[PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

[Thomas Hellstrom]

On 06/14/2018 02:48 PM, Thomas Hellstrom wrote:
> Hi, Peter,
>
> On 06/14/2018 02:41 PM, Peter Zijlstra wrote:
>> On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote:
>>> +static bool __ww_mutex_wound(struct mutex *lock,
>>> +                 struct ww_acquire_ctx *ww_ctx,
>>> +                 struct ww_acquire_ctx *hold_ctx)
>>> +{
>>> +    struct task_struct *owner = __mutex_owner(lock);
>>> +
>>> +    lockdep_assert_held(&lock->wait_lock);
>>> +
>>> +    if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) &&
>>> +        ww_ctx->acquired > 0) {
>>> +        hold_ctx->wounded = 1;
>>> +
>>> +        /*
>>> +         * wake_up_process() paired with set_current_state() inserts
>>> +         * sufficient barriers to make sure @owner either sees it's
>>> +         * wounded or has a wakeup pending to re-read the wounded
>>> +         * state.
>>> +         *
>>> +         * The value of hold_ctx->wounded in
>>> +         * __ww_mutex_lock_check_stamp();
>>> +         */
>>> +        if (owner != current)
>>> +            wake_up_process(owner);
>>> +
>>> +        return true;
>>> +    }
>>> +
>>> +    return false;
>>> +}
>>> @@ -338,12 +377,18 @@ ww_mutex_set_context_fastpath(struct ww_mutex 
>>> *lock, struct ww_acquire_ctx *ctx)
>>>        * and keep spinning, or it will acquire wait_lock, add itself
>>>        * to waiter list and sleep.
>>>        */
>>> -    smp_mb(); /* ^^^ */
>>> +    smp_mb(); /* See comments above and below. */
>>>         /*
>>> -     * Check if lock is contended, if not there is nobody to wake up
>>> +     * Check if lock is contended, if not there is nobody to wake up.
>>> +     * We can use list_empty() unlocked here since it only compares a
>>> +     * list_head field pointer to the address of the list head
>>> +     * itself, similarly to how list_empty() can be considered 
>>> RCU-safe.
>>> +     * The memory barrier above pairs with the memory barrier in
>>> +     * __ww_mutex_add_waiter and makes sure lock->ctx is visible 
>>> before
>>> +     * we check for waiters.
>>>        */
>>> -    if (likely(!(atomic_long_read(&lock->base.owner) & 
>>> MUTEX_FLAG_WAITERS)))
>>> +    if (likely(list_empty(&lock->base.wait_list)))
>>>           return;
>> OK, so what happens is that if we see !empty list, we take wait_lock,
>> if we end up in __ww_mutex_wound() we must really have !empty wait-list.
>>
>> It can however still see !owner because __mutex_unlock_slowpath() can
>> clear the owner field. But if owner is set, it must stay valid because
>> FLAG_WAITERS and we're holding wait_lock.
>
> If __ww_mutex_wound() is called from ww_mutex_set_context_fastpath() 
> owner is the current process so we can never see !owner. However if 
> __ww_mutex_wound() is called from __ww_mutex_add_waiter() then the 
> above is true.

Or actually it was intended to be true, but FLAG_WAITERS is set too 
late. It needs to be moved to just after we actually add the waiter to 
the list.

Then the hunk that replaces a FLAG_WAITERS read with a lockless 
list_empty() can also be ditched.

/Thomas


>
>>
>> So the wake_up_process() is in fact safe.
>>
>> Let me put that in a comment.
>
>
> Thanks,
>
> Thomas
>
>