Re: Bisected GFP in bfq_bfqq_expire on v5.1-rc1

Dmitrii Tcvetkov
2019-04-09T16:14:30Z

On Tue, 9 Apr 2019 11:55:21 +0200
Paolo Valente <paolo.valente@linaro.org> wrote:

>
>
> > Il giorno 4 apr 2019, alle ore 21:22, Dmitrii Tcvetkov
> > <demfloro@demfloro.ru> ha scritto:
> >
> > On Mon, 1 Apr 2019 12:35:11 +0200
> > Paolo Valente <paolo.valente@linaro.org> wrote:
> >
> >>
> >>
> >>> Il giorno 1 apr 2019, alle ore 11:22, Dmitrii Tcvetkov
> >>> <demfloro@demfloro.ru> ha scritto:
> >>>
> >>> On Mon, 1 Apr 2019 11:01:27 +0200
> >>> Paolo Valente <paolo.valente@linaro.org> wrote:
> >>>> Ok, thank you. Could you please do a
> >>>>
> >>>> list *(bfq_bfqq_expire+0x1f3)
> >>>>
> >>>> for me?
> >>>>
> >>>> Thanks,
> >>>> Paolo
> >>>>
> >>>>>
> >>>>> <gpf.txt><gpf-w-bfq-group-iosched.txt><config.txt>
> >>>
> >>> Reading symbols from vmlinux...done.
> >>> (gdb) list *(bfq_bfqq_expire+0x1f3)
> >>> 0xffffffff813d02c3 is in bfq_bfqq_expire
> >>> (block/bfq-iosched.c:3390). 3385 * even in case bfqq
> >>> and thus parent entities go on receiving 3386 *
> >>> service with the same budget. 3387 */
> >>> 3388 entity = entity->parent;
> >>> 3389 for_each_entity(entity)
> >>> 3390 entity->service = 0;
> >>> 3391 }
> >>> 3392
> >>> 3393 /*
> >>> 3394 * Budget timeout is not implemented through a dedicated
> >>> timer, but
> >>
> >> Thank you very much. Unfortunately this doesn't ring any bell.
> >> I'm trying to reproduce the failure. It will probably take a
> >> little time. If I don't make it, I'll ask you to kindly retry
> >> after applying some instrumentation patch.
> >>
> >
> > I looked at what git is doing just before panic and it's doing a
> > lot of lstat() syscalls on working tree.
> >
> > I've attached a python script which reproduces the crash in about
> > 10 seconds after it prepares testdir, git checkout
> > origin/linux-5.0.y reproduces it in about 2 seconds. I have to use
> > multiprocessing Pool as I couldn't reproduce the crash using
> > ThreadPool, probably due to Python GIL.
> >
>
> Unfortunately this failure doesn't reproduce on my systems. But I
> have a suspect. Could you please test this patch? (also attached as a
> compressed file):
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index fac188dd78fa..0a435bcfed20 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2822,7 +2822,7 @@ static void bfq_dispatch_remove(struct
> request_queue *q, struct request *rq) bfq_remove_request(q, rq);
> }
>
> -static void __bfq_bfqq_expire(struct bfq_data *bfqd, struct
> bfq_queue *bfqq) +static bool __bfq_bfqq_expire(struct bfq_data
> *bfqd, struct bfq_queue *bfqq) {
> /*
> * If this bfqq is shared between multiple processes, check
> @@ -2857,7 +2857,7 @@ static void __bfq_bfqq_expire(struct bfq_data
> *bfqd, struct bfq_queue *bfqq)
> * or requeued before executing the next function, which
> * resets all in-service entites as no more in service.
> */
> - __bfq_bfqd_reset_in_service(bfqd);
> + return __bfq_bfqd_reset_in_service(bfqd);
> }
>
> /**
> @@ -3262,7 +3262,6 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
> bool slow;
> unsigned long delta = 0;
> struct bfq_entity *entity = &bfqq->entity;
> - int ref;
>
> /*
> * Check whether the process is slow (see bfq_bfqq_is_slow).
> @@ -3347,10 +3346,8 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
> * reason.
> */
> __bfq_bfqq_recalc_budget(bfqd, bfqq, reason);
> - ref = bfqq->ref;
> - __bfq_bfqq_expire(bfqd, bfqq);
> -
> - if (ref == 1) /* bfqq is gone, no more actions on it */
> + if (__bfq_bfqq_expire(bfqd, bfqq))
> + /* bfqq is gone, no more actions on it */
> return;
>
> bfqq->injected_service = 0;
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 062e1c4787f4..86394e503ca9 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -995,7 +995,7 @@ bool __bfq_deactivate_entity(struct bfq_entity
> *entity, bool ins_into_idle_tree);
> bool next_queue_may_preempt(struct bfq_data *bfqd);
> struct bfq_queue *bfq_get_next_queue(struct bfq_data *bfqd);
> -void __bfq_bfqd_reset_in_service(struct bfq_data *bfqd);
> +bool __bfq_bfqd_reset_in_service(struct bfq_data *bfqd);
> void bfq_deactivate_bfqq(struct bfq_data *bfqd, struct bfq_queue
> *bfqq, bool ins_into_idle_tree, bool expiration);
> void bfq_activate_bfqq(struct bfq_data *bfqd, struct bfq_queue
> *bfqq); diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
> index a11bef75483d..a0c60c47ed1c 100644
> --- a/block/bfq-wf2q.c
> +++ b/block/bfq-wf2q.c
> @@ -1605,7 +1605,7 @@ struct bfq_queue *bfq_get_next_queue(struct
> bfq_data *bfqd) return bfqq;
> }
>
> -void __bfq_bfqd_reset_in_service(struct bfq_data *bfqd)
> +bool __bfq_bfqd_reset_in_service(struct bfq_data *bfqd)
> {
> struct bfq_queue *in_serv_bfqq = bfqd->in_service_queue;
> struct bfq_entity *in_serv_entity = &in_serv_bfqq->entity;
> @@ -1629,8 +1629,18 @@ void __bfq_bfqd_reset_in_service(struct
> bfq_data *bfqd)
> * service tree either, then release the service reference to
> * the queue it represents (taken with bfq_get_entity).
> */
> - if (!in_serv_entity->on_st)
> + if (!in_serv_entity->on_st) {
> + /*
> + * bfqq may be freed here, if bfq_exit_bfqq(bfqq) has
> + * already been executed
> + */
> + int ref = in_serv_bfqq->ref;
> bfq_put_queue(in_serv_bfqq);
> + if (ref == 1)
> + return true;
> + }
> +
> + return false;
> }
>
> void bfq_deactivate_bfqq(struct bfq_data *bfqd, struct bfq_queue
> *bfqq,
>

Awesome! I can't reproduce the panic with the patch on top of current
master (869e3305f23dfe) in my VM and on baremetal machine. Reverting
the patch allows to reproduce it.