linux-kernel - Re: bug in elevator/cfq regarding start

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sun, 4 Oct 2009 22:59:55 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: bug in elevator/cfq regarding start_time?

On Sun, Oct 04 2009, Corrado Zoccolo wrote:
> Hi Jens,
> I found that the start_time is not monotonic in the cfqq fifo_list.
> The problem seems to be that:
> * attempt_merge overrides one of the start_time:
>         if (time_after(req->start_time, next->start_time))
>                 req->start_time = next->start_time;
> * then it calls elv_merge_requests, that in turn invokes cfq's
> elevator_merge_req_fn, i.e. cfq_merged_requests
> * cfq_merged_requests checks start_times to decide whether requests
> need to be repositioned
>         if (!list_empty(&rq->queuelist) && !list_empty(&next->queuelist) &&
>             time_before(next->start_time, rq->start_time))
>                 list_move(&rq->queuelist, &next->queuelist);
> 
> Other I/O schedulers are not affected, since none of them uses
> start_time explicitly, but they have a copy in one of the private
> fields.

Good spotting! So we can either add the same copy in cfq, or we can just
move the ->start_time check and assignment in attempt_merge() to post
the elv_merge_requests() call. I think the latter would work fine, but
I'll check and fix tomorrow.

-- 
Jens Axboe

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/