On Tue, Mar 14, 2017 at 05:18:52PM +0900, Byungchul Park wrote:
> +/*
> + * Only access local task's data, so irq disable is only required.
A comment describing what it does; record a hist_lock entry; would be
more useful.
> + */
> +static void add_xhlock(struct held_lock *hlock)
> +{
> + unsigned int idx = current->xhlock_idx++;
> + struct hist_lock *xhlock = &xhlock(idx);
> +
> + /* Initialize hist_lock's members */
> + xhlock->hlock = *hlock;
> + xhlock->work_id = current->work_id;
> +
> + xhlock->trace.nr_entries = 0;
> + xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
> + xhlock->trace.entries = xhlock->trace_entries;
> + xhlock->trace.skip = 3;
> + save_stack_trace(&xhlock->trace);
> +}
> +/*
> + * This should be lockless as far as possible because this would be
> + * called very frequently.
idem; explain why depend_before().
> + */
> +static void check_add_xhlock(struct held_lock *hlock)
> +{
The other thing could be done like:
#ifdef CONFIG_DEBUG_LOCKDEP
/*
* This can be done locklessly because its all task-local state,
* we must however ensure IRQs are disabled.
*/
WARN_ON_ONCE(!irqs_disabled());
#endif
> + if (!current->xhlocks || !depend_before(hlock))
> + return;
> +
> + add_xhlock(hlock);
> +}
> +
> +/*
> + * For crosslock.
> + */
> +static int add_xlock(struct held_lock *hlock)
> +{
> + struct cross_lock *xlock;
> + unsigned int gen_id;
> +
> + if (!graph_lock())
> + return 0;
> +
> + xlock = &((struct lockdep_map_cross *)hlock->instance)->xlock;
> +
> + gen_id = (unsigned int)atomic_inc_return(&cross_gen_id);
> + xlock->hlock = *hlock;
> + xlock->hlock.gen_id = gen_id;
> + graph_unlock();
What does graph_lock protect here?
> +
> + return 1;
> +}
> +
> +/*
> + * return 0: Stop. Failed to acquire graph_lock.
> + * return 1: Done. No more acquire ops is needed.
> + * return 2: Need to do normal acquire operation.
> + */
> +static int lock_acquire_crosslock(struct held_lock *hlock)
> +{
> + /*
> + * CONTEXT 1 CONTEXT 2
> + * --------- ---------
> + * lock A (cross)
> + * X = atomic_inc_return(&cross_gen_id)
> + * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> + * Y = atomic_read_acquire(&cross_gen_id)
> + * lock B
> + *
> + * atomic_read_acquire() is for ordering between A and B,
> + * IOW, A happens before B, when CONTEXT 2 see Y >= X.
> + *
> + * Pairs with atomic_inc_return() in add_xlock().
> + */
> + hlock->gen_id = (unsigned int)atomic_read_acquire(&cross_gen_id);
> +
> + if (cross_lock(hlock->instance))
> + return add_xlock(hlock);
> +
> + check_add_xhlock(hlock);
> + return 2;
> +}
So I was wondering WTH we'd call into this with a !xlock to begin with.
Maybe something like:
/*
* Called for both normal and crosslock acquires. Normal locks will be
* pushed on the hist_lock queue. Cross locks will record state and
* stop regular lock_acquire() to avoid being placed on the held_lock
* stack.
*
* Returns: 0 - failure;
* 1 - cross-lock, done;
* 2 - normal lock, continue to held_lock[].
*/
> +static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock)
> +{
> + unsigned int xid, pid;
> + u64 chain_key;
> +
> + xid = xlock_class(xlock) - lock_classes;
> + chain_key = iterate_chain_key((u64)0, xid);
> + pid = xhlock_class(xhlock) - lock_classes;
> + chain_key = iterate_chain_key(chain_key, pid);
> +
> + if (lookup_chain_cache(chain_key))
> + return 1;
> +
> + if (!add_chain_cache_classes(xid, pid, xhlock->hlock.irq_context,
> + chain_key))
> + return 0;
> +
> + if (!check_prev_add(current, &xlock->hlock, &xhlock->hlock, 1,
> + &xhlock->trace, copy_trace))
> + return 0;
> +
> + return 1;
> +}
> +
> +static int commit_xhlocks(struct cross_lock *xlock)
> +{
> + unsigned int cur = current->xhlock_idx;
> + unsigned int i;
> +
> + if (!graph_lock())
> + return 0;
> +
> + for (i = cur - 1; !xhlock_same(i, cur); i--) {
> + struct hist_lock *xhlock = &xhlock(i);
*blink*, you mean this?
for (i = 0; i < MAX_XHLOCKS_NR; i++) {
struct hist_lock *xhlock = &xhlock(cur - i);
Except you seem to skip over the most recent element (@cur), why?
> +
> + if (!xhlock_used(xhlock))
> + break;
> +
> + if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id))
> + break;
> +
> + if (same_context_xhlock(xhlock) &&
> + !commit_xhlock(xlock, xhlock))
return with graph_lock held?
> + return 0;
> + }
> +
> + graph_unlock();
> + return 1;
> +}
> +
> +void lock_commit_crosslock(struct lockdep_map *lock)
> +{
> + struct cross_lock *xlock;
> + unsigned long flags;
> +
> + if (unlikely(!debug_locks || current->lockdep_recursion))
> + return;
> +
> + if (!current->xhlocks)
> + return;
> +
> + /*
> + * We have to check this here instead of in add_xlock(), since
> + * otherwise invalid cross_lock might be accessed on commit. In
> + * other words, building xlock in add_xlock() should not be
> + * skipped in order to access valid cross_lock on commit.
> + */
> + if (!depend_after(&((struct lockdep_map_cross *)lock)->xlock.hlock))
> + return;
> +
> + raw_local_irq_save(flags);
> + check_flags(flags);
> + current->lockdep_recursion = 1;
> + xlock = &((struct lockdep_map_cross *)lock)->xlock;
> + commit_xhlocks(xlock);
We don't seem to use the return value much..
> + current->lockdep_recursion = 0;
> + raw_local_irq_restore(flags);
> +}
> +EXPORT_SYMBOL_GPL(lock_commit_crosslock);
