On 06/14/2018 01:49 PM, Andrea Parri wrote:
[...]+ /* + * 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.IIUC, "sufficient barriers" = full memory barriers (here). (You may want to be more specific.)Thanks for reviewing! OK. What about if someone relaxes that in the future?This is actually one of my main concerns ;-) as, IIUC, those barriers are not only sufficient but also necessary: anything "less than a full barrier" (in either wake_up_process() or set_current_state()) would _not_ guarantee the "condition" above unless I'm misunderstanding it. But am I misunderstanding it? Which barriers/guarantee do you _need_ from the above mentioned pairing? (hence my comment...) Andrea
No you are probably not misunderstanding me at all. My comment originated from the reading of the kerneldoc of set_current_state()
* set_current_state() includes a barrier so that the write of current->state
* is correctly serialised wrt the caller's subsequent test of whether to
* actually sleep:
*
* for (;;) {
* set_current_state(TASK_UNINTERRUPTIBLE);
* if (!need_sleep)
* break;
*
* schedule();
* }
* __set_current_state(TASK_RUNNING);
*
* If the caller does not need such serialisation (because, for instance, the
* condition test and condition change and wakeup are under the same
lock) then
* use __set_current_state(). * * The above is typically ordered against the wakeup, which does: * * need_sleep = false; * wake_up_state(p, TASK_UNINTERRUPTIBLE); * * Where wake_up_state() (and all other wakeup primitives) imply enough* barriers to order the store of the variable against wakeup. And with ctx->wounded := !need_sleep this exactly matches what's happening in my code. So what I was trying to say in the comment was that this above contract is sufficient to guarantee the "condition" above, whitout me actually knowing exactly what barriers are required. Thanks, Thomas
