Date:	Mon, 14 Apr 2008 18:58:10 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Miles Lane <miles.lane@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.25-rc9 -- INFO: possible circular locking dependency
	detected

On Mon, Apr 14, 2008 at 02:42:27PM +0200, Peter Zijlstra wrote:
> On Mon, 2008-04-14 at 17:57 +0530, Gautham R Shenoy wrote:
> 
> > > Ok, so cpu_hotplug has a few issues imho:
> > > 
> > >  - access to active_writer isn't serialized and thus racey
> > >  - holding the lock over the 'write' section generates the stuff above
> > > 
> > > So basically we want a reader/writer lock, where get/put_online_cpu is
> > > the read side and cpu_hotplug_begin/done the write side.
> > > 
> > > We want:
> > >  - readers to recurse in readers (code excluding cpu-hotplug can
> > >    call code needing the same).
> > > 
> > >  - readers to recurse in the writer (the code changing the state can
> > >    call code needing a stable state)
> > > 
> > > rwlock_t isn't quite suitable because it doesn't allow reader in writer
> > > recursion and doesn't allow sleeping.
> > > 
> > > No lockdep annotation _yet_, because current lockdep recursive reader
> > > support is:
> > >  - broken (have a patch for that)
> > >  - doesn't support reader in writer recursion (will have to do something
> > >    about that)
> > > 
> > > Ok, so aside from the obviuos disclaimers, I've only compiled this and
> > > might have made things way too complicated - but a slightly feverish
> > > brain does that at times. What do people think?
> > 
> > You beat me to it! 
> > 
> > I just whipped up a similar patch, with slight differences, and lockdep
> > annotations :)
> 
> lockdep doesn't quite acecpt reader in writer recursion without a little
> patch like so:

For the recursive reads, one could use the rwlock semantics.
That coupled with the lockdep fix you provided
http://lkml.org/lkml/2008/3/12/110
should suffice.

For read after write, we're anyway performing an explicit check to see
if the active_writer is current, in which case, we just return without
troubling the lockdep.

Besides, I was not sure if we had any other scenario where we had a
similar requirement to warrant for the addition within lockdep.

> 
> (fold of two patches - one fixing the recursion another adding
> reader-writer recursion)
> 
> cpu_hotplug should use 3.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
> Index: linux-2.6-2/kernel/lockdep.c
> ===================================================================
> --- linux-2.6-2.orig/kernel/lockdep.c
> +++ linux-2.6-2/kernel/lockdep.c
> @@ -1281,6 +1281,13 @@ check_deadlock(struct task_struct *curr,
>  		 */
>  		if ((read == 2) && prev->read)
>  			return 2;
> +		/*
> +		 * Allow read-after-write recursion of the same
> +		 * lock class (i.e. write_lock(lock)+read_lock(lock)):
> +		 */
> +		if (read == 3)
> +			return 2;
> +
>  		return print_deadlock_bug(curr, prev, next);
>  	}
>  	return 1;
> @@ -1557,12 +1564,11 @@ static int validate_chain(struct task_st
>  		if (!ret)
>  			return 0;
>  		/*
> -		 * Mark recursive read, as we jump over it when
> -		 * building dependencies (just like we jump over
> -		 * trylock entries):
> +		 * If we are the first recursive read, don't jump over our
> +		 * dependency.
>  		 */
> -		if (ret == 2)
> -			hlock->read = 2;
> +		if (hlock->read >= 2 && ret != 2)
> +			hlock->read = 1;
>  		/*
>  		 * Add dependency only if this lock is not the head
>  		 * of the chain, and if it's not a secondary read-lock:
> Index: linux-2.6-2/lib/locking-selftest.c
> ===================================================================
> --- linux-2.6-2.orig/lib/locking-selftest.c
> +++ linux-2.6-2/lib/locking-selftest.c
> @@ -1135,12 +1135,12 @@ void locking_selftest(void)
>  	debug_locks_silent = !debug_locks_verbose;
> 
>  	DO_TESTCASE_6R("A-A deadlock", AA);
> -	DO_TESTCASE_6R("A-B-B-A deadlock", ABBA);
> -	DO_TESTCASE_6R("A-B-B-C-C-A deadlock", ABBCCA);
> -	DO_TESTCASE_6R("A-B-C-A-B-C deadlock", ABCABC);
> -	DO_TESTCASE_6R("A-B-B-C-C-D-D-A deadlock", ABBCCDDA);
> -	DO_TESTCASE_6R("A-B-C-D-B-D-D-A deadlock", ABCDBDDA);
> -	DO_TESTCASE_6R("A-B-C-D-B-C-D-A deadlock", ABCDBCDA);
> +	DO_TESTCASE_6("A-B-B-A deadlock", ABBA);
> +	DO_TESTCASE_6("A-B-B-C-C-A deadlock", ABBCCA);
> +	DO_TESTCASE_6("A-B-C-A-B-C deadlock", ABCABC);
> +	DO_TESTCASE_6("A-B-B-C-C-D-D-A deadlock", ABBCCDDA);
> +	DO_TESTCASE_6("A-B-C-D-B-D-D-A deadlock", ABCDBDDA);
> +	DO_TESTCASE_6("A-B-C-D-B-C-D-A deadlock", ABCDBCDA);
>  	DO_TESTCASE_6("double unlock", double_unlock);
>  	DO_TESTCASE_6("initialize held", init_held);
>  	DO_TESTCASE_6_SUCCESS("bad unlock order", bad_unlock_order);
> Index: linux-2.6-2/include/linux/lockdep.h
> ===================================================================
> --- linux-2.6-2.orig/include/linux/lockdep.h
> +++ linux-2.6-2/include/linux/lockdep.h
> @@ -291,6 +291,7 @@ extern void lockdep_init_map(struct lock
>   *   0: exclusive (write) acquire
>   *   1: read-acquire (no recursion allowed)
>   *   2: read-acquire with same-instance recursion allowed
> + *   3: 2 + reader in writer recursion
>   *
>   * Values for check:
>   *
> 
> > comments below
> > 
> > > 
> > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > ---
> > > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > > index 2011ad8..119d837 100644
> > > --- a/kernel/cpu.c
> > > +++ b/kernel/cpu.c
> > > @@ -27,12 +27,13 @@ static int cpu_hotplug_disabled;
> > > 
> > >  static struct {
> > >  	struct task_struct *active_writer;
> > > -	struct mutex lock; /* Synchronizes accesses to refcount, */
> > > +	spinlock_t lock; /* Synchronizes accesses to refcount, */
> > >  	/*
> > >  	 * Also blocks the new readers during
> > >  	 * an ongoing cpu hotplug operation.
> > >  	 */
> > >  	int refcount;
> > > +	wait_queue_head_t reader_queue;
> > >  	wait_queue_head_t writer_queue;
> > >  } cpu_hotplug;
> > > 
> > > @@ -41,8 +42,9 @@ static struct {
> > >  void __init cpu_hotplug_init(void)
> > >  {
> > >  	cpu_hotplug.active_writer = NULL;
> > > -	mutex_init(&cpu_hotplug.lock);
> > > +	spin_lock_init(&cpu_hotplug.lock);
> > >  	cpu_hotplug.refcount = 0;
> > > +	init_waitqueue_head(&cpu_hotplug.reader_queue);
> > >  	init_waitqueue_head(&cpu_hotplug.writer_queue);
> > >  }
> > > 
> > > @@ -51,27 +53,42 @@ void __init cpu_hotplug_init(void)
> > >  void get_online_cpus(void)
> > >  {
> > >  	might_sleep();
> > > +
> > > +	spin_lock(&cpu_hotplug.lock);
> > >  	if (cpu_hotplug.active_writer == current)
> > > -		return;
> > We don't need to perform this check holding the spinlock.
> > The reason being, this check should succeed only for get_online_cpus()
> > invoked from the CPU-hotplug callback path. and by that time,
> > the writer thread would have set active_writer to it's task struct
> > value.
> > 
> > For every other thread, when it's trying to check if it is the
> > active_writer, the value is either NULL, or the value of the currently
> > active writer's task_struct, but never current.
> > 
> > Am I missing something ?
> 
> I guess you're right - inside makes me feel better though :-) And its
> not like its a fast path or something like that.

I guess yes. But as of now, I don't see any point why we should take the
spinlock to perform that particular check.

> 
> > > -	mutex_lock(&cpu_hotplug.lock);
> > > +		goto unlock;
> > > +
> > > +	if (cpu_hotplug.active_writer) {
> > > +		DEFINE_WAIT(wait);
> > > +
> > > +		for (;;) {
> > > +			prepare_to_wait(&cpu_hotplug.reader_queue, &wait,
> > > +					TASK_UNINTERRUPTIBLE);
> > > +			if (!cpu_hotplug.active_writer)
> > > +				break;
> > > +			spin_unlock(&cpu_hotplug.lock);
> > > +			schedule();
> > > +			spin_lock(&cpu_hotplug.lock);
> > > +		}
> > > +		finish_wait(&cpu_hotplug.reader_queue, &wait);
> > > +	}
> > >  	cpu_hotplug.refcount++;
> > > -	mutex_unlock(&cpu_hotplug.lock);
> > > -
> > > + unlock:
> > > +	spin_unlock(&cpu_hotplug.lock);
> > >  }
> > >  EXPORT_SYMBOL_GPL(get_online_cpus);
> > > 
> > >  void put_online_cpus(void)
> > >  {
> > > +	spin_lock(&cpu_hotplug.lock);
> > >  	if (cpu_hotplug.active_writer == current)
> > > -		return;
> > 
> > ditto!
> > 
> > > -	mutex_lock(&cpu_hotplug.lock);
> > > -	cpu_hotplug.refcount--;
> > > +		goto unlock;
> > > 
> > > -	if (unlikely(writer_exists()) && !cpu_hotplug.refcount)
> > > +	cpu_hotplug.refcount--;
> > > +	if (!cpu_hotplug.refcount)
> > >  		wake_up(&cpu_hotplug.writer_queue);
> > 	hmm.. when there is no active writer, can't we avoid this
> > 	additional wake up ??
> > > -
> > > -	mutex_unlock(&cpu_hotplug.lock);
> > > -
> > > + unlock:
> > > +	spin_unlock(&cpu_hotplug.lock);
> > >  }
> > >  EXPORT_SYMBOL_GPL(put_online_cpus);
> > > 
> > > @@ -95,45 +112,41 @@ void cpu_maps_update_done(void)
> > >   * This ensures that the hotplug operation can begin only when the
> > >   * refcount goes to zero.
> > >   *
> > > - * Note that during a cpu-hotplug operation, the new readers, if any,
> > > - * will be blocked by the cpu_hotplug.lock
> > > - *
> > > - * Since cpu_maps_update_begin is always called after invoking
> > > - * cpu_maps_update_begin, we can be sure that only one writer is active.
> > > - *
> > > - * Note that theoretically, there is a possibility of a livelock:
> > > - * - Refcount goes to zero, last reader wakes up the sleeping
> > > - *   writer.
> > > - * - Last reader unlocks the cpu_hotplug.lock.
> > > - * - A new reader arrives at this moment, bumps up the refcount.
> > > - * - The writer acquires the cpu_hotplug.lock finds the refcount
> > > - *   non zero and goes to sleep again.
> > > - *
> > > - * However, this is very difficult to achieve in practice since
> > > - * get_online_cpus() not an api which is called all that often.
> > > - *
> > > + * cpu_hotplug is basically an unfair recursive reader/writer lock that
> > > + * allows reader in writer recursion.
> > >   */
> > >  static void cpu_hotplug_begin(void)
> > >  {
> > > -	DECLARE_WAITQUEUE(wait, current);
> > > -
> > > -	mutex_lock(&cpu_hotplug.lock);
> > > +	might_sleep();
> > > 
> > > -	cpu_hotplug.active_writer = current;
> > > -	add_wait_queue_exclusive(&cpu_hotplug.writer_queue, &wait);
> > > -	while (cpu_hotplug.refcount) {
> > > -		set_current_state(TASK_UNINTERRUPTIBLE);
> > > -		mutex_unlock(&cpu_hotplug.lock);
> > > -		schedule();
> > > -		mutex_lock(&cpu_hotplug.lock);
> > > +	spin_lock(&cpu_hotplug.lock);
> > > +	if (cpu_hotplug.refcount || cpu_hotplug.active_writer) {
> > 	if we reach this point, there can be only one writer, i.e.
> > 	ourselves!
> > 
> > 	Because, the other writers are serialized by the
> > 	cpu_add_remove_lock in cpu_up()/cpu_down().
> > 
> > 	Hence we can safely assign cpu_hotplug.active_writer to current.
> 
> Ah, missed that. Does it make sense to keep it like this, in case this
> outer lock goes away?

I wouldn't want to see the outer lock go away.
It serializes the access to the cpu_present_map, which is
updated on addition of logical cpus, in the Dynamic LPAR scenario.

> 
> > > +		DEFINE_WAIT(wait);
> > > +
> > > +		for (;;) {
> > > +			prepare_to_wait(&cpu_hotplug.writer_queue, &wait,
> > > +					TASK_UNINTERRUPTIBLE);
> > > +			if (!cpu_hotplug.refcount && !cpu_hotplug.active_writer)
> > > +				break;
> > > +			spin_unlock(&cpu_hotplug.lock);
> > > +			schedule();
> > > +			spin_lock(&cpu_hotplug.lock);
> > > +		}
> > > +		finish_wait(&cpu_hotplug.writer_queue, &wait);
> > >  	}
> > > -	remove_wait_queue_locked(&cpu_hotplug.writer_queue, &wait);
> > > +	cpu_hotplug.active_writer = current;
> > 
> > 
> > > +	spin_unlock(&cpu_hotplug.lock);
> > >  }
> > > 
> > >  static void cpu_hotplug_done(void)
> > >  {
> > > +	spin_lock(&cpu_hotplug.lock);
> > >  	cpu_hotplug.active_writer = NULL;
> > > -	mutex_unlock(&cpu_hotplug.lock);
> > > +	if (!list_empty(&cpu_hotplug.writer_queue.task_list))
> > > +		wake_up(&cpu_hotplug.writer_queue);
> > > +	else
> > > +		wake_up_all(&cpu_hotplug.reader_queue);
> > > +	spin_unlock(&cpu_hotplug.lock);
> > >  }
> > >  /* Need to know about CPUs going up/down? */
> > >  int __cpuinit register_cpu_notifier(struct notifier_block *nb)
> > >
> > 
> > Looks good otherwise!
> 
> Thanks..
> 
> lockdep annotations:
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
> Index: linux-2.6-2/include/linux/lockdep.h
> ===================================================================
> --- linux-2.6-2.orig/include/linux/lockdep.h
> +++ linux-2.6-2/include/linux/lockdep.h
> @@ -458,4 +458,19 @@ static inline void print_irqtrace_events
>  # define rwsem_release(l, n, i)			do { } while (0)
>  #endif
> 
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +# ifdef CONFIG_PROVE_LOCKING
> +#  define cpu_hotplug_acquire(l, s, t, i)	lock_acquire(l, s, t, 0, 2,
> i)
> +#  define cpu_hotplug_acquire_read(l, s, t, i)	lock_acquire(l, s, t, 3,
> 2, i)
> +# else
> +#  define cpu_hotplug_acquire(l, s, t, i)	lock_acquire(l, s, t, 0, 1,
> i)
> +#  define cpu_hotplug_acquire_read(l, s, t, i)	lock_acquire(l, s, t, 3,
> 1, i)
> +# endif
> +# define cpu_hotplug_release(l, n, i)		lock_release(l, n, i)
> +#else
> +# define cpu_hotplug_acquire(l, s, t, i)	do { } while (0)
> +# define cpu_hotplug_acquire_read(l, s, t, i)	do { } while (0)
> +# define cpu_hotplug_release(l, n, i)		do { } while (0)
> +#endif
> +
>  #endif /* __LINUX_LOCKDEP_H */
> Index: linux-2.6-2/kernel/cpu.c
> ===================================================================
> --- linux-2.6-2.orig/kernel/cpu.c
> +++ linux-2.6-2/kernel/cpu.c
> @@ -35,12 +35,20 @@ static struct {
>  	int refcount;
>  	wait_queue_head_t reader_queue;
>  	wait_queue_head_t writer_queue;
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +	struct lockdep_map	dep_map;
> +#endif
>  } cpu_hotplug;
> 
>  #define writer_exists() (cpu_hotplug.active_writer != NULL)
> 
>  void __init cpu_hotplug_init(void)
>  {
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +	static struct lockdep_class_key __key;
> +
> +	lockdep_init_map(&cpu_hotplug.dep_map, "cpu_hotplug", &__key, 0);
> +#endif
>  	cpu_hotplug.active_writer = NULL;
>  	spin_lock_init(&cpu_hotplug.lock);
>  	cpu_hotplug.refcount = 0;
> @@ -54,6 +62,8 @@ void get_online_cpus(void)
>  {
>  	might_sleep();
> 
> +	cpu_hotplug_acquire_read(&cpu_hotplug.dep_map, 0, 0, _THIS_IP_);
> +
>  	spin_lock(&cpu_hotplug.lock);
>  	if (cpu_hotplug.active_writer == current)
>  		goto unlock;
> @@ -80,6 +90,8 @@ EXPORT_SYMBOL_GPL(get_online_cpus);
> 
>  void put_online_cpus(void)
>  {
> +	cpu_hotplug_release(&cpu_hotplug.lock, 1, _THIS_IP_);
> +
>  	spin_lock(&cpu_hotplug.lock);
>  	if (cpu_hotplug.active_writer == current)
>  		goto unlock;
> @@ -119,6 +131,8 @@ static void cpu_hotplug_begin(void)
>  {
>  	might_sleep();
> 
> +	cpu_hotplug_acquire(&cpu_hotplug.dep_map, 0, 0, _THIS_IP_);
> +
>  	spin_lock(&cpu_hotplug.lock);
>  	if (cpu_hotplug.refcount || cpu_hotplug.active_writer) {
>  		DEFINE_WAIT(wait);
> @@ -140,6 +154,8 @@ static void cpu_hotplug_begin(void)
> 
>  static void cpu_hotplug_done(void)
>  {
> +	cpu_hotplug_release(&cpu_hotplug.lock, 1, _THIS_IP_);
> +
>  	spin_lock(&cpu_hotplug.lock);
>  	cpu_hotplug.active_writer = NULL;
>  	if (!list_empty(&cpu_hotplug.writer_queue.task_list))
> 

Looks good. Will test the series and report back.

But w.r.t the problem at hand, I susupect
some more changes are required in the
cpufreq-cpuhotplug interations.

-- 
Thanks and Regards
gautham
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