linux-kernel - Re: [RFC][PATCH 7/7] sched: Replace rq::wake

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Date:   Fri, 5 Jun 2020 09:41:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Guenter Roeck <linux@...ck-us.net>, tglx@...utronix.de,
        frederic@...nel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        cai@....pw, mgorman@...hsingularity.net
Subject: Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list

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@...radead.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