Re: [PATCH] 2.4.21-pre7 ide request races

[Andrew Morton]

Jens Axboe <[e-mail]> wrote:
>
> We've had some problems with request corruption on IDE in the past, IBM
> traced these to stack corruption. In various places, the IDE code does
> something ala:
> 
> submission:
> 	struct request rq;
> 
> 	...
> 	ide_do_drive_cmd(drive, &rq, ide_wait);
> 
> ide_end_request:
> 	...
> 	blkdev_release_request()
> 
> which works fine, as long as the stack persists for the
> blkdev_release_request() call, but it may not if the task has already
> exited (CPU0 may be waiting in ide_do_drive_cmd(), CPU1 gets the
> completion interrupt, task is woken, and exits, CPU0 now calls
> blkdev_release_request()). The result is random stack corruption (or
> request list corruption, rq->q may have been scrippled!), not good.
> 

Those locally allocated requests are foul, and the patch is a good cleanup,
but is a simpler fix more appropriate?

The bug is in end_that_request_last(), yes?

void end_that_request_last(struct request *req)
{
	if (req->waiting != NULL)
		complete(req->waiting);
	req_finished_io(req);

	blkdev_release_request(req);
}

Wouldn't it be simpler to just do:

void end_that_request_last(struct request *req)
{
	struct completion *c = req->waiting;

	req_finished_io(req);
	blkdev_release_request(req);
	if (c)
		complete(c);
}

I vaguely seem to remember being told months ago why this wasn't right ;)