Re: [PATCH 000 of 4] Introduction

Peter Zijlstra
2006-10-11T08:44:29Z

On Wed, 2006-10-11 at 16:09 +1000, NeilBrown wrote:
> Following 4 patches address issues with lockdep, particularly around bd_mutex.
>
> They are against 2.6.18-mm3 and do *not* apply against -linus as -mm already has
> some changes to the handling of bd_mutex nesting. 2-4 probably apply on top of -linus plus
> -mm/broken-out/remove-the-old-bd_mutex-lockdep-annotation.patch
>
> I believe they are probably ok for 2.6.19.
>
> The core issue is that blkdev_get when called on a partition needs to
> lock (bd_mutex) the partition and the whole device. lockdep would
> normally see this as a possible deadlock and needs to be told that
> this particular nesting is known to be safe.
>
> The code to do this is in -linus is rather messy, largely because the
> locking itself is messy. The bd_mutex for the whole is taken several
> times while bd_mutex for the partition is held, and it is taken at
> both levels of the recursion (blkdev_get calls blkdev_get - only to
> one level).
>
> As key observation to simplifying the locking is to observer that a
> lot of the locking is there to protect the updating of bd_part_count.
> If those updates are moved, the locking can become simpler.
>
> The first patch removes the current approach in -mm to handling this
> nesting and explains why it is not ideal.
> This reverts new-bd_mutex-lockdep-annotation.patch
>
> The second simplifies the locking as explained above.
>
> The third adds the mutex_lock_nested annotations, which are now trivial.
>
> The last fixes a tangentially related lockdep problem in md - there is
> a false relationship between bd_mutex and md->reconfig_mutex which
> needs to be clarified.
>
> [PATCH 000 of 4] Introduction
> [PATCH 001 of 4] Remove lock_key approach to managing nested bd_mutex locks.
> [PATCH 002 of 4] Simplify some aspects of bd_mutex nesting.
> [PATCH 003 of 4] Use mutex_lock_nested for bd_mutex to avoid lockdep warning.
> [PATCH 004 of 4] Avoid lockdep warning in md.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>