Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list

[Eric Biggers]

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@xxxxxxxxxxxxx>
> > ---
> ...
> > +	/*
> > +	 * 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.

  CC      kernel/smp.o
In file included from ./arch/x86/include/asm/atomic.h:5,
                 from ./include/linux/atomic.h:7,
                 from ./include/linux/llist.h:51,
                 from ./include/linux/irq_work.h:5,
                 from kernel/smp.c:10:
kernel/smp.c: In function âsmp_initâ:
./include/linux/compiler.h:403:38: error: call to â__compiletime_assert_68â declared with attribute error: BUILD_BUG_ON failed: 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)
  403 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                      ^
./include/linux/compiler.h:384:4: note: in definition of macro â__compiletime_assertâ
  384 |    prefix ## suffix();    \
      |    ^~~~~~
./include/linux/compiler.h:403:2: note: in expansion of macro â_compiletime_assertâ
  403 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro âcompiletime_assertâ
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:50:2: note: in expansion of macro âBUILD_BUG_ON_MSGâ
   50 |  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |  ^~~~~~~~~~~~~~~~
kernel/smp.c:687:2: note: in expansion of macro âBUILD_BUG_ONâ
  687 |  BUILD_BUG_ON(offsetof(struct task_struct, wake_entry_type) - offsetof(struct task_struct, wake_entry) !=
      |  ^~~~~~~~~~~~
make[1]: *** [scripts/Makefile.build:267: kernel/smp.o] Error 1
make: *** [Makefile:1735: kernel] Error 2