Re: [PATCH v2 1/1] scsi: Fix racing between dev init and dev reset

[Alice Chao]

Hi Miles,

On Wed, 2022-03-09 at 07:23 +0800, Miles Chen wrote:
> Hi Alice,
> 
> Thanks for your patch. I have some questions.
> 
> > Device reset thread uses kobject_uevent_env() to get kobj.parent
> > after scsi_evt_emit(), and it races with device init thread which
> > calls device_add() to create kobj.parent before
> > kobject_uevent_env().
> > 
> > Device reset call trace:
> > fill_kobj_path
> > kobject_get_path
> > kobject_uevent_env
> > scsi_evt_emit			<- add wait_event()
> > scsi_evt_thread
> > 
> > Device init call trace:
> > fill_kobj_path
> > kobject_get_path
> > kobject_uevent_env
> > device_add				<- create kobj.parent
> > scsi_target_add
> > scsi_sysfs_add_sdev
> > scsi_add_lun
> > scsi_probe_and_add_lun
> 
> These are backtraces, and the things above looks like your solution,
> not
> the problem.
> It's better to describe the problem first and solution later:
> 
> Device init call:                              Device reset call:   
> scsi_probe_and_add_lun()                       scsi_evt_thread()
>   scsi_add_lun()
>     scsi_sysfs_add_sdev()                        scsi_evt_emit()
>       scsi_target_add()                            kobject_uevent_env
> () 
>         device_add()                                 kobject_get_path
> () // something wrong?
> 	                                               fill_kobj_path()
> 	  get_device_parent() // I see a get, not create parent kobj,
> could you show me that point?
> 	  kobject_uevent()
>             kobject_uevent_env()
>               kobject_get_path()
>                 fill_kobj_path()
> 

Device init call:	   Device reset call:
scsi_probe_and_add_lun()   scsi_evt_thread()
  scsi_add_lun()	    scsi_evt_emit()
   scsi_sysfs_add_sdev()     kobject_uevent_env() //get kobj.parent
    scsi_target_add()	      kobject_get_path() //get wrong
kobj.parent     device_add() // add kobj.parent  fill_kobj_path()
      kobject_uevent_env()
       kobject_get_path()
        fill_kobj_path()
Above backtrace describes the problem, device reset thread will get
wrong kobj.parent when device init thread didn’t add kobj.parent yet.

> > 
> > These two jobs are scheduled asynchronously, we can't guaranteed
> > that
> > kobj.parent will be created in device init thread before device
> > reset
> > thread calls kobj_get_path().
> 
> What happens if device reset thread calls kobj_get_path() before
> kobj.parent's
> creation? a crash or something?
> 

It will cause slab-out-of-bounds in kobject_get_path and kernel panic
than crash.

> > 
> > To resolve the racing issue between device init thread and device
> > reset thread, we use wait_event() in scsi_evt_emit() to wait for
> > device_add() to complete the creation of kobj.parent.
> > 
> > Signed-off-by: Alice Chao <alice.chao@xxxxxxxxxxxx>
> 
> When we send patch N+1, we have to put the change log between patch
> v1 to vN
> under a "---", so others can understand the changes between patch N
> and N+1.
> e.g.,
> 
> ---
> 
> Change since v1
> - remove Change-Id
> 
> > ---
> >  drivers/scsi/scsi_lib.c  | 1 +
> >  drivers/scsi/scsi_scan.c | 1 +
> >  2 files changed, 2 insertions(+)
> > 
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index 0a70aa763a96..abf9a71ed77c 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -2461,6 +2461,7 @@ static void scsi_evt_emit(struct scsi_device
> > *sdev, struct scsi_event *evt)
> >  		break;
> >  	case SDEV_EVT_POWER_ON_RESET_OCCURRED:
> >  		envp[idx++] = "SDEV_UA=POWER_ON_RESET_OCCURRED";
> > +		wait_event(sdev->host->host_wait, sdev-
> > >sdev_gendev.kobj.parent != NULL);
> >  		break;
> >  	default:
> >  		/* do nothing */
> > diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
> > index f4e6c68ac99e..431f229ac435 100644
> > --- a/drivers/scsi/scsi_scan.c
> > +++ b/drivers/scsi/scsi_scan.c
> > @@ -1904,6 +1904,7 @@ static void do_scsi_scan_host(struct
> > Scsi_Host *shost)
> >  	} else {
> >  		scsi_scan_host_selected(shost, SCAN_WILD_CARD,
> > SCAN_WILD_CARD,
> >  				SCAN_WILD_CARD, 0);
> > +		wake_up(&shost->host_wait);
> 
> do_scsi_scan_host() is not in the call trace above, could you show
> the relationship
> of do_scsi_scan_host in the call flow?
> 

ufshcd_async_scan()
scsi_scan_host()
do_scsi_scan_host()  << here
scsi_scan_host_selected()
 __scsi_scan_target()
scsi_probe_and_add_lun()
scsi_sysfs_add_sdev()
device_add()
kobject_uevent_env()
kobject_get_path()
fill_kobj_path()

After we add wake_up here, we can ensure that device reset thread will
get kobject after device init thread finishes adding parent.

> thanks,
> Miles
> 
> >  	}
> >  }