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

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Jun 09, 2020 at 01:21:34PM -0700, Eric Biggers wrote:
> 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"...)
> 

How about the hack below ?

Guenter

---
diff --git a/include/linux/sched.h b/include/linux/sched.h
index c5d96e3e7fff..df1cbb04f9b3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -629,6 +629,15 @@ struct wake_q_node {
 	struct wake_q_node *next;
 };
 
+/*
+ * Hack around assumption that wake_entry_type follows wake_entry even with
+ * CONFIG_GCC_PLUGIN_RANDSTRUCT=y.
+ */
+struct _wake_entry {
+	struct llist_node	wake_entry;
+	unsigned int		wake_entry_type;
+};
+
 struct task_struct {
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 	/*
@@ -653,8 +662,9 @@ struct task_struct {
 	unsigned int			ptrace;
 
 #ifdef CONFIG_SMP
-	struct llist_node		wake_entry;
-	unsigned int			wake_entry_type;
+	struct _wake_entry		_we;
+#define wake_entry		_we.wake_entry
+#define wake_entry_type		_we.wake_entry_type
 	int				on_cpu;
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 	/* Current CPU: */