linux-kernel - Re: [PATCH v2] Btrfs, lockdep: get_restripe_target: use lockdep in BUG

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Date:	Fri, 6 Apr 2012 21:06:15 -0400
From:	Bobby Powers <bobbypowers@...il.com>
To:	Ilya Dryomov <idryomov@...il.com>
Cc:	linux-btrfs@...r.kernel.org, Chris Mason <chris.mason@...cle.com>,
	Andi Kleen <ak@...ux.intel.com>, Jeff Mahoney <jeffm@...e.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Btrfs, lockdep: get_restripe_target: use lockdep in BUG_ON

On Fri, Apr 6, 2012 at 4:05 PM, Ilya Dryomov <idryomov@...il.com> wrote:
> On Wed, Apr 04, 2012 at 10:04:12PM -0400, Bobby Powers wrote:
>> spin_is_locked always returns 0 on non-SMP systems, which causes btrfs
>> to fail the mount.  There is documentation pending as to why checking
>> for spin_is_locked is a bad idea:
>>
>> https://lkml.org/lkml/2012/3/27/413
>>
>> The suggested lockdep_assert_held() is not appropriate in this case,
>> as what get_restripe_target() is checking for is that either
>> volume_mutex is held or balance_lock is held.  Luckily
>> lockdep_assert_held() is a simple macro:
>>
>> WARN_ON(debug_locks && !lockdep_is_held(l))
>>
>> We can mimic the structure in get_restripe_target(), but we need to
>> make sure lockdep_is_held() is defined for the !LOCKDEP case.
>>
>> CC: Ilya Dryomov <idryomov@...il.com>
>> CC: Chris Mason <chris.mason@...cle.com>
>> CC: Andi Kleen <ak@...ux.intel.com>
>> CC: Jeff Mahoney <jeffm@...e.de>
>> CC: Ingo Molnar <mingo@...hat.com>
>> CC: linux-kernel@...r.kernel.org
>> Signed-off-by: Bobby Powers <bobbypowers@...il.com>
>> ---
>>  fs/btrfs/extent-tree.c  |    5 +++--
>>  include/linux/lockdep.h |    1 +
>>  2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index a844204..4d13eb1 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -24,6 +24,7 @@
>>  #include <linux/kthread.h>
>>  #include <linux/slab.h>
>>  #include <linux/ratelimit.h>
>> +#include <linux/lockdep.h>
>>  #include "compat.h"
>>  #include "hash.h"
>>  #include "ctree.h"
>> @@ -3158,8 +3159,8 @@ static u64 get_restripe_target(struct btrfs_fs_info *fs_info, u64 flags)
>>       struct btrfs_balance_control *bctl = fs_info->balance_ctl;
>>       u64 target = 0;
>>
>> -     BUG_ON(!mutex_is_locked(&fs_info->volume_mutex) &&
>> -            !spin_is_locked(&fs_info->balance_lock));
>> +     BUG_ON(debug_locks && !lockdep_is_held(&fs_info->volume_mutex) &&
>> +            !lockdep_is_held(&fs_info->balance_lock));
>>
>>       if (!bctl)
>>               return 0;
>> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
>> index d36619e..94c0edb 100644
>> --- a/include/linux/lockdep.h
>> +++ b/include/linux/lockdep.h
>> @@ -392,6 +392,7 @@ struct lock_class_key { };
>>
>>  #define lockdep_depth(tsk)   (0)
>>
>> +#define lockdep_is_held(l)   (0)
>>  #define lockdep_assert_held(l)                       do { } while (0)
>>
>>  #define lockdep_recursing(tsk)                       (0)
>> --
>> 1.7.10.rc3.3.g19a6c
>
> OK, Mitch's report prompted me to actually take a look.  This patch is
> wrong: by defining lockdep_is_held(l) to 0 in !LOCKDEP case you
> effectively mimic the behaviour of spin_is_locked() which is what
> getting us in the first place.
>
> get_restripe_target() interface is WIP so I will replace BUG_ON with a
> comment and send a patch through btrfs tree.

Hah, good point...

> Thanks,
>
>                Ilya
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