Re: [PATCH] dmaengine: altera-msgdma: fix descriptors freeing logic

[Eric Schwarz]

Hello Olivier,

> > Am 20.09.2023 um 21:58 schrieb Olivier Dautricourt:
> > > Sparse complains because we first take the lock in msgdma_tasklet -> move
> > > locking to msgdma_chan_desc_cleanup.
> > > In consequence, move calling of msgdma_chan_desc_cleanup outside of the
> > > critical section of function msgdma_tasklet.
> > >
> > > Use spin_unlock_irqsave/restore instead of just spinlock/unlock to keep
> > > state of irqs while executing the callbacks.
> >
> > What about the locking in the IRQ handler msgdma_irq_handler() itself? -
> > Shouldn't spin_unlock_irqsave/restore() be used there as well instead of
> > just spinlock/unlock()?
>
> IMO no:
> It is covered by [1]("Locking Between Hard IRQ and Softirqs/Tasklets")
> The irq handler cannot be preempted by the tasklet, so the
> spin_lock/unlock version is ok. However the tasklet could be interrupted
> by the Hard IRQ hence the disabling of irqs with save/restore when
> entering critical section.
>
> It should not be needed to keep interrupts locally disabled while invoking
> callbacks, will add this to the commit description.
>
> [1] https://www.kernel.org/doc/Documentation/kernel-hacking/locking.rst

Thanks for the link. I have read differently here [2] w/ special 
emphasis on "Lesson 3: spinlocks revisited.".

[2] https://www.kernel.org/doc/Documentation/locking/spinlocks.txt

Cheers
Eric