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SubjectRe: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order
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On 06.12.2016 16:36, Peter Zijlstra wrote:
> On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
>> +static inline int __sched
>> +__ww_mutex_add_waiter(struct mutex_waiter *waiter,
>> + struct mutex *lock,
>> + struct ww_acquire_ctx *ww_ctx)
>> +{
>> + struct mutex_waiter *cur;
>> +
>> + if (!ww_ctx) {
>> + list_add_tail(&waiter->list, &lock->wait_list);
>> + return 0;
>> + }
>> +
>> + /*
>> + * Add the waiter before the first waiter with a higher stamp.
>> + * Waiters without a context are skipped to avoid starving
>> + * them.
>> + */
>> + list_for_each_entry(cur, &lock->wait_list, list) {
>> + if (!cur->ww_ctx)
>> + continue;
>> +
>> + if (__ww_mutex_stamp_after(ww_ctx, cur->ww_ctx)) {
>> + /* Back off immediately if necessary. */
>> + if (ww_ctx->acquired > 0) {
>> +#ifdef CONFIG_DEBUG_MUTEXES
>> + struct ww_mutex *ww;
>> +
>> + ww = container_of(lock, struct ww_mutex, base);
>> + DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
>> + ww_ctx->contending_lock = ww;
>> +#endif
>> + return -EDEADLK;
>> + }
>> +
>> + continue;
>> + }
>> +
>> + list_add_tail(&waiter->list, &cur->list);
>> + return 0;
>> + }
>> +
>> + list_add_tail(&waiter->list, &lock->wait_list);
>> + return 0;
>> +}
>
> So you keep the list in order of stamp, and in general stamps come in,
> in-order. That is, barring races on concurrent ww_mutex_lock(), things
> are already ordered.
>
> So it doesn't make sense to scan the entire list forwards, that's almost
> guarantees you scan the entire list every single time.
>
> Or am I reading this wrong? Which in itself is a hint a comment might be
> in place.

No, it's a reasonable question. Some things to keep in mind:

1. Each ww_acquire_ctx may be used with hundreds of locks. It's not that
clear that things will be ordered in a contention scenario, especially
since the old stamp is re-used when a context backs off and goes into
the slow path (with ww_ctx->acquired == 0).

2. We want to add waiters directly before the first waiter with a higher
stamp. Keeping in mind that there may be non-ww_ctx waiters in between,
and we don't want to starve them, traversing the list backwards would
require keeping the best insertion point around in a separate variable.
Clearly possible, but it seemed more awkward.

In hindsight, backwards iteration may not be so bad.

Nicolai

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