mutex_cpu_relax(); smp_mb(); } static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node) { struct mcs_spin_node *next = ACCESS_ONCE(node->next); if (likely(!next)) { /* * Release the lock by setting it to NULL */ if (cmpxchg(lock, node, NULL) == node) return; /* Wait until the next pointer is set */ while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); } smp_wmb(); ACCESS_ONCE(next->locked) = 1; } -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>