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Subject: Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when
 !CONFIG_DEBUG_LOCK_ALLOC
From:   Alan Huang <mmpgouride@gmail.com>
In-Reply-To: <7d433fac-a62d-4e81-b8e5-57cf5f2d1d55@paulmck-laptop>
Date:   Fri, 14 Jul 2023 23:35:41 +0800
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
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> 2023=E5=B9=B47=E6=9C=8814=E6=97=A5 10:16=EF=BC=8CPaul E. McKenney =
<paulmck@kernel.org> =E5=86=99=E9=81=93=EF=BC=9A
>=20
> 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=E2=80=AFAM Gao Xiang =
<hsiangkao@linux.alibaba.com> wrote:
>>>> On 2023/7/13 22:07, Joel Fernandes wrote:
>>>>> On Thu, Jul 13, 2023 at 12:59=E2=80=AFAM 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:
>>>>>>=20
>>>>>> ...
>>>>>>=20
>>>>>>>>=20
>>>>>>>> There are lots of performance issues here and even a plumber
>>>>>>>> topic last year to show that, see:
>>>>>>>>=20
>>>>>>>> [1] =
https://lore.kernel.org/r/20230519001709.2563-1-tj@kernel.org
>>>>>>>> [2] =
https://lore.kernel.org/r/CAHk-=3DwgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03=
qCOGg@mail.gmail.com
>>>>>>>> [3] =
https://lore.kernel.org/r/CAB=3DBE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyU=
qFneQ@mail.gmail.com
>>>>>>>> [4] https://lpc.events/event/16/contributions/1338/
>>>>>>>> and more.
>>>>>>>>=20
>>>>>>>> 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.)
>>>>>>>=20
>>>>>>> Hmmm...  Please let me try again.
>>>>>>>=20
>>>>>>> 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.
>>>>>>>=20
>>>>>>> And of course, for the approach to make sense, it must avoid =
breaking
>>>>>>> the existing lockdep-RCU debugging code.
>>>>>>>=20
>>>>>>> Is that more clear?
>>>>>>=20
>>>>>> 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.
>>>>>>=20
>>>>>=20
>>>>> 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);
>>>>>=20
>>>>> 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.
>>>>>=20
>>>>> z_erofs_decompressqueue_endio
>>>>> -> z_erofs_decompress_kickoff
>>>>>  ->z_erofs_decompressqueue_work
>>>>>   ->z_erofs_decompress_queue
>>>>>    -> z_erofs_decompress_pcluster
>>>>>     -> mutex_lock
>>>>>=20
>>>>=20
>>>> Why does the softirq path not trigger a workqueue instead?
>>>=20
>>> I said "if it is". I was giving a scenario. mutex_lock() is not
>>> allowed in softirq context or in an RCU-reader.
>>>=20
>>>>> Per Sandeep in [1], this stack happens under RCU read-lock in:
>>>>>=20
>>>>> #define __blk_mq_run_dispatch_ops(q, check_sleep, dispatch_ops) \
>>>>> [...]
>>>>>                 rcu_read_lock();
>>>>>                 (dispatch_ops);
>>>>>                 rcu_read_unlock();
>>>>> [...]
>>>>>=20
>>>>> Coming from:
>>>>> blk_mq_flush_plug_list ->
>>>>>                            blk_mq_run_dispatch_ops(q,
>>>>>                                 __blk_mq_flush_plug_list(q, =
plug));
>>>>>=20
>>>>> and __blk_mq_flush_plug_list does this:
>>>>>           q->mq_ops->queue_rqs(&plug->mq_list);
>>>>>=20
>>>>> This somehow ends up calling the bio_endio and the
>>>>> z_erofs_decompressqueue_endio which grabs the mutex.
>>>>>=20
>>>>> 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:
>>>>=20
>>>> 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.
>>>>=20
>>>> 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.
>>>=20
>>>> =46rom 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().
>>=20
>> How can this be solved?
>>=20
>> 1. Always use a workqueue.  Simple, but is said to have performance
>> issues.
>>=20
>> 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!)
>>=20
>> 3. Create a function in z_erofs that gives you a decent
>> approximation, maybe something like the following.
>>=20
>> 4. Other ideas here.
>=20
> 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=3Dy.)

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.

Not sure if this is feasible or acceptable. :)

>=20
> Thanx, Paul
>=20
>> The following is untested, and is probably quite buggy, but it should
>> provide you with a starting point.
>>=20
>> 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;
>> }
>>=20
>> You break it, you buy it!  ;-)
>>=20
>> Thanx, Paul