netdev - Re: [Bugme-new] [Bug 38102] New: BUG kmalloc-2048: Poison overwritten

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Date:	Tue, 5 Jul 2011 12:42:02 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexey Zaytsev <alexey.zaytsev@...il.com>,
	Michael Büsch <m@...s.ch>,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev@...r.kernel.org, Gary Zambrano <zambrano@...adcom.com>,
	bugme-daemon@...zilla.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Pekka Pietikainen <pp@...oulu.fi>,
	Florian Schirmer <jolt@...box.org>,
	Felix Fietkau <nbd@...nwrt.org>, Michael Buesch <mb@...sch.de>
Subject: Re: [Bugme-new] [Bug 38102] New: BUG kmalloc-2048: Poison overwritten

On Tue, Jul 05, 2011 at 06:12:32PM +0200, Eric Dumazet wrote:
> Le mardi 05 juillet 2011 à 12:05 -0400, Neil Horman a écrit :
> > On Tue, Jul 05, 2011 at 07:59:33AM +0200, Eric Dumazet wrote:
> > > Le mardi 05 juillet 2011 à 07:33 +0200, Eric Dumazet a écrit :
> > > > Le mardi 05 juillet 2011 à 09:18 +0400, Alexey Zaytsev a écrit :
> > > > 
> > > > > Actually, I've added a trace to show b44_init_rings and b44_free_rings
> > > > > calls, and they are only called once, right after the driver is
> > > > > loaded. So it can't be related to START_RFO. Will attach the diff and
> > > > > dmesg.
> > > > 
> > > > Thanks
> > > > 
> > > > I was wondering if DMA could be faster if providing word aligned
> > > > addresses, could you try :
> > > > 
> > > > -#define RX_PKT_OFFSET          (RX_HEADER_LEN + 2)
> > > > +#define RX_PKT_OFFSET          (RX_HEADER_LEN + NET_IP_ALIGN)
> > > > 
> > > > (On x86, we now have NET_IP_ALIGN = 0 since commit ea812ca1)
> > > > 
> > > 
> > > I suspect a hardware bug.
> > > 
> > I'm not sure if this helps, but I've been reading over this bug, and it seems
> > that the rx path never checks the status of a buffers rx header prior to
> > unmapping it or otherwise modifying it in hardware.  If we were to start munging
> > pointers in the rx channel while a dma was active in it still, it sems like the
> > observed corruption might be the result.  The docs aren't super clear on this,
> > but I think a descriptor needs to be in the idle wait or stopped state prior to
> > being acessed.  This patch might help out there (although I don't have hardware
> > to test)
> > Neil
> > 
> > diff --git a/drivers/net/b44.c b/drivers/net/b44.c
> > index 3d247f3..48540ad 100644
> > --- a/drivers/net/b44.c
> > +++ b/drivers/net/b44.c
> > @@ -769,7 +769,19 @@ static int b44_rx(struct b44 *bp, int budget)
> >  		dma_addr_t map = rp->mapping;
> >  		struct rx_header *rh;
> >  		u16 len;
> > -
> > +		u32 state = br32(bp, B44_DMARX_STAT) & DMARX_STAT_SMASK;
> > +		state >>= 12;
> > +
> > +		/*
> > + 		 * I _think_ descriptors need to be in the idle or stopped state
> > + 		 * before its safe to access them.  If the current buffer
> > + 		 * pointed to by the dma channel is in state 1 or lower (active
> > + 		 * or disabled), then we should just stop receving until the
> > + 		 * next interrupt kicks us again (I think)
> > + 		 */
> > +		if (state < 2)
> > +			return;
> > + 
> >  		dma_sync_single_for_cpu(bp->sdev->dev, map,
> >  					    RX_PKT_BUF_SZ,
> >  					    DMA_FROM_DEVICE);
> 
> Hmm... We are in a NAPI handler... There wont be a new interrupt.
> 
Not until we're done with the napi handler, no.  But if we encounter a dma
descriptor that isn't idle, then we know that either we're clearing out the ring
(ie. for a shutdown), or we'll get another interrupt when the descriptor we
failed on completes.

> Plus, we do at start of b44_rx() :
> 
> prod  = br32(bp, B44_DMARX_STAT) & DMARX_STAT_CDMASK;
> 
Yes, that just tells us which is the current dma index.  After that we loop
through subsequent dma descriptor incrementing the index each time.

> So all descriptors before prod are guaranteed to be ready for host
> consume... Fact that a dma access is running on 'next descriptor' should
> be irrelevant.
> 
But we handle more than one descriptor per b44_rx call - theres a while loop in
there where we do advance to the next descriptor.

> IMHO Peeking B44_DMARX_STAT for each packet would be a waste of time.
> 
> 
> 
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