Re: [PATCH] hwmon: (nzxt-kraken2) mark and order concurrent accesses

[Jonas Malaco]

On Mon, Mar 29, 2021 at 06:01:00PM -0700, Guenter Roeck wrote:
> On 3/29/21 5:21 PM, Jonas Malaco wrote:
> > On Mon, Mar 29, 2021 at 02:53:39PM -0700, Guenter Roeck wrote:
> >> On Mon, Mar 29, 2021 at 05:22:01AM -0300, Jonas Malaco wrote:
> >>> To avoid a spinlock, the driver explores concurrent memory accesses
> >>> between _raw_event and _read, having the former updating fields on a
> >>> data structure while the latter could be reading from them.  Because
> >>> these are "plain" accesses, those are data races according to the Linux
> >>> kernel memory model (LKMM).
> >>>
> >>> Data races are undefined behavior in both C11 and LKMM.  In practice,
> >>> the compiler is free to make optimizations assuming there is no data
> >>> race, including load tearing, load fusing and many others,[1] most of
> >>> which could result in corruption of the values reported to user-space.
> >>>
> >>> Prevent undesirable optimizations to those concurrent accesses by
> >>> marking them with READ_ONCE() and WRITE_ONCE().  This also removes the
> >>> data races, according to the LKMM, because both loads and stores to each
> >>> location are now "marked" accesses.
> >>>
> >>> As a special case, use smp_load_acquire() and smp_load_release() when
> >>> loading and storing ->updated, as it is used to track the validity of
> >>> the other values, and thus has to be stored after and loaded before
> >>> them.  These imply READ_ONCE()/WRITE_ONCE() but also ensure the desired
> >>> order of memory accesses.
> >>>
> >>> [1] https://lwn.net/Articles/793253/
> >>>
> >>
> >> I think you lost me a bit there. What out-of-order accesses that would be
> >> triggered by a compiler optimization are you concerned about here ?
> >> The only "problem" I can think of is that priv->updated may have been
> >> written before the actual values. The impact would be ... zero. An
> >> attribute read would return "stale" data for a few microseconds.
> >> Why is that a concern, and what difference does it make ?
> > 
> > The impact of out-of-order accesses to priv->updated is indeed minimal.
> > 
> > That said, smp_load_acquire() and smp_store_release() were meant to
> > prevent reordering at runtime, and only affect architectures other than
> > x86.  READ_ONCE() and WRITE_ONCE() would already prevent reordering from
> > compiler optimizations, and x86 provides the load-acquire/store-release
> > semantics by default.
> > 
> > But the reordering issue is not a concern to me, I got carried away when
> > adding READ_ONCE()/WRITE_ONCE().  While smp_load_acquire() and
> > smp_store_release() make the code work more like I intend it to, they
> > are (small) costs we can spare.
> > 
> > I still think that READ_ONCE()/WRITE_ONCE() are necessary, including for
> > priv->updated.  Do you agree?
> > 
> 
> No. What is the point ? The order of writes doesn't matter, the writes won't
> be randomly dropped, and it doesn't matter if the reader reports old values
> for a couple of microseconds either. This would be different if the values
> were used as synchronization primitives or similar, but that isn't the case
> here. As for priv->updated, if you are concerned about lost reports and
> the 4th report is received a few microseconds before the read, I'd suggest
> to loosen the interval a bit instead.
> 
> Supposedly we are getting reports every 500ms. We have two situations:
> - More than three reports are lost, making priv->updated somewhat relevant.
>   In this case, it doesn't matter if outdated values are reported for
>   a few uS since most/many/some reports are outdated more than a second
>   anyway.
> - A report is received but old values are reported for a few uS. That
>   doesn't matter either because reports are always outdated anyway by
>   much more than a few uS anyway, and the code already tolerates up to
>   2 seconds of lost reports.
> 
> Sorry, I completely fail to see the problem you are trying to solve here.

Please disregard the out-of-order accesses, I agree that preventing them
"are a (small) cost we can spare".

The main problem I still would like to address are the data races.
While the stores and loads cannot be dropped, and we can tolerate their
reordering, they could still be teared, fused, perhaps invented...
According to [1] these types of optimizations are not unheard.

Load tearing alone could easily produce values that are not stale, but
wrong.  Do we also tolerate wrong values, even if they are infrequent?

Another detail I should have mentioned sooner is that READ_ONCE() and
WRITE_ONCE() cause only minor (gcc) to no (clang) changes to the
generated code for x86_64 and i386.[2]  While this seems contrary to the
point I am trying to make, I want to show that, for the most part, these
changes just lock in a reasonable compiler behavior.

Specifically, on x86_64/gcc (the most relevant arch/compiler for this
driver) the changes are restricted to kraken2_read:

1.	Loading of priv->updated and jiffies are reordered, because
	(with READ_ONCE()) both are volatile and time_after(a, b) is
	defined as b - a.

2.	When loading priv->fan_input[channel],
		movzx  eax,WORD PTR [rdx+rcx*2+0x14]
	is split into
		add rcx,0x8
		movzx  eax,WORD PTR [rdx+rcx*2+0x4]
	for no reason I could find in the x86 manual.

3.	Similarly, when loading priv->temp_input[channel]
		movsxd rax,DWORD PTR [rdx+rcx*4+0x10]
	turns into
		add    rcx,0x4
		movsxd rax,DWORD PTR [rdx+rcx*4]

Both 2 and 3 admittedly get a bit worse with READ_ONCE()/WRITE_ONCE().
But that is on gcc, and with the data race it could very well decide to
produce much worse code than that at any moment.

On Arm64 the code does get a lot more ordered, which we have already
agreed is not really necessary.  But removing smp_load_acquire() and
smp_store_release() should still allow the CPU to reorder those,
mitigating some of the impact.

I hope this email clarifies what I am concerned about.

Thanks for the patience,
Jonas

P.S. Tested with gcc 10.2.0 and clang 11.1.0.

[1] https://lwn.net/Articles/793253/
[2] (outdated, still with smp_*()): https://github.com/jonasmalacofilho/patches/tree/master/linux/nzxt-kraken2-mark-and-order-concurrent-accesses/objdumps