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io_uring: use signal based task_work running

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Since 5.7, we've been using task_work to trigger async running of
requests in the context of the original task. This generally works
great, but there's a case where if the task is currently blocked
in the kernel waiting on a condition to become true, it won't process
task_work. Even though the task is woken, it just checks whatever
condition it's waiting on, and goes back to sleep if it's still false.

This is a problem if that very condition only becomes true when that
task_work is run. An example of that is the task registering an eventfd
with io_uring, and it's now blocked waiting on an eventfd read. That
read could depend on a completion event, and that completion event
won't get trigged until task_work has been run.

Use the TWA_SIGNAL notification for task_work, so that we ensure that
the task always runs the work when queued.

Cc: stable@vger.kernel.org # v5.7
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2020-06-30 12:39:05 -06:00
parent e91b481623
commit ce593a6c48
1 changed files with 24 additions and 8 deletions
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32
fs/io_uring.c
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@ -4072,6 +4072,21 @@ struct io_poll_table {
int error;
};
static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb,
int notify)
{
struct task_struct *tsk = req->task;
int ret;
if (req->ctx->flags & IORING_SETUP_SQPOLL)
notify = 0;
ret = task_work_add(tsk, cb, notify);
if (!ret)
wake_up_process(tsk);
return ret;
}
static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,
__poll_t mask, task_work_func_t func)
{
@ -4095,13 +4110,13 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,
* of executing it. We can't safely execute it anyway, as we may not
* have the needed state needed for it anyway.
*/
ret = task_work_add(tsk, &req->task_work, true);
ret = io_req_task_work_add(req, &req->task_work, TWA_SIGNAL);
if (unlikely(ret)) {
WRITE_ONCE(poll->canceled, true);
tsk = io_wq_get_task(req->ctx->io_wq);
task_work_add(tsk, &req->task_work, true);
task_work_add(tsk, &req->task_work, 0);
wake_up_process(tsk);
}
wake_up_process(tsk);
return 1;
}
@ -6182,19 +6197,20 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
do {
prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,
TASK_INTERRUPTIBLE);
/* make sure we run task_work before checking for signals */
if (current->task_works)
task_work_run();
if (signal_pending(current)) {
ret = -ERESTARTSYS;
break;
}
if (io_should_wake(&iowq, false))
break;
schedule();
if (signal_pending(current)) {
ret = -EINTR;
break;
}
} while (1);
finish_wait(&ctx->wait, &iowq.wq);
restore_saved_sigmask_unless(ret == -EINTR);
restore_saved_sigmask_unless(ret == -ERESTARTSYS);
return READ_ONCE(rings->cq.head) == READ_ONCE(rings->cq.tail) ? ret : 0;
}