Re: [PATCH] scsi: take module reference during async scan

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2020-09-07 at 17:32 -0400, Douglas Gilbert wrote:
> On 2020-09-07 1:46 p.m., James Bottomley wrote:
> > On Mon, 2020-09-07 at 17:47 +0200, Tomas Henzl wrote:
> > > During an async scan the driver shost->hostt structures are used,
> > > that may cause issues when the driver is removed at that time.
> > > As protection take the module reference.
> > 
> > Can I just ask what issues?  Today, our module model is that
> > scsi_device_get() bumps the module refcount and therefore makes the
> > module ineligible to be removed.  scsi_host_get() doesn't do this
> > because the way the host model is supposed to be coded, we can call
> > remove at any time but the module won't get freed until the last
> > put of the host.  I can see we have a potential problem with
> > scsi_forget_host() racing with the async scan thread ... is that
> > what you see? What's supposed to happen is that scsi_device_get()
> > starts failing as soon as the module begins it's exit routine, so
> > if a scan is in progress, it can't add any new devices ... in
> > theory this means that the list is stable for scsi_forget_host(),
> > so knowing how that assumption is breaking would be useful.
> 
> James,
> If you think it is bullet-proof try using 

I'm not saying it's got no bugs, just that the above is the way it's
supposed to work.

> CONFIG_DEBUG_TEST_DRIVER_REMOVE=y .

The problem with this option is it basically gives you a thundering
herd of removal reinsertions ... trying to do it for a single driver
(or set of drivers) is likely a better way to get actionable debugging
information.

> John Garry reported that:
> 
>   # insmod scsi_debug.ko
> 
> Gave errors like this:
> 
> [  140.115244] debugfs: Directory 'sde' with parent 'block' already
> present!
> [  140.376426] debugfs: Directory 'sde' with parent 'block' already
> present!
> [  140.420613] sd 3:0:0:0: [sde] tag#40 access beyond end of device
> [  140.426655] blk_update_request: I/O error, dev sde, sector 15984
> op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
> [  140.437319] sd 3:0:0:0: [sde] tag#41 access beyond end of device
> [  140.443368] blk_update_request: I/O error, dev sde, sector 15984
> op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
> ...
> 
> Which wasn't the scsi_debug driver directly as it doesn't use
> debugfs. So I suspect something is rotten in the mid-level.
> 
> When I tried to replicate John's config I couldn't even boot my
> Ubuntu 20.04 based system (with a MKP kernel). Seemed to fail/lockup
> before any kernel prints came out to the serial port (yes, still
> useful), perhaps in initrd. I'm guessing another, non-SCSI module
> caused the lockup. So I gave up and turned off that config setting.

If that can be distilled down to a better test case, I can look into
it.

James