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SubjectRe: [PATCH -mm 4/5][AIO] - AIO completion signal notification
I'm a little bit unhappy about the usage of the notify flag.  The usage
seems correct but very confusing:

In io_submit_one we set ki_notify.notify to SIGEV_NONE and possibly
call aio_setup_sigevent:

> + /* handle setting up the sigevent for POSIX AIO signals */
> + req->ki_notify.notify = SIGEV_NONE;
> +
> + if (iocb->aio_sigeventp) {
> + ret = aio_setup_sigevent(&req->ki_notify,
> + (struct sigevent __user *)(unsigned
> long)
> + iocb->aio_sigeventp);
> + if (ret)
> + goto out_put_req;
> + }
> +

aio_setup_sigevent then checks the user passed even for which notify type
we have, and returns if it's none or otherwise sets notify->notify to it.

> + if (event.sigev_notify == SIGEV_NONE)
> + return 0;
> +
> + notify->notify = event.sigev_notify;

Later aio_setup_sigevent gets a reference to the target task_structure
if notify->notify is (SIGEV_SIGNAL|SIGEV_THREAD_ID) but _always_ stores
the target pointer.

> + if (notify->notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID)) {
> + /*
> + * This reference will be dropped in really_put_req() when
> + * we're done with the request.
> + */
> + get_task_struct(target);
> + }
> +
> + notify->target = target;


Once we're done with the iocb aio_complete aclls aio_send_signal if
notify.notify is not SIGEV_NONE.

> + if (iocb->ki_notify.notify != SIGEV_NONE) {
> + ret = aio_send_signal(&iocb->ki_notify);
> +
> + /* If signal generation failed, release the sigqueue */
> + if (ret)
> + sigqueue_free(iocb->ki_notify.sigq);
> + }
> +

Which then uses notify->target to send the signal:
> + if (notify->notify & SIGEV_THREAD_ID)
> + ret = send_sigqueue(notify->signo, sigq, notify->target);
> + else
> + ret = send_group_sigqueue(notify->signo, sigq, notify->target);

And finally really_put_req puts the target if notify.notify contains
either SIGEV_SIGNAL or SIGEV_THREAD_ID.

> + /* Release task ref */
> + if (req->ki_notify.notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID))
> + put_task_struct(req->ki_notify.target);

Do you see the confusing? I think all the notify.notify != SIGEV_NONE
in the above code should be replaces by the much more descriptive
notify.notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID). In addition we should
only store the target pointer inside the (SIGEV_SIGNAL|SIGEV_THREAD_ID)
if block that gets a reference to it.
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