Subject: Re: [PATCH 2/2] sysctl: lockdep support for sysctl reference counting.

From: [email-masked] (Eric W. Biederman)
Date: Tue, 31 Mar 2009 06:40:54 -0700

Peter Zijlstra <[email-masked]> writes:

> On Sat, 2009-03-21 at 00:42 -0700, Eric W. Biederman wrote:
>> It is possible for get lock ordering deadlocks between locks
>> and waiting for the sysctl used count to drop to zero.  We have
>> recently observed one of these in the networking code.
>> 
>> So teach the sysctl code how to speak lockdep so the kernel
>> can warn about these kinds of rare issues proactively.
>
> It would be very good to extend this changelog with a more detailed
> explanation of the deadlock in question.
>
> Let me see if I got it right:
>
> We're holding a lock, while waiting for the refcount to drop to 0.
> Dropping that refcount is blocked on that lock.
>
> Something like that?

Exactly.

I must have written an explanation so many times that it got
lost when I wrote that commit message.

In particular the problem can be see with /proc/sys/net/ipv4/conf/*/forwarding.

The problem is that the handler for fowarding takes the rtnl_lock
with the reference count held.

Then we call unregister_sysctl_table under the rtnl_lock.
which waits for the reference count to go to zero.

>> Signed-off-by: Eric Biederman <[email-masked]>
>> ---
>>  include/linux/sysctl.h |    4 ++
>>  kernel/sysctl.c        |  108 ++++++++++++++++++++++++++++++++++++++---------
>>  2 files changed, 91 insertions(+), 21 deletions(-)
>> 
>> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
>> index 39d471d..ec9b1dd 100644
>> --- a/include/linux/sysctl.h
>> +++ b/include/linux/sysctl.h
>> @@ -28,6 +28,7 @@
>>  #include <linux/kernel.h>
>>  #include <linux/types.h>
>>  #include <linux/compiler.h>
>> +#include <linux/lockdep.h>
>>  
>>  struct file;
>>  struct completion;
>> @@ -1087,6 +1088,9 @@ struct ctl_table_header
>>  	struct ctl_table *attached_by;
>>  	struct ctl_table *attached_to;
>>  	struct ctl_table_header *parent;
>> +#ifdef CONFIG_PROVE_LOCKING
>> +	struct lockdep_map dep_map;
>> +#endif
>>  };
>>  
>>  /* struct ctl_path describes where in the hierarchy a table is added */
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index c5ef44f..ea8cc39 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1454,12 +1454,63 @@ static struct ctl_table dev_table[] = {
>>  
>>  static DEFINE_SPINLOCK(sysctl_lock);
>>  
>> +#ifndef CONFIG_PROVE_LOCKING
>> +
>> +# define lock_sysctl() spin_lock(&sysctl_lock)
>> +# define unlock_sysctl() spin_unlock(&sysctl_lock)
>> +
>> +static inline void table_acquire_use(struct ctl_table_header *hdr) { }
>> +static inline void table_release_use(struct ctl_table_header *hdr) { }
>> +static inline void table_acquire(struct ctl_table_header *hdr) { }
>> +static inline void table_contended(struct ctl_table_header *hdr) { }
>> +static inline void table_acquired(struct ctl_table_header *hdr) { }
>> +static inline void table_release(struct ctl_table_header *hdr) { }
>> +
>> +#else	/* CONFIG_PROVE_LOCKING */
>> +
>> +#  define lock_sysctl() __raw_spin_lock(&sysctl_lock.raw_lock)
>> +#  define unlock_sysctl() __raw_spin_unlock(&sysctl_lock.raw_lock)
>
> Uhmm, Please explain that -- without a proper explanation this is a NAK.

If the refcount is to be considered a lock.  sysctl_lock must be considered
the internals of that lock.  lockdep gets extremely confused otherwise.
Since the spinlock is static to this file I'm not especially worried
about it.

>> +static inline void table_acquire_use(struct ctl_table_header *hdr)
>> +{
>> +	lock_acquire(&hdr->dep_map, 0, 0, 1, 2, NULL, _RET_IP_);
>> +	lock_acquired(&hdr->dep_map, _RET_IP_);
>> +}
>> +
>> +static inline void table_release_use(struct ctl_table_header *hdr)
>> +{
>> +	lock_release(&hdr->dep_map, 0, _RET_IP_);
>> +}
>> +
>> +static inline void table_acquire(struct ctl_table_header *hdr)
>> +{
>> +	lock_acquire(&hdr->dep_map, 0, 0, 0, 2, NULL, _RET_IP_);
>> +}
>> +
>> +static inline void table_contended(struct ctl_table_header *hdr)
>> +{
>> +	lock_contended(&hdr->dep_map, _RET_IP_);
>> +}
>> +
>> +static inline void table_acquired(struct ctl_table_header *hdr)
>> +{
>> +	lock_acquired(&hdr->dep_map, _RET_IP_);
>> +}
>> +
>> +static inline void table_release(struct ctl_table_header *hdr)
>> +{
>> +	lock_release(&hdr->dep_map, 0, _RET_IP_);
>> +}
>> +
>> +#endif /* CONFIG_PROVE_LOCKING */
>> +
>>  /* called under sysctl_lock */
>>  static int use_table(struct ctl_table_header *p)
>>  {
>>  	if (unlikely(p->unregistering))
>>  		return 0;
>>  	p->used++;
>> +	table_acquire_use(p);
>>  	return 1;
>>  }
>>  
>> @@ -1469,6 +1520,8 @@ static void unuse_table(struct ctl_table_header *p)
>>  	if (!--p->used)
>>  		if (unlikely(p->unregistering))
>>  			complete(p->unregistering);
>> +
>> +	table_release_use(p);
>>  }
>>  
>>  /* called under sysctl_lock, will reacquire if has to wait */
>> @@ -1478,47 +1531,54 @@ static void start_unregistering(struct ctl_table_header *p)
>>  	 * if p->used is 0, nobody will ever touch that entry again;
>>  	 * we'll eliminate all paths to it before dropping sysctl_lock
>>  	 */
>> +	table_acquire(p);
>>  	if (unlikely(p->used)) {
>>  		struct completion wait;
>> +		table_contended(p);
>> +
>>  		init_completion(&wait);
>>  		p->unregistering = &wait;
>> -		spin_unlock(&sysctl_lock);
>> +		unlock_sysctl();
>>  		wait_for_completion(&wait);
>> -		spin_lock(&sysctl_lock);
>> +		lock_sysctl();
>>  	} else {
>>  		/* anything non-NULL; we'll never dereference it */
>>  		p->unregistering = ERR_PTR(-EINVAL);
>>  	}
>> +	table_acquired(p);
>> +
>>  	/*
>>  	 * do not remove from the list until nobody holds it; walking the
>>  	 * list in do_sysctl() relies on that.
>>  	 */
>>  	list_del_init(&p->ctl_entry);
>> +
>> +	table_release(p);
>>  }
>>  
>
>> @@ -1951,7 +2011,13 @@ struct ctl_table_header *__register_sysctl_paths(
>>  		return NULL;
>>  	}
>>  #endif
>> -	spin_lock(&sysctl_lock);
>> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
>> +	{
>> +		static struct lock_class_key __key;
>> +		lockdep_init_map(&header->dep_map, "sysctl_used", &__key, 0);
>> +	}
>> +#endif	
>
> This means every sysctl thingy gets the same class, is that
> intended/desired?

There is only one place we initialize it, and as far as I know really
only one place we take it.  Which is the definition of a lockdep
class as far as I know.

Eric