Re: [PATCH V2] scsi: core: put LLD module refcnt after SCSI device is released

Ming Lei
2021-09-30T08:21:18Z

On Thu, Sep 30, 2021 at 10:07:44AM +0200, Greg Kroah-Hartman wrote:
> On Thu, Sep 30, 2021 at 03:40:26PM +0800, Ming Lei wrote:
> > SCSI host release is triggered when SCSI device is freed, and we have to
> > make sure that LLD module won't be unloaded before SCSI host instance is
> > released because shost->hostt is required in host release handler.
> >
> > So put LLD module refcnt after SCSI device is released.
> >
> > The real release handler can be run from wq context in case of
> > in_interrupt(), so add one atomic counter for serializing putting
> > module via current and wq context. This way is fine since we don't
> > call scsi_device_put() in fast IO path.
> >
> > Reported-by: Changhui Zhong <czhong@redhat.com>
> > Reported-by: Yi Zhang <yi.zhang@redhat.com>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> > drivers/scsi/scsi.c | 8 +++++++-
> > drivers/scsi/scsi_sysfs.c | 10 ++++++++++
> > include/scsi/scsi_device.h | 2 ++
> > 3 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
> > index b241f9e3885c..b6612161587f 100644
> > --- a/drivers/scsi/scsi.c
> > +++ b/drivers/scsi/scsi.c
> > @@ -553,8 +553,14 @@ EXPORT_SYMBOL(scsi_device_get);
> > */
> > void scsi_device_put(struct scsi_device *sdev)
> > {
> > - module_put(sdev->host->hostt->module);
> > + struct module *mod = sdev->host->hostt->module;
> > +
> > + atomic_inc(&sdev->put_dev_cnt);
>
> Ick, no! Why are you making a new lock and reference count for no
> reason?

The reason is to make sure that the LLD module is only put from either
scsi_device_put() and scsi_device_dev_release_usercontext().

>
> > +
> > put_device(&sdev->sdev_gendev);
> > +
> > + if (atomic_dec_if_positive(&sdev->put_dev_cnt) >= 0)
> > + module_put(mod);
>
> How do you know if your module pointer is still valid here?

module refcnt is grabbed in scsi_device_get(), so it is valid.

>
> Why do you care?
>
> What problem are you trying to solve and why is it unique to scsi
> devices?

See it from the commit log:

SCSI host release is triggered when SCSI device is freed, and we have to
make sure that LLD module won't be unloaded before SCSI host instance is
released because shost->hostt is required in host release handler.

So put LLD module refcnt after SCSI device is released.

and the upstream report on the issue:

https://lore.kernel.org/linux-block/CAHj4cs8XNtkzbbiLnFmVu82wYeQpLkVp6_wCtrnbhODay+OP9w@mail.gmail.com/

Thanks,
Ming