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

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Date:   Fri, 22 Sep 2023 09:49:59 +0200
From:   Eric Schwarz <eas@...optimization.com>
To:     Olivier Dautricourt <olivierdautricourt@...il.com>
Cc:     dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vinod Koul <vkoul@...nel.org>, Stefan Roese <sr@...x.de>
Subject: Re: [PATCH] dmaengine: altera-msgdma: fix descriptors freeing logic

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