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

[Gao Xiang]

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? why here
it triggers "schedule-while-atomic" in the softirq context?

> 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.


Thanks,
Gao Xiang