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Date:   Thu, 20 Feb 2020 22:33:17 +0100
From:   Marco Elver <elver@google.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     andreyknvl@google.com, glider@google.com, dvyukov@google.com,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kcsan: Add option to allow watcher interruptions
Message-ID: <20200220213317.GA35033@google.com>
References: <20200220141551.166537-1-elver@google.com>
 <20200220185855.GY2935@paulmck-ThinkPad-P72>
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On Thu, 20 Feb 2020, Paul E. McKenney wrote:

> On Thu, Feb 20, 2020 at 03:15:51PM +0100, Marco Elver wrote:
> > Add option to allow interrupts while a watchpoint is set up. This can be
> > enabled either via CONFIG_KCSAN_INTERRUPT_WATCHER or via the boot
> > parameter 'kcsan.interrupt_watcher=1'.
> > 
> > Note that, currently not all safe per-CPU access primitives and patterns
> > are accounted for, which could result in false positives. For example,
> > asm-generic/percpu.h uses plain operations, which by default are
> > instrumented. On interrupts and subsequent accesses to the same
> > variable, KCSAN would currently report a data race with this option.
> > 
> > Therefore, this option should currently remain disabled by default, but
> > may be enabled for specific test scenarios.
> > 
> > Signed-off-by: Marco Elver <elver@google.com>
> 
> Queued for review and testing, thank you!
> 
> > ---
> > 
> > As an example, the first data race that this found:
> > 
> > write to 0xffff88806b3324b8 of 4 bytes by interrupt on cpu 0:
> >  rcu_preempt_read_enter kernel/rcu/tree_plugin.h:353 [inline]
> >  __rcu_read_lock+0x3c/0x50 kernel/rcu/tree_plugin.h:373
> >  rcu_read_lock include/linux/rcupdate.h:599 [inline]
> >  cpuacct_charge+0x36/0x80 kernel/sched/cpuacct.c:347
> >  cgroup_account_cputime include/linux/cgroup.h:773 [inline]
> >  update_curr+0xe2/0x1d0 kernel/sched/fair.c:860
> >  enqueue_entity+0x130/0x5d0 kernel/sched/fair.c:4005
> >  enqueue_task_fair+0xb0/0x420 kernel/sched/fair.c:5260
> >  enqueue_task kernel/sched/core.c:1302 [inline]
> >  activate_task+0x6d/0x110 kernel/sched/core.c:1324
> >  ttwu_do_activate.isra.0+0x40/0x60 kernel/sched/core.c:2266
> >  ttwu_queue kernel/sched/core.c:2411 [inline]
> >  try_to_wake_up+0x3be/0x6c0 kernel/sched/core.c:2645
> >  wake_up_process+0x10/0x20 kernel/sched/core.c:2669
> >  hrtimer_wakeup+0x4c/0x60 kernel/time/hrtimer.c:1769
> >  __run_hrtimer kernel/time/hrtimer.c:1517 [inline]
> >  __hrtimer_run_queues+0x274/0x5f0 kernel/time/hrtimer.c:1579
> >  hrtimer_interrupt+0x22d/0x490 kernel/time/hrtimer.c:1641
> >  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1119 [inline]
> >  smp_apic_timer_interrupt+0xdc/0x280 arch/x86/kernel/apic/apic.c:1144
> >  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
> >  delay_tsc+0x38/0xc0 arch/x86/lib/delay.c:68                   <--- interrupt while delayed
> >  __delay arch/x86/lib/delay.c:161 [inline]
> >  __const_udelay+0x33/0x40 arch/x86/lib/delay.c:175
> >  __udelay+0x10/0x20 arch/x86/lib/delay.c:181
> >  kcsan_setup_watchpoint+0x17f/0x400 kernel/kcsan/core.c:428
> >  check_access kernel/kcsan/core.c:550 [inline]
> >  __tsan_read4+0xc6/0x100 kernel/kcsan/core.c:685               <--- Enter KCSAN runtime
> >  rcu_preempt_read_enter kernel/rcu/tree_plugin.h:353 [inline]  <---+
> >  __rcu_read_lock+0x2a/0x50 kernel/rcu/tree_plugin.h:373            |
> >  rcu_read_lock include/linux/rcupdate.h:599 [inline]               |
> >  lock_page_memcg+0x31/0x110 mm/memcontrol.c:1972                   |
> >                                                                    |
> > read to 0xffff88806b3324b8 of 4 bytes by task 6131 on cpu 0:       |
> >  rcu_preempt_read_enter kernel/rcu/tree_plugin.h:353 [inline]  ----+
> >  __rcu_read_lock+0x2a/0x50 kernel/rcu/tree_plugin.h:373
> >  rcu_read_lock include/linux/rcupdate.h:599 [inline]
> >  lock_page_memcg+0x31/0x110 mm/memcontrol.c:1972
> > 
> > The writer is doing 'current->rcu_read_lock_nesting++'. The read is as
> > vulnerable to compiler optimizations and would therefore conclude this
> > is a valid data race.
> 
> Heh!  That one is a fun one!  It is on a very hot fastpath.  READ_ONCE()
> and WRITE_ONCE() are likely to be measurable at the system level.
> 
> Thoughts on other options?

Would this be a use-case for local_t? Don't think this_cpu ops work
here.

See below idea. This would avoid the data race (KCSAN stopped
complaining) and seems to generate reasonable code.

Version before:

 <__rcu_read_lock>:
     130	mov    %gs:0x0,%rax
     137
     139	addl   $0x1,0x370(%rax)
     140	retq   
     141	data16 nopw %cs:0x0(%rax,%rax,1)
     148
     14c	nopl   0x0(%rax)

Version after:

 <__rcu_read_lock>:
     130	mov    %gs:0x0,%rax
     137
     139	incq   0x370(%rax)
     140	retq   
     141	data16 nopw %cs:0x0(%rax,%rax,1)
     148
     14c	nopl   0x0(%rax)

I haven't checked the other places where it is used, though.
(Can send it as a patch if you think this might work.)

Thanks,
-- Marco

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 2678a37c31696..3d8586ee7ae64 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -50,7 +50,7 @@ void __rcu_read_unlock(void);
  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
  */
-#define rcu_preempt_depth() (current->rcu_read_lock_nesting)
+#define rcu_preempt_depth() local_read(&current->rcu_read_lock_nesting)
 
 #else /* #ifdef CONFIG_PREEMPT_RCU */
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0918904c939d2..70d7e3257feed 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -10,6 +10,7 @@
 #include <uapi/linux/sched.h>
 
 #include <asm/current.h>
+#include <asm/local.h>
 
 #include <linux/pid.h>
 #include <linux/sem.h>
@@ -708,7 +709,7 @@ struct task_struct {
 	cpumask_t			cpus_mask;
 
 #ifdef CONFIG_PREEMPT_RCU
-	int				rcu_read_lock_nesting;
+	local_t				rcu_read_lock_nesting;
 	union rcu_special		rcu_read_unlock_special;
 	struct list_head		rcu_node_entry;
 	struct rcu_node			*rcu_blocked_node;
diff --git a/init/init_task.c b/init/init_task.c
index 096191d177d5c..941777fce11e5 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -130,7 +130,7 @@ struct task_struct init_task
 	.perf_event_list = LIST_HEAD_INIT(init_task.perf_event_list),
 #endif
 #ifdef CONFIG_PREEMPT_RCU
-	.rcu_read_lock_nesting = 0,
+	.rcu_read_lock_nesting = LOCAL_INIT(0),
 	.rcu_read_unlock_special.s = 0,
 	.rcu_node_entry = LIST_HEAD_INIT(init_task.rcu_node_entry),
 	.rcu_blocked_node = NULL,
diff --git a/kernel/fork.c b/kernel/fork.c
index 60a1295f43843..43af326081b06 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1669,7 +1669,7 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
 static inline void rcu_copy_process(struct task_struct *p)
 {
 #ifdef CONFIG_PREEMPT_RCU
-	p->rcu_read_lock_nesting = 0;
+	local_set(&p->rcu_read_lock_nesting, 0);
 	p->rcu_read_unlock_special.s = 0;
 	p->rcu_blocked_node = NULL;
 	INIT_LIST_HEAD(&p->rcu_node_entry);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c6ea81cd41890..e0595abd50c0f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -350,17 +350,17 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 
 static void rcu_preempt_read_enter(void)
 {
-	current->rcu_read_lock_nesting++;
+	local_inc(&current->rcu_read_lock_nesting);
 }
 
 static void rcu_preempt_read_exit(void)
 {
-	current->rcu_read_lock_nesting--;
+	local_dec(&current->rcu_read_lock_nesting);
 }
 
 static void rcu_preempt_depth_set(int val)
 {
-	current->rcu_read_lock_nesting = val;
+	local_set(&current->rcu_read_lock_nesting, val);
 }
 
 /*