On Mon, 2014-07-28 at 19:55 -0700, Davidlohr Bueso wrote:
> +static bool mutex_optimistic_spin(struct mutex *lock,
> + struct ww_acquire_ctx *ww_ctx, const bool
> use_ww_ctx)
> +{
> + struct task_struct *task = current;
> +
> + if (!mutex_can_spin_on_owner(lock))
> + return false;
> +
> + if (!osq_lock(&lock->osq))
> + return false;
In the !osq_lock() case, we could exit the cancellable MCS spinlock due
to need_resched(). However, this would return from the function rather
than doing the need_resched() check below. Perhaps we can add something
like "goto out" which goes to the below check?
The mutex_can_spin_on_owner() also returns false if need_resched().
> + while (true) {
> + struct task_struct *owner;
> +
> + if (use_ww_ctx && ww_ctx->acquired > 0) {
> + struct ww_mutex *ww;
> +
> + ww = container_of(lock, struct ww_mutex, base);
> + /*
> + * If ww->ctx is set the contents are undefined, only
> + * by acquiring wait_lock there is a guarantee that
> + * they are not invalid when reading.
> + *
> + * As such, when deadlock detection needs to be
> + * performed the optimistic spinning cannot be done.
> + */
> + if (ACCESS_ONCE(ww->ctx))
> + break;
> + }
> +
> + /*
> + * If there's an owner, wait for it to either
> + * release the lock or go to sleep.
> + */
> + owner = ACCESS_ONCE(lock->owner);
> + if (owner && !mutex_spin_on_owner(lock, owner))
> + break;
> +
> + /* Try to acquire the mutex if it is unlocked. */
> + if (mutex_try_to_acquire(lock)) {
> + if (use_ww_ctx) {
> + struct ww_mutex *ww;
> + ww = container_of(lock, struct ww_mutex, base);
> +
> + ww_mutex_set_context_fastpath(ww, ww_ctx);
> + }
> +
> + mutex_set_owner(lock);
> + osq_unlock(&lock->osq);
> + return true;
> + }
> +
> + /*
> + * When there's no owner, we might have preempted between the
> + * owner acquiring the lock and setting the owner field. If
> + * we're an RT task that will live-lock because we won't let
> + * the owner complete.
> + */
> + if (!owner && (need_resched() || rt_task(task)))
> + break;
> +
> + /*
> + * The cpu_relax() call is a compiler barrier which forces
> + * everything in this loop to be re-loaded. We don't need
> + * memory barriers as we'll eventually observe the right
> + * values at the cost of a few extra spins.
> + */
> + cpu_relax_lowlatency();
> + }
> +
> + osq_unlock(&lock->osq);
> +
> + /*
> + * If we fell out of the spin path because of need_resched(),
> + * reschedule now, before we try-lock the mutex. This avoids getting
> + * scheduled out right after we obtained the mutex.
> + */
> + if (need_resched())
> + schedule_preempt_disabled();
> +
> + return false;
> +}
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