linux-kernel - Re: [PATCH v1 3/7] dmaengine: tegra-apb: Prevent race conditions on channel's freeing

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Date:   Fri, 3 Jan 2020 09:16:04 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Laxman Dewangan <ldewangan@...dia.com>,
        Vinod Koul <vkoul@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        dmaengine@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/7] dmaengine: tegra-apb: Prevent race conditions on
 channel's freeing

On Thu, Jan 02, 2020 at 06:09:45PM +0300, Dmitry Osipenko wrote:
> 30.12.2019 23:50, Michał Mirosław пишет:
> > On Mon, Dec 30, 2019 at 09:45:55PM +0100, Michał Mirosław wrote:
> >> On Sat, Dec 28, 2019 at 11:46:36PM +0300, Dmitry Osipenko wrote:
> >>> It's unsafe to check the channel's "busy" state without taking a lock,
> >>> it is also unsafe to assume that tasklet isn't in-fly.
> >>
> >> 'in-flight'. Also, the patch seems to have two independent bug-fixes
> >> in it. Second one doesn't look right, at least not without an explanation.
> >>
> >> First:
> >>
> >>> -	if (tdc->busy)
> >>> -		tegra_dma_terminate_all(dc);
> >>> +	tegra_dma_terminate_all(dc);
> >>
> >> Second:
> >>
> >>> +	tasklet_kill(&tdc->tasklet);
> > 
> > BTW, maybe you can convert the code to threaded interrupt handler and
> > just get rid of the tasklet instead of fixing it?
> 
> This shouldn't bring much benefit because the the code's logic won't be
> changed since we will still have to use the threaded ISR part as the
> bottom-half and then IRQ API doesn't provide a nice way to synchronize
> interrupt's execution, while tasklet_kill() is a nice way to sync it.

What about synchronize_irq()?

BTW, does tegra_dma_terminate_all() prevent further interrupts that might
cause the tasklet to be scheduled again?

Best Regards,
Michał Mirosław