linux-kernel - Re: [PATCH RFC stable 4.14 1/1] mmc: core: fix hung task caused by race condition on context

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Date:   Thu, 29 Sep 2022 10:13:49 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "dinggao.pan" <dinggao.pan@...izon.ai>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "ming.yu" <ming.yu@...izon.ai>,
        "yunqian.wang" <yunqian.wang@...izon.ai>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [PATCH RFC stable 4.14 1/1] mmc: core: fix hung task caused by
 race condition on context_info

On Thu, 29 Sep 2022 10:07:50 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Thu, 29 Sep 2022 14:41:26 +0200
> Ulf Hansson <ulf.hansson@...aro.org> wrote:
> 
> > On Mon, 5 Sept 2022 at 08:22, dinggao.pan <dinggao.pan@...izon.ai> wrote:  
> > >
> > > Hi,
> > > After applying rt patches to our 4.14 kernel and enabling preempt-rt, we met a hung task during boot caused by race condition on context_info stored in struct mmc_host.
> > > From our investigation, context_info should not be changed by threads that have not claimed the host, hence the following fix.
> > >
> > > Any comments are much appreciated.
> > > Dinggao Pan    
> > 
> > Hi Dinggao,
> > 
> > Apologize for the delay.
> > 
> > The 4.14 kernel is too old for me to be able to comment. In
> > particular, the mmc block layer moved to blk-mq in v4.16, which means
> > the path you are investigating doesn't exist any more, sorry.  
> 
> Luis (Cc'd) is still supporting the 4.14-rt kernel.

It appears that I have an old email address for Luis, and it bounced.
Updated with his redhat one.

-- Steve

> 
> > 
> > Kind regards
> > Uffe
> >   
> > >
> > > From: "Dinggao Pan" <mailto:dinggao.pan@...izon.ai>
> > >
> > > 　　A race condition happens under following circumstances:
> > >     (mmc_thread1)               |              (mmc_thread2)
> > >     mmc_issue_rq(req1)          |    
> > >       > qcnt++ for req1         |    
> > >         host handling req1      |
> > >     mmc_queue_thread(req=null)  |    
> > >       > enter queue thread      |    
> > >         again, fetches blk req  |
> > >         (return null), sets     |
> > >         is_waiting_last_req 1   |  mmc_request_fn(req1) -> set is_new_req 1
> > >                                 |                   and wake_up wait_queue
> > >     mmc_issue_rq(req2)          |   > mmc_thread2 tries to claim host    
> > >       > **qcnt++ for req2**     |    
> > >       mmc_finalize_req(req2)    |    
> > >         > should wait for req1  |    
> > >           done but req2 return  |
> > >           MMC_BLK_NEW_REQ       |
> > >           due to is_new_req     |
> > >           already set to 1      |
> > >                                 |
> > >                                 |
> > >     req1 done                   |    
> > >       > qcnt-- for req1         |    
> > >     mmc_issue_rq(req3)          |    
> > >       > qcnt++ for req3         |    
> > > req2 is not handled but qcnt is already added(noted by **),
> > > thus mmc_thread1 will never release host, causing mmc_threads
> > > except thread1 to hung. Fix race by moving wake_up to the front of
> > > context_info update.
> > >
> > > Reviewed By: Yunqian Wang <mailto:yunqian.wang@...izon.ai>
> > > Signed-off-by: Dinggao Pan <mailto:dinggao.pan@...izon.ai>
> > > Signed-off-by: Ming Yu <mailto:ming.yu@...izon.ai>
> > > ---
> > > drivers/mmc/core/queue.c | 7 +++++--
> > > 1 file changed, 5 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c
> > > index 0a4e77a5b..58318c102 100644
> > > --- a/drivers/mmc/core/queue.c
> > > +++ b/drivers/mmc/core/queue.c
> > > @@ -107,6 +107,11 @@ static void mmc_request_fn(struct request_queue *q)
> > >                return;
> > >       }
> > >
> > > +      if (mq->asleep) {
> > > +               wake_up_process(mq->thread);
> > > +               return;
> > > +      }
> > > +
> > >       cntx = &mq->card->host->context_info;
> > >
> > >       if (cntx->is_waiting_last_req) {
> > > @@ -114,8 +119,6 @@ static void mmc_request_fn(struct request_queue *q)
> > >                wake_up_interruptible(&cntx->wait);
> > >       }
> > >
> > > -       if (mq->asleep)
> > > -                wake_up_process(mq->thread);
> > > }
> > >
> > > static struct scatterlist *mmc_alloc_sg(int sg_len, gfp_t gfp)
> > > --
> > > 2.36.1    
>