On Mon, Nov 28, 2016 at 01:20:06PM +0100, Nicolai Hähnle wrote:
> From: Nicolai Hähnle <Nicolai.Haehnle@xxxxxxx>
>
> Add regular waiters in stamp order. Keep adding waiters that have no
> context in FIFO order and take care not to starve them.
>
> While adding our task as a waiter, back off if we detect that there is a
> waiter with a lower stamp in front of us.
>
> Make sure to call lock_contended even when we back off early.
I'm hitting
[ 86.202749] WARNING: CPU: 1 PID: 813 at ./include/linux/ww_mutex.h:292 stress_inorder_work+0x436/0x4b5 [test_ww_mutex]
[ 86.202885] DEBUG_LOCKS_WARN_ON(!ctx->contending_lock)
which if I understand correctly is due to
> +static inline int __sched
> +__ww_mutex_add_waiter(struct mutex_waiter *waiter,
> + struct mutex *lock,
> + struct ww_acquire_ctx *ww_ctx)
> +{
> + if (ww_ctx) {
> + struct mutex_waiter *cur;
> +
> + /*
> + * 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)
> + return -EDEADLK;
not setting ww_ctx->contending_lock here.
> +
> + continue;
> + }
> +
> + list_add_tail(&waiter->list, &cur->list);
> + return 0;
> + }
> + }
> +
> + list_add_tail(&waiter->list, &lock->wait_list);
> + return 0;
> +}
--
Chris Wilson, Intel Open Source Technology Centre
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