Date:	Fri, 24 Jun 2016 08:37:13 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	peterz@...radead.org, mingo@...nel.org
Cc:	linux-kernel@...r.kernel.org, npiggin@...e.de, walken@...gle.com,
	ak@...e.de, tglx@...elltoy.tec.linutronix.de
Subject: Re: [RFC 00/12] lockdep: Implement crossrelease feature

On Mon, Jun 20, 2016 at 01:55:15PM +0900, Byungchul Park wrote:

Hello,

I have a plan to resend this patchset after reinforcement of
documentation. However I am wondering what you think about the
main concept of this. A main motivation is to be able to detect
several problems which I describes with examples below.

ex.1)

PROCESS X		PROCESS Y
---------		---------
mutext_lock A
			lock_page B
lock_page B
			mutext_lock A // DEADLOCK
unlock_page B
			mutext_unlock A
mutex_unlock A
			unlock_page B

ex.2)

PROCESS X		PROCESS Y		PROCESS Z
---------		---------		---------
lock_page B		mutex_lock A

			lock_page B
						mutext_lock A // DEADLOCK
						mutext_unlock A
						unlock_page B
			mutex_unlock A

ex.3)

PROCESS X		PROCESS Y
---------		---------
			mutex_lock A
mutex_lock A
mutex_unlock A		wait_for_complete B // DEADLOCK

complete B
			mutex_unlock A

and so on...

Whatever lockdep can detect can be detected by my implementation
except AA deadlock in a context, which is of course not a deadlock
by nature, for locks releasable by difference context. Fortunately,
current kernel code is robust enough not to be detected on my machine,
I am sure this can be a good navigator to developers.

Thank you.
Byungchul

> Crossrelease feature calls a lock which is releasable by a
> different context from the context having acquired the lock,
> crosslock. For crosslock, all locks having been held in the
> context unlocking the crosslock, until eventually the crosslock
> will be unlocked, have dependency with the crosslock. That's a
> key idea to implement crossrelease feature.
> 
> Crossrelease feature introduces 2 new data structures.
> 
> 1. pend_lock (== plock)
> 
> 	This is for keeping locks waiting to commit those so
> 	that an actual dependency chain is built, when commiting
> 	a crosslock.
> 
> 	Every task_struct has an array of this pending lock to
> 	keep those locks. These pending locks will be added
> 	whenever lock_acquire() is called for normal(non-crosslock)
> 	lock and will be flushed(committed) at proper time.
> 
> 2. cross_lock (== xlock)
> 
> 	This keeps some additional data only for crosslock. There
> 	is one cross_lock per one lockdep_map for crosslock.
> 	lockdep_init_map_crosslock() should be used instead of
> 	lockdep_init_map() to use the lock as a crosslock.
> 
> Acquiring and releasing sequence for crossrelease feature:
> 
> 1. Acquire
> 
> 	All validation check is performed for all locks.
> 
> 	1) For non-crosslock (normal lock)
> 
> 		The hlock will be added not only to held_locks
> 		of the current's task_struct, but also to
> 		pend_lock array of the task_struct, so that
> 		a dependency chain can be built with the lock
> 		when doing commit.
> 
> 	2) For crosslock
> 
> 		The hlock will be added only to the cross_lock
> 		of the lock's lockdep_map instead of held_locks,
> 		so that a dependency chain can be built with
> 		the lock when doing commit. And this lock is
> 		added to the xlocks_head list.
> 
> 2. Commit (only for crosslock)
> 
> 	This establishes a dependency chain between the lock
> 	unlocking it now and all locks having held in the context
> 	unlocking it since the lock was held, even though it tries
> 	to avoid building a chain unnecessarily as far as possible.
> 
> 3. Release
> 
> 	1) For non-crosslock (normal lock)
> 
> 		No change.
> 
> 	2) For crosslock
> 
> 		Just Remove the lock from xlocks_head list. Release
> 		operation should be used with commit operation
> 		together for crosslock, in order to build a
> 		dependency chain properly.
> 
> Byungchul Park (12):
>   lockdep: Refactor lookup_chain_cache()
>   lockdep: Add a function building a chain between two hlocks
>   lockdep: Make check_prev_add can use a stack_trace of other context
>   lockdep: Make save_trace can copy from other stack_trace
>   lockdep: Implement crossrelease feature
>   lockdep: Apply crossrelease to completion
>   pagemap.h: Remove trailing white space
>   lockdep: Apply crossrelease to PG_locked lock
>   cifs/file.c: Remove trailing white space
>   mm/swap_state.c: Remove trailing white space
>   lockdep: Call lock_acquire(release) when accessing PG_locked manually
>   x86/dumpstack: Optimize save_stack_trace
> 
>  arch/x86/include/asm/stacktrace.h |   1 +
>  arch/x86/kernel/dumpstack.c       |   2 +
>  arch/x86/kernel/dumpstack_32.c    |   2 +
>  arch/x86/kernel/stacktrace.c      |   7 +
>  fs/cifs/file.c                    |   6 +-
>  include/linux/completion.h        | 121 +++++-
>  include/linux/irqflags.h          |  16 +-
>  include/linux/lockdep.h           | 139 +++++++
>  include/linux/mm_types.h          |   9 +
>  include/linux/pagemap.h           | 104 ++++-
>  include/linux/sched.h             |   5 +
>  kernel/fork.c                     |   4 +
>  kernel/locking/lockdep.c          | 846 +++++++++++++++++++++++++++++++++++---
>  kernel/sched/completion.c         |  55 +--
>  lib/Kconfig.debug                 |  30 ++
>  mm/filemap.c                      |  10 +-
>  mm/ksm.c                          |   1 +
>  mm/migrate.c                      |   1 +
>  mm/page_alloc.c                   |   3 +
>  mm/shmem.c                        |   2 +
>  mm/swap_state.c                   |  12 +-
>  mm/vmscan.c                       |   1 +
>  22 files changed, 1255 insertions(+), 122 deletions(-)
> 
> -- 
> 1.9.1

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