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    SubjectRe: [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@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

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