Re: [RFC][PATCH 5/7] irq_work, smp: Allow irq_work on call_single_queue

From: Peter Zijlstra
Date: Fri Jun 05 2020 - 05:37:20 EST

On Fri, May 29, 2020 at 03:36:41PM +0200, Peter Zijlstra wrote:
> Maybe I can anonymous-union my way around it, dunno. I'll think about
> it. I'm certainly not proud of this. But at least the BUILD_BUG_ON()s
> should catch the more blatant breakage here.

How's this then? Differently ugly, but at least it compiles with that
horrible struct randomization junk enabled.

---
include/linux/irq_work.h | 28 ++++++-------------
include/linux/sched.h | 4 +-
include/linux/smp.h | 25 ++++++-----------
include/linux/smp_types.h | 66 ++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched/core.c | 6 ++--
kernel/smp.c | 18 ------------
6 files changed, 89 insertions(+), 58 deletions(-)

--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -2,7 +2,7 @@
#ifndef _LINUX_IRQ_WORK_H
#define _LINUX_IRQ_WORK_H

-#include <linux/llist.h>
+#include <linux/smp_types.h>

/*
* An entry can be in one of four states:
@@ -13,26 +13,16 @@
* busy NULL, 2 -> {free, claimed} : callback in progress, can be claimed
*/

-/* flags share CSD_FLAG_ space */
-
-#define IRQ_WORK_PENDING BIT(0)
-#define IRQ_WORK_BUSY BIT(1)
-
-/* Doesn't want IPI, wait for tick: */
-#define IRQ_WORK_LAZY BIT(2)
-/* Run hard IRQ context, even on RT */
-#define IRQ_WORK_HARD_IRQ BIT(3)
-
-#define IRQ_WORK_CLAIMED (IRQ_WORK_PENDING | IRQ_WORK_BUSY)
-
-/*
- * structure shares layout with single_call_data_t.
- */
struct irq_work {
- struct llist_node llnode;
- atomic_t flags;
+ union {
+ struct __call_single_node node;
+ struct {
+ struct llist_node llnode;
+ atomic_t flags;
+ };
+ };
void (*func)(struct irq_work *);
-};
+} __no_randomize_layout;

static inline
void init_irq_work(struct irq_work *work, void (*func)(struct irq_work *))
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -32,6 +32,7 @@
#include <linux/posix-timers.h>
#include <linux/rseq.h>
#include <linux/kcsan.h>
+#include <linux/smp_types.h>

/* task_struct member predeclarations (sorted alphabetically): */
struct audit_context;
@@ -654,9 +655,8 @@ struct task_struct {
unsigned int ptrace;

#ifdef CONFIG_SMP
- struct llist_node wake_entry;
- unsigned int wake_entry_type;
int on_cpu;
+ struct __call_single_node wake_entry;
#ifdef CONFIG_THREAD_INFO_IN_TASK
/* Current CPU: */
unsigned int cpu;
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -12,32 +12,25 @@
#include <linux/list.h>
#include <linux/cpumask.h>
#include <linux/init.h>
-#include <linux/llist.h>
+#include <linux/smp_types.h>

typedef void (*smp_call_func_t)(void *info);
typedef bool (*smp_cond_func_t)(int cpu, void *info);

-enum {
- CSD_FLAG_LOCK = 0x01,
-
- /* IRQ_WORK_flags */
-
- CSD_TYPE_ASYNC = 0x00,
- CSD_TYPE_SYNC = 0x10,
- CSD_TYPE_IRQ_WORK = 0x20,
- CSD_TYPE_TTWU = 0x30,
- CSD_FLAG_TYPE_MASK = 0xF0,
-};
-
/*
* structure shares (partial) layout with struct irq_work
*/
struct __call_single_data {
- struct llist_node llist;
- unsigned int flags;
+ union {
+ struct __call_single_node node;
+ struct {
+ struct llist_node llist;
+ unsigned int flags;
+ };
+ };
smp_call_func_t func;
void *info;
-};
+} __no_randomize_layout;

/* Use __aligned() to avoid to use 2 cache lines for 1 csd */
typedef struct __call_single_data call_single_data_t
--- /dev/null
+++ b/include/linux/smp_types.h
@@ -0,0 +1,66 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_SMP_TYPES_H
+#define __LINUX_SMP_TYPES_H
+
+#include <linux/llist.h>
+
+enum {
+ CSD_FLAG_LOCK = 0x01,
+
+ IRQ_WORK_PENDING = 0x01,
+ IRQ_WORK_BUSY = 0x02,
+ IRQ_WORK_LAZY = 0x04, /* No IPI, wait for tick */
+ IRQ_WORK_HARD_IRQ = 0x08, /* IRQ context on PREEMPT_RT */
+
+ IRQ_WORK_CLAIMED = (IRQ_WORK_PENDING | IRQ_WORK_BUSY),
+
+ CSD_TYPE_ASYNC = 0x00,
+ CSD_TYPE_SYNC = 0x10,
+ CSD_TYPE_IRQ_WORK = 0x20,
+ CSD_TYPE_TTWU = 0x30,
+
+ CSD_FLAG_TYPE_MASK = 0xF0,
+};
+
+/*
+ * struct __call_single_node is the primary type on
+ * smp.c:call_single_queue.
+ *
+ * flush_smp_call_function_queue() only reads the type from
+ * __call_single_node::u_flags as a regular load, the above
+ * (anonymous) enum defines all the bits of this word.
+ *
+ * Other bits are not modified until the type is known.
+ *
+ * CSD_TYPE_SYNC/ASYNC:
+ * struct {
+ * struct llist_node node;
+ * unsigned int flags;
+ * smp_call_func_t func;
+ * void *info;
+ * };
+ *
+ * CSD_TYPE_IRQ_WORK:
+ * struct {
+ * struct llist_node node;
+ * atomic_t flags;
+ * void (*func)(struct irq_work *);
+ * };
+ *
+ * CSD_TYPE_TTWU:
+ * struct {
+ * struct llist_node node;
+ * unsigned int flags;
+ * };
+ *
+ */
+
+struct __call_single_node {
+ struct llist_node llist;
+ union {
+ unsigned int u_flags;
+ atomic_t a_flags;
+ };
+} __no_randomize_layout;
+
+#endif /* __LINUX_SMP_TYPES_H */
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2293,7 +2293,7 @@ void sched_ttwu_pending(void *arg)
rq_lock_irqsave(rq, &rf);
update_rq_clock(rq);

- llist_for_each_entry_safe(p, t, llist, wake_entry)
+ llist_for_each_entry_safe(p, t, llist, wake_entry.llist)
ttwu_do_activate(rq, p, p->sched_remote_wakeup ? WF_MIGRATED : 0, &rf);

rq_unlock_irqrestore(rq, &rf);
@@ -2322,7 +2322,7 @@ static void __ttwu_queue_wakelist(struct
p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED);

WRITE_ONCE(rq->ttwu_pending, 1);
- __smp_call_single_queue(cpu, &p->wake_entry);
+ __smp_call_single_queue(cpu, &p->wake_entry.llist);
}

void wake_up_if_idle(int cpu)
@@ -2763,7 +2763,7 @@ static void __sched_fork(unsigned long c
#endif
init_numa_balancing(clone_flags, p);
#ifdef CONFIG_SMP
- p->wake_entry_type = CSD_TYPE_TTWU;
+ p->wake_entry.u_flags = CSD_TYPE_TTWU;
#endif
}

--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -669,24 +669,6 @@ void __init smp_init(void)
{
int num_nodes, num_cpus;

- /*
- * Ensure struct irq_work layout matches so that
- * flush_smp_call_function_queue() can do horrible things.
- */
- BUILD_BUG_ON(offsetof(struct irq_work, llnode) !=
- offsetof(struct __call_single_data, llist));
- BUILD_BUG_ON(offsetof(struct irq_work, func) !=
- offsetof(struct __call_single_data, func));
- BUILD_BUG_ON(offsetof(struct irq_work, flags) !=
- offsetof(struct __call_single_data, flags));
-
- /*
- * 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));
-
idle_threads_init();
cpuhp_threads_init();