Messages in this thread | | | From | John Ogness <> | Subject | Re: [PATCH printk v4 06/27] printk: nbcon: Add callbacks to synchronize with driver | Date | Thu, 18 Apr 2024 14:16:16 +0206 |
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On 2024-04-18, Petr Mladek <pmladek@suse.com> wrote: > I am not sure how it is done in other parts of kernel code where > RT needed to introduce some tricks. But I think that we should > really start mentioning RT behavior in the commit messages and > and comments where the RT mode makes huge changes.
Yes, our motivation is RT. But these semantics are not RT-specific. They apply to the general kernel locking model. For example, even for a !RT system, it is semantically incorrect to take a spin_lock while holding a raw_spin_lock.
In the full PREEMPT_RT series I have tried to be careful about only mentioning PREEMPT_RT when it is really PREEMPT_RT-specific. For example [0][1][2].
[0] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-6.9.y-rt-rebase&id=1564af55a92c32fe215af35cf55cb9359c5fff30
[1] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-6.9.y-rt-rebase&id=033b416ad25b17dc60d5f71c1a0b33a5fbc17639
[2] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=linux-6.9.y-rt-rebase&id=7929ba9e5c110148a1fcd8bd93d6a4eff37aa265
> The race could NOT happen in: > > + NBCON_PRIO_PANIC context because it does not schedule
Yes.
> + NBCON_PRIO_EMERGENCY context because we explicitly disable > preemption there
Yes.
> + NBCON_NORMAL_PRIO context when we ALWAYS do nbcon_try_acquire() > under con->device() lock. Here the con->device_lock() serializes > nbcon_try_acquire() calls even between running tasks.
The nbcon_legacy_emit_next_record() printing as NBCON_NORMAL_PRIO is a special situation where write_atomic() is used. It is safe because it disables hard interrupts and is never called from NMI context.
nbcon_atomic_flush_pending() as NBCON_NORMAL_PRIO is safe in !NMI because it also disables hard interrupts. However, nbcon_atomic_flush_pending() could be called in NMI with NBCON_NORMAL_PRIO. I need to think about this case.
John
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