Date:	Fri, 24 Jun 2016 14:26:45 +0300
From:	Nikolay Borisov <kernel@...p.com>
To:	Byungchul Park <byungchul.park@....com>, 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 06/24/2016 02:37 AM, Byungchul Park wrote:
> 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

Am I correct in assuming that in ex2 PROCESS Z holds page B lock? If so
can you make it a bit more explicit if this is going to go into the
documentation, that is.

> 
> 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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