Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC

[Paul E. McKenney]

On Thu, Jul 13, 2023 at 11:33:24AM -0400, Joel Fernandes wrote:
> On Thu, Jul 13, 2023 at 10:34 AM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
> > On 2023/7/13 22:07, Joel Fernandes wrote:
> > > On Thu, Jul 13, 2023 at 12:59 AM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
> > >> On 2023/7/13 12:52, Paul E. McKenney wrote:
> > >>> On Thu, Jul 13, 2023 at 12:41:09PM +0800, Gao Xiang wrote:
> > >>
> > >> ...
> > >>
> > >>>>
> > >>>> There are lots of performance issues here and even a plumber
> > >>>> topic last year to show that, see:
> > >>>>
> > >>>> [1] https://lore.kernel.org/r/20230519001709.2563-1-tj@xxxxxxxxxx
> > >>>> [2] https://lore.kernel.org/r/CAHk-=wgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03qCOGg@xxxxxxxxxxxxxx
> > >>>> [3] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@xxxxxxxxxxxxxx
> > >>>> [4] https://lpc.events/event/16/contributions/1338/
> > >>>> and more.
> > >>>>
> > >>>> I'm not sure if it's necessary to look info all of that,
> > >>>> andSandeep knows more than I am (the scheduling issue
> > >>>> becomes vital on some aarch64 platform.)
> > >>>
> > >>> Hmmm...  Please let me try again.
> > >>>
> > >>> Assuming that this approach turns out to make sense, the resulting
> > >>> patch will need to clearly state the performance benefits directly in
> > >>> the commit log.
> > >>>
> > >>> And of course, for the approach to make sense, it must avoid breaking
> > >>> the existing lockdep-RCU debugging code.
> > >>>
> > >>> Is that more clear?
> > >>
> > >> Personally I'm not working on Android platform any more so I don't
> > >> have a way to reproduce, hopefully Sandeep could give actually
> > >> number _again_ if dm-verity is enabled and trigger another
> > >> workqueue here and make a comparsion why the scheduling latency of
> > >> the extra work becomes unacceptable.
> > >>
> > >
> > > Question from my side, are we talking about only performance issues or
> > > also a crash? It appears z_erofs_decompress_pcluster() takes
> > > mutex_lock(&pcl->lock);
> > >
> > > So if it is either in an RCU read-side critical section or in an
> > > atomic section, like the softirq path, then it may
> > > schedule-while-atomic or trigger RCU warnings.
> > >
> > > z_erofs_decompressqueue_endio
> > > -> z_erofs_decompress_kickoff
> > >   ->z_erofs_decompressqueue_work
> > >    ->z_erofs_decompress_queue
> > >     -> z_erofs_decompress_pcluster
> > >      -> mutex_lock
> > >
> >
> > Why does the softirq path not trigger a workqueue instead?
> 
> I said "if it is". I was giving a scenario. mutex_lock() is not
> allowed in softirq context or in an RCU-reader.
> 
> > > Per Sandeep in [1], this stack happens under RCU read-lock in:
> > >
> > > #define __blk_mq_run_dispatch_ops(q, check_sleep, dispatch_ops) \
> > > [...]
> > >                  rcu_read_lock();
> > >                  (dispatch_ops);
> > >                  rcu_read_unlock();
> > > [...]
> > >
> > > Coming from:
> > > blk_mq_flush_plug_list ->
> > >                             blk_mq_run_dispatch_ops(q,
> > >                                  __blk_mq_flush_plug_list(q, plug));
> > >
> > > and __blk_mq_flush_plug_list does this:
> > >            q->mq_ops->queue_rqs(&plug->mq_list);
> > >
> > > This somehow ends up calling the bio_endio and the
> > > z_erofs_decompressqueue_endio which grabs the mutex.
> > >
> > > So... I have a question, it looks like one of the paths in
> > > __blk_mq_run_dispatch_ops() uses SRCU.  Where are as the alternate
> > > path uses RCU. Why does this alternate want to block even if it is not
> > > supposed to? Is the real issue here that the BLK_MQ_F_BLOCKING should
> > > be set? It sounds like you want to block in the "else" path even
> > > though BLK_MQ_F_BLOCKING is not set:
> >
> > BLK_MQ_F_BLOCKING is not a flag that a filesystem can do anything with.
> > That is block layer and mq device driver stuffs. filesystems cannot set
> > this value.
> >
> > As I said, as far as I understand, previously,
> > .end_io() can only be called without RCU context, so it will be fine,
> > but I don't know when .end_io() can be called under some RCU context
> > now.
> 
> >From what Sandeep described, the code path is in an RCU reader. My
> question is more, why doesn't it use SRCU instead since it clearly
> does so if BLK_MQ_F_BLOCKING. What are the tradeoffs? IMHO, a deeper
> dive needs to be made into that before concluding that the fix is to
> use rcu_read_lock_any_held().

How can this be solved?

1.	Always use a workqueue.  Simple, but is said to have performance
	issues.

2.	Pass a flag in that indicates whether or not the caller is in an
	RCU read-side critical section.  Conceptually simple, but might
	or might not be reasonable to actually implement in the code as
	it exists now.	(You tell me!)

3.	Create a function in z_erofs that gives you a decent
	approximation, maybe something like the following.

4.	Other ideas here.

The following is untested, and is probably quite buggy, but it should
provide you with a starting point.

static bool z_erofs_wq_needed(void)
{
	if (IS_ENABLED(CONFIG_PREEMPTION) && rcu_preempt_depth())
		return true;  // RCU reader
	if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && !preemptible())
		return true;  // non-preemptible
	if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
		return true;  // non-preeemptible kernel, so play it safe
	return false;
}

You break it, you buy it!  ;-)

							Thanx, Paul