Messages in this thread | | | Date | Thu, 4 Jun 2020 07:18:37 -0700 | From | Guenter Roeck <> | Subject | Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list |
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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
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