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Date:   Tue, 9 Jun 2020 13:21:34 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Peter Zijlstra <peterz@infradead.org>, tglx@linutronix.de,
        frederic@kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org,
        cai@lca.pw, mgorman@techsingularity.net
Subject: Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list
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On Sat, Jun 06, 2020 at 04:13:33PM -0700, Guenter Roeck wrote:
> On 6/5/20 9:15 AM, Eric Biggers wrote:
> > On Fri, Jun 05, 2020 at 09:41:54AM +0200, Peter Zijlstra wrote:
> >> On Thu, Jun 04, 2020 at 05:24:33PM -0700, Eric Biggers wrote:
> >>> On Thu, Jun 04, 2020 at 07:18:37AM -0700, Guenter Roeck wrote:
> >>>> On Tue, May 26, 2020 at 06:11:04PM +0200, Peter Zijlstra wrote:
> >>>>> The recent commit: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
> >>>>> got smp_call_function_single_async() subtly wrong. Even though it will
> >>>>> return -EBUSY when trying to re-use a csd, that condition is not
> >>>>> atomic and still requires external serialization.
> >>>>>
> >>>>> The change in ttwu_queue_remote() got this wrong.
> >>>>>
> >>>>> While on first reading ttwu_queue_remote() has an atomic test-and-set
> >>>>> that appears to serialize the use, the matching 'release' is not in
> >>>>> the right place to actually guarantee this serialization.
> >>>>>
> >>>>> The actual race is vs the sched_ttwu_pending() call in the idle loop;
> >>>>> that can run the wakeup-list without consuming the CSD.
> >>>>>
> >>>>> Instead of trying to chain the lists, merge them.
> >>>>>
> >>>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> >>>>> ---
> >>>> ...
> >>>>> +	/*
> >>>>> +	 * Assert the CSD_TYPE_TTWU layout is similar enough
> >>>>> +	 * for task_struct to be on the @call_single_queue.
> >>>>> +	 */
> >>>>> +	BUILD_BUG_ON(offsetof(struct task_struct, wake_entry_type) - offsetof(struct task_struct, wake_entry) !=
> >>>>> +		     offsetof(struct __call_single_data, flags) - offsetof(struct __call_single_data, llist));
> >>>>> +
> >>>>
> >>>> There is no guarantee in C that
> >>>>
> >>>> 	type1 a;
> >>>> 	type2 b;
> >>>>
> >>>> in two different data structures means that offsetof(b) - offsetof(a)
> >>>> is the same in both data structures unless attributes such as
> >>>> __attribute__((__packed__)) are used.
> >>>>
> >>>> As result, this does and will cause a variety of build errors depending
> >>>> on the compiler version and compile flags.
> >>>>
> >>>> Guenter
> >>>
> >>> Yep, this breaks the build for me.
> >>
> >> -ENOCONFIG
> > 
> > For me, the problem seems to be randstruct.  To reproduce, you can use
> > (on x86_64):
> > 
> > 	make defconfig
> > 	echo CONFIG_GCC_PLUGIN_RANDSTRUCT=y >> .config
> > 	make olddefconfig
> > 	make kernel/smp.o
> > 
> 
> I confirmed that disabling CONFIG_GCC_PLUGIN_RANDSTRUCT "fixes" the problem
> in my test builds. Maybe it would make sense to mark that configuration option
> for the time being as BROKEN.
> 

Still occurring on Linus' tree.  This needs to be fixed.  (And not by removing
support for randstruct; that's not a "fix"...)

Shouldn't the kbuild test robot have caught this?

- Eric