linux-kernel - Re: [PATCH PREEMPT_RT] kcov: fix locking splat from kcov_remote

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Date:   Tue, 10 Aug 2021 22:38:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Clark Williams <williams@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH PREEMPT_RT] kcov:  fix locking splat from
 kcov_remote_start()

On Tue, Aug 10 2021 at 11:50, Sebastian Andrzej Siewior wrote:
> On 2021-08-09 15:59:09 [-0500], Clark Williams wrote:
>> Saw the following splat on 5.14-rc4-rt5 with:
> …
>> Change kcov_remote_lock from regular spinlock_t to raw_spinlock_t so that
>> we don't get "sleeping function called from invalid context" on PREEMPT_RT kernel.
>
> I'm not entirely happy with that:
> - kcov_remote_start() decouples spin_lock_irq() and does local_irq_save()
>   + spin_lock() which shouldn't be done as per
>       Documentation/locking/locktypes.rst
>   I would prefer to see the local_irq_save() replaced by
>   local_lock_irqsave() so we get a context on what is going on.

Which does not make it raw unless we create a raw_local_lock.

> - kcov_remote_reset() has a kfree() with that irq-off lock acquired.

That free needs to move out obviously

> - kcov_remote_add() has a kmalloc() and is invoked with that irq-off
>   lock acquired.

So does the kmalloc.

> - kcov_remote_area_put() uses INIT_LIST_HEAD() for no reason (just
>   happen to notice).
>
> - kcov_remote_stop() does local_irq_save() + spin_lock(&kcov->lock);.
>   This should also create a splat.
>
> - With lock kcov_remote_lock acquired there is a possible
>   hash_for_each_safe() and list_for_each() iteration. I don't know what
>   the limits are here but with a raw_spinlock_t it will contribute to
>   the maximal latency. 

And that matters because? kcov has a massive overhead and with that
enabled you care as much about latencies as you do when running with
lockdep enabled.

Thanks,

        tglx