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Date:   Fri, 14 Jul 2023 10:02:10 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Huang <mmpgouride@gmail.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Gao Xiang <hsiangkao@linux.alibaba.com>,
        Sandeep Dhavale <dhavale@google.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        linux-erofs@lists.ozlabs.org, xiang@kernel.org,
        Will Shiu <Will.Shiu@mediatek.com>, kernel-team@android.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
Subject: Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when
 !CONFIG_DEBUG_LOCK_ALLOC
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On Fri, Jul 14, 2023 at 11:54:47PM +0800, Alan Huang wrote:
> 
> > 2023年7月14日 23:35，Alan Huang <mmpgouride@gmail.com> 写道：
> > 
> >> 
> >> 2023年7月14日 10:16，Paul E. McKenney <paulmck@kernel.org> 写道：
> >> 
> >> On Thu, Jul 13, 2023 at 09:33:35AM -0700, Paul E. McKenney wrote:
> >>> 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@linux.alibaba.com> wrote:
> >>>>> On 2023/7/13 22:07, Joel Fernandes wrote:
> >>>>>> On Thu, Jul 13, 2023 at 12:59 AM Gao Xiang <hsiangkao@linux.alibaba.com> 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@kernel.org
> >>>>>>>>> [2] https://lore.kernel.org/r/CAHk-=wgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03qCOGg@mail.gmail.com
> >>>>>>>>> [3] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@mail.gmail.com
> >>>>>>>>> [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.
> >> 
> >> 5. #3 plus make the corresponding Kconfig option select
> >> PREEMPT_COUNT, assuming that any users needing compression in
> >> non-preemptible kernels are OK with PREEMPT_COUNT being set.
> >> (Some users of non-preemptible kernels object strenuously
> >> to the added overhead from CONFIG_PREEMPT_COUNT=y.)
> > 
> > 6. Set one bit in bio->bi_private, check the bit and flip it in rcu_read_lock() path,
> > then in z_erofs_decompressqueue_endio, check if the bit has changed.
> 
> Seems bad, read and modify bi_private is a bad idea.

Is there some other field that would work?

							Thanx, Paul

> > Not sure if this is feasible or acceptable. :)
> > 
> >> 
> >> Thanx, Paul
> >> 
> >>> 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
> 
>