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

[Joel Fernandes]

On Thu, Jul 13, 2023 at 10:34 AM Gao Xiang <hsiangkao at 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 at 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().

 - Joel