[RFC V2 PATCH 0/3] timer: patchset focus on del_timer_sync()

[RFC V2 PATCH 0/3] timer: patchset focus on del_timer_sync()

[Yong Zhang]

From: Yong Zhang <yong.zhang@windriver.com>

This is inspired by http://lkml.org/lkml/2010/8/16/291
which catch a lockdep false positive on fake_timer_lock.
When I go into del_timer_sync(), but don't find anything
which prevent del_timer_sync() from using in softirq context,
and indeed it's been used in softirq for some timer(such as
__dst_free()).

Thus, tell others it can't be used in softirq context, and
teach lockdep about that. It's realized by patch-0002.

Your comments are very appreciated.

Thanks,
Yong

V2 <- V1
  Update patch 0001, the new version make try_to_del_timer_sync()
  safe on both hardirq context and UP. Thanks Oleg.

---
Yong Zhang (3):
      timer: make try_to_del_timer_sync() safe on both hardirq context and UP
      timer: del_timer_sync() can be used in softirq context
      timer: warn when del_timer_sync() used in hardirq context

 include/linux/timer.h |    4 ++--
 kernel/timer.c        |   20 +++++++++-----------
 2 files changed, 11 insertions(+), 13 deletions(-)

[RFC V2 PATCH 1/3] timer: make try_to_del_timer_sync() safe on both hardirq context and UP

[Yong Zhang]

From: Yong Zhang <yong.zhang@windriver.com>

In commit fd450b7318b75343fd76b3d95416853e34e72c95, it was saying
try_to_del_timer_sync() can be used in interrupt context.

But because base->running_timer is SMP special, this lead to
unsymmetry try_to_del_timer_sync() on UP and SMP.

We can make running_timer live on everywhere, then try_to_del_timer_sync()
is also UP safe and can be used in hardirq context.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@redhat.com>
---
Hi Oleg,

How do you think about this one?

Yong

 include/linux/timer.h |    4 ++--
 kernel/timer.c        |    6 +-----
 2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 38cf093..225c4ce 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -248,11 +248,11 @@ static inline void timer_stats_timer_clear_start_info(struct timer_list *timer)
 
 extern void add_timer(struct timer_list *timer);
 
+extern int try_to_del_timer_sync(struct timer_list *timer);
+
 #ifdef CONFIG_SMP
-  extern int try_to_del_timer_sync(struct timer_list *timer);
   extern int del_timer_sync(struct timer_list *timer);
 #else
-# define try_to_del_timer_sync(t)	del_timer(t)
 # define del_timer_sync(t)		del_timer(t)
 #endif
 
diff --git a/kernel/timer.c b/kernel/timer.c
index 97bf05b..e031eb4 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -347,9 +347,7 @@ EXPORT_SYMBOL_GPL(set_timer_slack);
 static inline void set_running_timer(struct tvec_base *base,
 					struct timer_list *timer)
 {
-#ifdef CONFIG_SMP
 	base->running_timer = timer;
-#endif
 }
 
 static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
@@ -936,15 +934,12 @@ int del_timer(struct timer_list *timer)
 }
 EXPORT_SYMBOL(del_timer);
 
-#ifdef CONFIG_SMP
 /**
  * try_to_del_timer_sync - Try to deactivate a timer
  * @timer: timer do del
  *
  * This function tries to deactivate a timer. Upon successful (ret >= 0)
  * exit the timer is not queued and the handler is not running on any CPU.
- *
- * It must not be called from interrupt contexts.
  */
 int try_to_del_timer_sync(struct timer_list *timer)
 {
@@ -973,6 +968,7 @@ out:
 }
 EXPORT_SYMBOL(try_to_del_timer_sync);
 
+#ifdef CONFIG_SMP
 /**
  * del_timer_sync - deactivate a timer and wait for the handler to finish.
  * @timer: the timer to be deactivated
-- 
1.7.0.4

Re: [RFC V2 PATCH 1/3] timer: make try_to_del_timer_sync() safe on both hardirq context and UP

[Oleg Nesterov]

On 08/25, Yong Zhang wrote:
>
> From: Yong Zhang <yong.zhang@windriver.com>
>
> In commit fd450b7318b75343fd76b3d95416853e34e72c95, it was saying
> try_to_del_timer_sync() can be used in interrupt context.
>
> But because base->running_timer is SMP special, this lead to
> unsymmetry try_to_del_timer_sync() on UP and SMP.
>
> We can make running_timer live on everywhere, then try_to_del_timer_sync()
> is also UP safe and can be used in hardirq context.


Acked-by: Oleg Nesterov <oleg@redhat.com>


> Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Oleg Nesterov <oleg@redhat.com>
> ---
> Hi Oleg,
> 
> How do you think about this one?
> 
> Yong
> 
>  include/linux/timer.h |    4 ++--
>  kernel/timer.c        |    6 +-----
>  2 files changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/timer.h b/include/linux/timer.h
> index 38cf093..225c4ce 100644
> --- a/include/linux/timer.h
> +++ b/include/linux/timer.h
> @@ -248,11 +248,11 @@ static inline void timer_stats_timer_clear_start_info(struct timer_list *timer)
>  
>  extern void add_timer(struct timer_list *timer);
>  
> +extern int try_to_del_timer_sync(struct timer_list *timer);
> +
>  #ifdef CONFIG_SMP
> -  extern int try_to_del_timer_sync(struct timer_list *timer);
>    extern int del_timer_sync(struct timer_list *timer);
>  #else
> -# define try_to_del_timer_sync(t)	del_timer(t)
>  # define del_timer_sync(t)		del_timer(t)
>  #endif
>  
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 97bf05b..e031eb4 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -347,9 +347,7 @@ EXPORT_SYMBOL_GPL(set_timer_slack);
>  static inline void set_running_timer(struct tvec_base *base,
>  					struct timer_list *timer)
>  {
> -#ifdef CONFIG_SMP
>  	base->running_timer = timer;
> -#endif
>  }
>  
>  static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
> @@ -936,15 +934,12 @@ int del_timer(struct timer_list *timer)
>  }
>  EXPORT_SYMBOL(del_timer);
>  
> -#ifdef CONFIG_SMP
>  /**
>   * try_to_del_timer_sync - Try to deactivate a timer
>   * @timer: timer do del
>   *
>   * This function tries to deactivate a timer. Upon successful (ret >= 0)
>   * exit the timer is not queued and the handler is not running on any CPU.
> - *
> - * It must not be called from interrupt contexts.
>   */
>  int try_to_del_timer_sync(struct timer_list *timer)
>  {
> @@ -973,6 +968,7 @@ out:
>  }
>  EXPORT_SYMBOL(try_to_del_timer_sync);
>  
> +#ifdef CONFIG_SMP
>  /**
>   * del_timer_sync - deactivate a timer and wait for the handler to finish.
>   * @timer: the timer to be deactivated
> -- 
> 1.7.0.4
> 

[RFC V2 PATCH 2/3] timer: del_timer_sync() can be used in softirq context

[Yong Zhang]

From: Yong Zhang <yong.zhang@windriver.com>

Actually we have used del_timer_sync() in softirq context for a long
time, such as: __dst_free()::cancel_delayed_work() and maybe the
DEBUG_OBJECTS_TIMERS fundamental code.

So change the comments of it to warn on hardirq context only,
and make lockdep know about this change.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 kernel/timer.c |    8 +++-----
 1 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/kernel/timer.c b/kernel/timer.c
index 55b6a2d..08c9559 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -981,7 +981,7 @@ EXPORT_SYMBOL(try_to_del_timer_sync);
  *
  * Synchronization rules: Callers must prevent restarting of the timer,
  * otherwise this function is meaningless. It must not be called from
- * interrupt contexts. The caller must not hold locks which would prevent
+ * hardirq contexts. The caller must not hold locks which would prevent
  * completion of the timer's handler. The timer's handler must not call
  * add_timer_on(). Upon exit the timer is not queued and the handler is
  * not running on any CPU.
@@ -991,12 +991,10 @@ EXPORT_SYMBOL(try_to_del_timer_sync);
 int del_timer_sync(struct timer_list *timer)
 {
 #ifdef CONFIG_LOCKDEP
-	unsigned long flags;
-
-	local_irq_save(flags);
+	local_bh_disable();
 	lock_map_acquire(&timer->lockdep_map);
 	lock_map_release(&timer->lockdep_map);
-	local_irq_restore(flags);
+	local_bh_enable();
 #endif
 
 	for (;;) {
-- 
1.7.0.4

[RFC V2 PATCH 3/3] timer: warn when del_timer_sync() used in hardirq context

[Yong Zhang]

From: Yong Zhang <yong.zhang@windriver.com>

Add explict warning to prevent del_timer_sync() from using
in hardirq context.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
---
 kernel/timer.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/kernel/timer.c b/kernel/timer.c
index 08c9559..1bc4b4a 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -996,7 +996,11 @@ int del_timer_sync(struct timer_list *timer)
 	lock_map_release(&timer->lockdep_map);
 	local_bh_enable();
 #endif
-
+	/*
+	 * don't use it in hardirq context, because it
+	 * could lead to deadlock.
+	 */
+	WARN_ON(in_irq());
 	for (;;) {
 		int ret = try_to_del_timer_sync(timer);
 		if (ret >= 0)
-- 
1.7.0.4

Re: [RFC V2 PATCH 3/3] timer: warn when del_timer_sync() used in hardirq context

[Oleg Nesterov]

On 08/25, Yong Zhang wrote:
>
> From: Yong Zhang <yong.zhang@windriver.com>
>
> Add explict warning to prevent del_timer_sync() from using
> in hardirq context.

Agreed, del_timer_sync() from softirq should be OK.

> Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ingo Molnar <mingo@elte.hu>
> ---
>  kernel/timer.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 08c9559..1bc4b4a 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -996,7 +996,11 @@ int del_timer_sync(struct timer_list *timer)
>  	lock_map_release(&timer->lockdep_map);
>  	local_bh_enable();
>  #endif
> -
> +	/*
> +	 * don't use it in hardirq context, because it
> +	 * could lead to deadlock.
> +	 */
> +	WARN_ON(in_irq());
>  	for (;;) {
>  		int ret = try_to_del_timer_sync(timer);
>  		if (ret >= 0)
> -- 
> 1.7.0.4
>