在 2021/8/21 上午6:09, Jens Axboe 写道:
On 8/20/21 3:32 PM, Pavel Begunkov wrote:
On 8/20/21 9:39 PM, Hao Xu wrote:
在 2021/8/21 上午2:59, Pavel Begunkov 写道:
On 8/20/21 7:40 PM, Hao Xu wrote:
coml_nr in ctx_flush_and_put() is not protected by uring_lock, this
may cause problems when accessing it parallelly.
Did you hit any problem? It sounds like it should be fine as is:
The trick is that it's only responsible to flush requests added
during execution of current call to tctx_task_work(), and those
naturally synchronised with the current task. All other potentially
enqueued requests will be of someone else's responsibility.
So, if nobody flushed requests, we're finely in-sync. If we see
0 there, but actually enqueued a request, it means someone
actually flushed it after the request had been added.
Probably, needs a more formal explanation with happens-before
and so.
I should put more detail in the commit message, the thing is:
say coml_nr > 0
ctx_flush_and put other context
if (compl_nr) get mutex
coml_nr > 0
do flush
coml_nr = 0
release mutex
get mutex
do flush (*)
release mutex
in (*) place, we do a bunch of unnecessary works, moreover, we
I wouldn't care about overhead, that shouldn't be much
call io_cqring_ev_posted() which I think we shouldn't.
IMHO, users should expect spurious io_cqring_ev_posted(),
though there were some eventfd users complaining before, so
for them we can do
It does sometimes cause issues, see:
commit b18032bb0a883cd7edd22a7fe6c57e1059b81ed0
Author: Jens Axboe <axboe@xxxxxxxxx>
Date: Sun Jan 24 16:58:56 2021 -0700
io_uring: only call io_cqring_ev_posted() if events were posted
I would tend to agree with Hao here, and the usual optimization idiom
looks like:
if (struct->nr) {
mutex_lock(&struct->lock);
if (struct->nr)
do_something();
mutex_unlock(&struct->lock);
}
like you posted, which would be fine and avoid this whole discussion :-)
Hao, care to spin a patch that does that?
no problem.
