linux-kernel - Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

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Message-Id: <20090126140116.35f9c173.akpm@linux-foundation.org>
Date:	Mon, 26 Jan 2009 14:01:16 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	oleg@...hat.com, a.p.zijlstra@...llo.nl, rusty@...tcorp.com.au,
	travis@....com, mingo@...hat.com, davej@...hat.com,
	cpufreq@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

On Mon, 26 Jan 2009 22:45:16 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Mon, 26 Jan 2009 22:27:27 +0100
> > Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > 
> > > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > 
> > > > > So if it's generic it ought to be implemented in a generic way - not a 
> > > > > "dont use from any codepath that has a lock held that might 
> > > > > occasionally also be held in a keventd worklet". (which is a totally 
> > > > > unmaintainable proposition and which would just cause repeat bugs 
> > > > > again and again.)
> > > > 
> > > > That's different.  The core fault here lies in the keventd workqueue 
> > > > handling code.  If we're flushing work A then we shouldn't go and 
> > > > block behind unrelated work B.
> > > 
> > > the blocking is inherent in the concept of "a queue of worklets 
> > > handled by a single thread".
> > > 
> > > If a worklet is blocked then all other work performed by that thread 
> > > is blocked as well. So by waiting on a piece of work in the queue, we 
> > > wait for all prior work queued up there as well.
> > > 
> > > The only way to decouple that and to make them independent (and hence 
> > > independently flushable) is to create more parallel flows of 
> > > execution: i.e. by creating another thread (another workqueue).
> > > 
> > 
> > Nope.  As I said, the caller of flush_work() can detach the work item 
> > and run it directly.
> 
> that would change the concept of execution but indeed it would be 
> interesting to try. It's outside the scope of late -rcs i guess, but 
> worthwile nevertheless.
> 

Well it turns out that I was having a less-than-usually-senile moment:

: commit b89deed32ccc96098bd6bc953c64bba6b847774f
: Author:     Oleg Nesterov <oleg@...sign.ru>
: AuthorDate: Wed May 9 02:33:52 2007 -0700
: Commit:     Linus Torvalds <torvalds@...dy.linux-foundation.org>
: CommitDate: Wed May 9 12:30:50 2007 -0700
: 
:     implement flush_work()
:     
:     A basic problem with flush_scheduled_work() is that it blocks behind _all_
:     presently-queued works, rather than just the work whcih the caller wants to
:     flush.  If the caller holds some lock, and if one of the queued work happens
:     to want that lock as well then accidental deadlocks can occur.
:     
:     One example of this is the phy layer: it wants to flush work while holding
:     rtnl_lock().  But if a linkwatch event happens to be queued, the phy code will
:     deadlock because the linkwatch callback function takes rtnl_lock.
:     
:     So we implement a new function which will flush a *single* work - just the one
:     which the caller wants to free up.  Thus we avoid the accidental deadlocks
:     which can arise from unrelated subsystems' callbacks taking shared locks.
:     
:     flush_work() non-blockingly dequeues the work_struct which we want to kill,
:     then it waits for its handler to complete on all CPUs.
:     
:     Add ->current_work to the "struct cpu_workqueue_struct", it points to
:     currently running "struct work_struct". When flush_work(work) detects
:     ->current_work == work, it inserts a barrier at the _head_ of ->worklist
:     (and thus right _after_ that work) and waits for completition. This means
:     that the next work fired on that CPU will be this barrier, or another
:     barrier queued by concurrent flush_work(), so the caller of flush_work()
:     will be woken before any "regular" work has a chance to run.
:     
:     When wait_on_work() unlocks workqueue_mutex (or whatever we choose to protect
:     against CPU hotplug), CPU may go away. But in that case take_over_work() will
:     move a barrier we queued to another CPU, it will be fired sometime, and
:     wait_on_work() will be woken.
:     
:     Actually, we are doing cleanup_workqueue_thread()->kthread_stop() before
:     take_over_work(), so cwq->thread should complete its ->worklist (and thus
:     the barrier), because currently we don't check kthread_should_stop() in
:     run_workqueue(). But even if we did, everything should be ok.

Why isn't that working in this case??

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