linux-kernel - Re: [PATCH v2 1/2] dmaengine: avalon: Intel Avalon-MM DMA Interface for PCIe

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Date:   Tue, 15 Oct 2019 16:19:55 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Alexander Gordeev <a.gordeev.box@...il.com>
Cc:     devel@...verdev.osuosl.org, Michael Chen <micchen@...era.com>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dmaengine: avalon: Intel Avalon-MM DMA Interface
 for PCIe

On Tue, Oct 15, 2019 at 01:24:50PM +0200, Alexander Gordeev wrote:
> On Thu, Oct 10, 2019 at 02:30:34PM +0300, Dan Carpenter wrote:
> > On Thu, Oct 10, 2019 at 10:51:45AM +0200, Alexander Gordeev wrote:
> > > On Wed, Oct 09, 2019 at 09:53:23PM +0300, Dan Carpenter wrote:
> > > > > > > +	u32 *rd_flags = hw->dma_desc_table_rd.cpu_addr->flags;
> > > > > > > +	u32 *wr_flags = hw->dma_desc_table_wr.cpu_addr->flags;
> > > > > > > +	struct avalon_dma_desc *desc;
> > > > > > > +	struct virt_dma_desc *vdesc;
> > > > > > > +	bool rd_done;
> > > > > > > +	bool wr_done;
> > > > > > > +
> > > > > > > +	spin_lock(lock);
> 
> [*]
> 
> > > > > > > +
> > > > > > > +	rd_done = (hw->h2d_last_id < 0);
> > > > > > > +	wr_done = (hw->d2h_last_id < 0);
> > > > > > > +
> > > > > > > +	if (rd_done && wr_done) {
> > > > > > > +		spin_unlock(lock);
> > > > > > > +		return IRQ_NONE;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	do {
> > > > > > > +		if (!rd_done && rd_flags[hw->h2d_last_id])
> > > > > > > +			rd_done = true;
> > > > > > > +
> > > > > > > +		if (!wr_done && wr_flags[hw->d2h_last_id])
> > > > > > > +			wr_done = true;
> > > > > > > +	} while (!rd_done || !wr_done);


Thinking about this some more, my initial instinct was still correct
actually.  If we're holding the lock to prevent the CPU from writing
to it then how is hw->d2h_last_id updated in the other thread?  Either
it must deadlock or it must be a race condition.

regards,
dan carpenter