From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: dgilbert@interlog.com, Tomas Henzl <thenzl@redhat.com>,
linux-scsi@vger.kernel.org
Subject: Re: [PATCH] scsi: take module reference during async scan
Date: Mon, 07 Sep 2020 15:56:14 -0700 [thread overview]
Message-ID: <1599519374.4232.71.camel@HansenPartnership.com> (raw)
In-Reply-To: <a31b8640-307f-a14f-7cf8-7673fa8a4ff1@interlog.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
next prev parent reply other threads:[~2020-09-07 22:56 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-07 15:47 [PATCH] scsi: take module reference during async scan Tomas Henzl
2020-09-07 16:57 ` Bart Van Assche
2020-09-07 21:12 ` Tomas Henzl
2020-09-07 17:46 ` James Bottomley
2020-09-07 20:09 ` Tomas Henzl
2020-09-07 20:24 ` James Bottomley
2020-09-07 21:02 ` Tomas Henzl
2020-09-07 22:02 ` James Bottomley
2020-09-08 8:22 ` Tomas Henzl
2020-09-07 21:32 ` Douglas Gilbert
2020-09-07 22:56 ` James Bottomley [this message]
2020-09-08 14:04 ` John Garry
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