[PATCH v3] serial: stm32: Merge hard IRQ and threaded IRQ handling into single IRQ handler

[Sebastian Andrzej Siewior]

On 2022-12-27 15:56:47 [+0100], Johan Hovold wrote:
> On Fri, Dec 16, 2022 at 12:53:38PM +0100, Marek Vasut wrote:
> > Requesting an interrupt with IRQF_ONESHOT will run the primary handler
> > in the hard-IRQ context even in the force-threaded mode. The
> > force-threaded mode is used by PREEMPT_RT in order to avoid acquiring
> > sleeping locks (spinlock_t) in hard-IRQ context. This combination
> > makes it impossible and leads to "sleeping while atomic" warnings.
> > 
> > Use one interrupt handler for both handlers (primary and secondary)
> > and drop the IRQF_ONESHOT flag which is not needed.
> > 
> > Fixes: e359b4411c283 ("serial: stm32: fix threaded interrupt handling")
> 
> I don't think a Fixes tag is warranted as this is only needed due to
> this undocumented quirk of PREEMPT_RT.

It is not an undocumented quirk of PREEMPT_RT. The part that might not
be well documented is that IRQF_ONESHOT won't run the primary handler as
a threaded handler. This is also the case for IRQF_NO_THREAD and
IRQF_PERCPU but might be more obvious.
Anyway, if the primary handler is not threaded then it runs in HARDIRQ
context and here you must not use a spinlock_t. This is documented in
Documentation/locking/locktypes.rst and there is also a LOCKDEP warning
for this on !RT which is off by default because it triggers with printk
(and this is worked on).

> And this should not be backported in any case.

Such things have been backported via -stable in the past. If you
disagree, please keep me in loop while is merged so I can poke people to
backport it as part of the RT patch for the relevant kernels.

> Johan

Sebastian