[PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes
Thomas Hellstrom
thellstrom at vmware.com
Thu Jun 14 12:48:16 UTC 2018
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.
>
> So the wake_up_process() is in fact safe.
>
> Let me put that in a comment.
Thanks,
Thomas
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