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

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Date:   Thu, 10 Oct 2019 10:51:45 +0200
From:   Alexander Gordeev <a.gordeev.box@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
        Michael Chen <micchen@...era.com>, dmaengine@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dmaengine: avalon: Intel Avalon-MM DMA Interface
 for PCIe

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);
> > > 
> > > This loop is very strange.  It feels like the last_id indexes needs
> > > to atomic or protected from racing somehow so we don't do an out of
> > > bounds read.

[...]

> You're missing my point.  When we set
> hw->d2h_last_id = 1;
[1]
> ...
> hw->d2h_last_id = 2;
[2]

> There is a tiny moment where ->d2h_last_id is transitioning from 1 to 2
> where its value is unknown.  We're in a busy loop here so we have a
> decent chance of hitting that 1/1000,000th of a second.  If we happen to
> hit it at exactly the right time then we're reading from a random
> address and it will cause an oops.
> 
> We have to use atomic_t types or something to handle race conditions.

Err.. I am still missing the point :( In your example I do see a chance
for a reader to read out 1 at point in time [2] - because of SMP race.
But what could it be other than 1 or 2?

Anyways, all code paths dealing with h2d_last_id and d2h_last_id indexes
are protected with a spinlock.

> regards,
> dan carpenter
>