linux-kernel - Re: [PATCH v2 0/8] Add rcu_sync infrastructure to avoid

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Date:	Sat, 22 Aug 2015 09:38:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/8] Add rcu_sync infrastructure to avoid _expedited()
	in percpu-rwsem

On Fri, Aug 21, 2015 at 07:42:30PM +0200, Oleg Nesterov wrote:
> Hello,
> 
> Now that sb->s_writers was changed to use percpu_rw_semaphore let me
> send v2.
> 
> Changes:
> 
> 	- whitespace fix in 1/8.
> 
> 	- remove EXPORT_SYMBOL() in 3/8, currently rcu_sync has no
> 	  modular users.
> 
> 	- 5/8 is new. This ugly hack pairs with another one:
> 	  "shift percpu_counter_destroy() into destroy_super_work()"
> 	  https://git.kernel.org/cgit/linux/kernel/git/viro/vfs.git/commit/?h=for-next&id=853b39a7c82826b8413048feec7bf08e98ce7a84
> 	  They both will be reverted later.
> 
> 	  The problem is that we have 2 series routed via different
> 	  trees, RCU and VFS. We need this hack to ensure that this
> 	  series won't break alloc_super() which currently assumes that
> 	  destroy_super()->percpu_free_rwsem() is safe after kzalloc().
> 	  This way these 2 series do not depend on each other, we can
> 	  test/change/revert/etc them independently.
> 
> 	  We will add rcu_sync_dtor() into deactivate_locked_super()
> 	  later and revert both hacks.
> Oleg.

Queued for testing, thank you, Oleg!

Right now, this is mostly relying on 0day and -next testing.  Any thoughts
for a useful torture test for this?  One approach would be to treat it
like a reader-writer lock.  Other thoughts?

							Thanx, Paul

>  include/linux/percpu-rwsem.h  |    3 +-
>  include/linux/rcusync.h       |   56 +++++++++++++++
>  kernel/locking/percpu-rwsem.c |   85 ++++++++---------------
>  kernel/rcu/Makefile           |    2 +-
>  kernel/rcu/sync.c             |  151 +++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 240 insertions(+), 57 deletions(-)
> 

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