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

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

On Mon, 2020-09-07 at 22:09 +0200, Tomas Henzl wrote:
> On 9/7/20 7:46 PM, 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.
> 
> I think that the problem is that async scan uses callbacks to the
> module and when the module is being removed during scan it is not
> protected.

As I said above: the module shouldn't be freed until the scans are
completed or aborted ... I don't think we have a use after free
problem.  What you show below seems to be a deadlock:

> modprobe mpt3sas && rmmod  mpt3sas
> 
> [  370.031614] INFO: task rmmod:3120 blocked for more than 120
> seconds.
> [  370.037967]       Not tainted 4.18.0-193.el8.x86_64 #1
> [  370.043105] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> disables this message.
> [  370.050931] rmmod           D    0  3120   2460 0x00004080
> [  370.056414] Call Trace:
> [  370.058889]  ? __schedule+0x24f/0x650
> [  370.062554]  schedule+0x2f/0xa0
> [  370.065738]  async_synchronize_cookie_domain+0xad/0x140
> [  370.070983]  ? finish_wait+0x80/0x80
> [  370.074580]  __x64_sys_delete_module+0x166/0x280
> [  370.079198]  do_syscall_64+0x5b/0x1a0
> [  370.082876]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> [  370.087946] RIP: 0033:0x7f6de460a7db
> [  370.091534] Code: Bad RIP value.
> [  370.094777] RSP: 002b:00007ffe9971e798 EFLAGS: 00000206 ORIG_RAX:
> 00000000000000b0
> [  370.102341] RAX: ffffffffffffffda RBX: 00005592370d37b0 RCX:
> 00007f6de460a7db
> [  370.109481] RDX: 000000000000000a RSI: 0000000000000800 RDI:
> 00005592370d3818
> [  370.116606] RBP: 0000000000000000 R08: 00007ffe9971d711 R09:
> 0000000000000000
> [  370.123748] R10: 00007f6de467c8e0 R11: 0000000000000206 R12:
> 00007ffe9971e9c0
> [  370.130888] R13: 00007ffe99720333 R14: 00005592370d32a0 R15:
> 00005592370d37b0

This seems to be showing something different: I think the
async_synchronize_full() in delete_module is where we're stuck. That
seems to indicate something has just stopped inside the async scan code
... likely due to something reacting badly to scsi_device_get()
failing.

James