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Date:   Tue, 19 Oct 2021 08:50:24 -0700
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, tj@kernel.org,
        gregkh@linuxfoundation.org, akpm@linux-foundation.org,
        minchan@kernel.org, jeyu@kernel.org, shuah@kernel.org,
        bvanassche@acm.org, dan.j.williams@intel.com, joe@perches.com,
        tglx@linutronix.de, keescook@chromium.org, rostedt@goodmis.org,
        linux-spdx@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate
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Sender: Luis Chamberlain <mcgrof@infradead.org>
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On Tue, Oct 19, 2021 at 10:34:41AM +0800, Ming Lei wrote:
> On Mon, Oct 18, 2021 at 12:32:11PM -0700, Luis Chamberlain wrote:
> > On Sat, Oct 16, 2021 at 07:28:39PM +0800, Ming Lei wrote:
> > > On Fri, Oct 15, 2021 at 10:31:31AM -0700, Luis Chamberlain wrote:
> > > > On Fri, Oct 15, 2021 at 04:36:11PM +0800, Ming Lei wrote:
> > > > > On Thu, Oct 14, 2021 at 05:22:40PM -0700, Luis Chamberlain wrote:
> > > > > > On Fri, Oct 15, 2021 at 07:52:04AM +0800, Ming Lei wrote:
> > > > > ...
> > > > > > > 
> > > > > > > We need to understand the exact reason why there is still cpuhp node
> > > > > > > left, can you share us the exact steps for reproducing the issue?
> > > > > > > Otherwise we may have to trace and narrow down the reason.
> > > > > > 
> > > > > > See my commit log for my own fix for this issue.
> > > > > 
> > > > > OK, thanks!
> > > > > 
> > > > > I can reproduce the issue, and the reason is that reset_store fails
> > > > > zram_remove() when unloading module, then the warning is caused.
> > > > > 
> > > > > The top 3 patches in the following tree can fix the issue:
> > > > > 
> > > > > https://github.com/ming1/linux/commits/my_v5.15-blk-dev
> > > > 
> > > > Thanks for trying an alternative fix! A crash stops yes, however this
> > > 
> > > I doubt it is alternative since your patchset doesn't mention the exact
> > > reason of 'Error: Removing state 63 which has instances left.', that is
> > > simply caused by failing to remove zram because ->claim is set during
> > > unloading module.
> > 
> > Well I disagree because it does explain how the race can happen, and it
> > also explains how since the sysfs interface is exposed until module
> > removal completes, it leaves exposed knobs to allow re-initializing of a
> > struct zcomp for a zram device before the exit.
> > 
> > > Yeah, you mentioned the race between disksize_store() vs. zram_remove(),
> > > however I don't think it is reproduced easily in the test because the race
> > > window is pretty small, also it can be fixed easily in my 3rd path
> > > without any complicated tricks.
> > 
> > Reproducing for me is... extremely easy.
> 
> In my observation, failing zram_remove() is extremely easy to trigger, which
> is caused by reset_store() which sets ->reclaim as true, so
> zram_remove() is failed and zram_reset_device() is bypassed , then the
> failure of 'Error: Removing state 63 which has instances left.' is caused.
> 
> We are in same page?

The actual first issue is the CPU hotplug remove callback is long gone and
in the meantime we allow a race to add a new "instance", in the zram
driver's case a cpu struct zcomp instance though the sysfs interface.

Regardless of if zram_remove() can fail or not, the above race needs to
be addressed.

> > > Not dig into details of your patchset via grabbing module reference
> > > count during show/store attribute of kernfs which is done in your patch
> > > 9, but IMO this way isn't necessary:
> > 
> > That's to address the deadlock only.
> > 
> > > 1) any driver module has to cleanup anything which may refer to symbols
> > > or data defined in module_exit of this driver
> > 
> > Yes, and as the cpu multistate hotplug documentation warns (although
> > such documentation is kind of hidden) that driver authors need to be
> > careful with module removal too, refer to the warning at the end of
> > __cpuhp_remove_state_cpuslocked() about module removal.
> 
> It is zram's bug. zram has to clean everything in module_exit(),
> unfortunately zram_remove() can be failed when calling from
> module_exit() because ->claim is set as true by reset_store(), then
> zram_reset_device()(->zcomp_destroy) isn't called, and this failure should
> not happen when unloading module, should it?

You're addressing a possible failig zram_remove() while I address not
allowing entry to muck with the zram driver at all once we're bailing
on module removal.

> > > 2) device_del() is often done in module_exit(), once device_del()
> > > returns, no any new show/store on the device's kobject attribute
> > > is possible.
> > 
> > Right and if a syfs knob is exposed before device_del() completely
> > and is allowed to do things, the driver should take care to prevent
> > races for CPU multistate support. The small state machine I added ensures
> 
> What is the race for CPU multistate support? If you mean 'Error: Removing
> state 63 which has instances left.', it is zram's bug since zram has to
> cleanup everything in module_exit().

Yes. And it is what my out of tree yet Acked patch, 'zram: fix     
crashes with cpu hotplug multistate' does.

> > we don't run over any expectations from cpu hotplug multistate support.
> > 
> > I've *never* suggested there cannot be alternatives to my solution with
> > the small state machine, but for you to say it is incorrect is simply
> > not right either.
> > 
> > > 3) it is _not_ a must or pattern for fixing bugs to hold one lock before
> > > calling device_del(), meantime the lock is required in the device's
> > > attribute show()/store(), which causes AA deadlock easily. Your approach
> > > just avoids the issue by not releasing module until all show/store are
> > > done.
> > 
> > Right, there are two approaches here:
> > 
> > a) Your approach is to accept the deadlock as a requirement and so
> > you would prefer to implement an alternative to using a shared lock
> > on module exit and sysfs op.
> 
> wrt. in-tree zram, there is neither any deadlock in linus tree, nor after
> applying my 3 patches. If you think there is, please share us the code
> or lockdep warning.

Right, 'zram: fix crashes with cpu hotplug multistate' is not yet
merged, my approach to fixing that does add a lock use on module removal
which does introduce a possible deadlock with syfs, which is later addressed
generically between sysfs and module removal for all drivers.

> > b) While I address such a deadlock head on as I think this sort of locking
> > be allowed for two reasons:
> >    b1) as we never documented such requirement otherwise.
> >    b2) There is a possibility that other drivers already exist too
> >        which *do* use a shared lock on module removal and sysfs ops
> >        (and I just confirmed this to be true)
> 
> The 'deadlock' is actually caused by your out-of-tree patch of 'zram: fix
> crashes with cpu hotplug multistate' which adds mutex_lock(zram_index_mutex)
> in destroy_devices().

Yes yes, but you are completely throwing out the window that other
possible deadlocks can exist in the kernel *and* that *new* cases of
the deadlock can easily also be added!

> We can fix this issue easily without needing the global lock, please see the
> attached(pre-V2) patch.

So far your patches do not fix the issues though...

> > So I *really* don't think it is wise for us to simply accept this new
> > found deadlock as a *new* requirement, specially if we can fix it easily.
> > 
> > A cursory review using Coccinelle potential issues with mutex lock
> > directly used on module exit (so this doesn't cover drivers like zram
> > which uses a routine and then grabs the lock through indirection) and a
> > sysfs op shows these drivers are also affected by this deadlock:
> > 
> >   * arch/powerpc/sysdev/fsl_mpic_timer_wakeup.c
> 
> In fsl_wakeup_sys_exit(), device_remove_file() is called before
> acquiring &sysfs_lock, so there shouldn't be such AA deadlock.
> 
> >   * lib/test_firmware.c
> 
> Yeah, there is the AA deadlock risk, but it should be fixed by moving
> misc_deregister() out of &test_fw_mutex.

And just like that you are ignoring other possible uses in the kernel
which might have similar deadlocks.

So do you want to take the position:

Hey driver authors: you cannot use any shared lock on module removal and
on sysfs ops?

  Luis