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Date: Sun, 22 May 2016 23:21:23 +0200 From: Francois Romieu <romieu@...zoreil.com> To: Lino Sanfilippo <LinoSanfilippo@....de> Cc: Shuyu Wei <wsy2220@...il.com>, David Miller <davem@...emloft.net>, wxt@...k-chips.com, heiko@...ech.de, linux-rockchip@...ts.infradead.org, netdev@...r.kernel.org Subject: Re: [PATCH v2] ethernet:arc: Fix racing of TX ring buffer Lino Sanfilippo <LinoSanfilippo@....de> : > On 21.05.2016 21:47, Francois Romieu wrote: > > Shuyu Wei <wsy2220@...il.com> : > > [...] > >> diff --git a/drivers/net/ethernet/arc/emac_main.c b/drivers/net/ethernet/arc/emac_main.c > >> index a3a9392..c2447b0 100644 > >> --- a/drivers/net/ethernet/arc/emac_main.c > >> +++ b/drivers/net/ethernet/arc/emac_main.c > >> @@ -686,6 +686,9 @@ static int arc_emac_tx(struct sk_buff *skb, struct net_device *ndev) > >> > >> skb_tx_timestamp(skb); > >> > >> + /* Make sure timestamp is set */ > >> + smp_wmb(); > > > > Should be dma_wmb() (see davem's message). > > > > It's completely unrelated to SMP. > > > > Its also completely unrelated to dma so I doubt that this is what davem meant. It's related to dma: nobody wants the device to perform dma from memory while the CPU is writing timestamp. The device must enforce a commit of any network buffer memory write before releasing control, i.e. before writing *info. See 'git log -p --grep=dma_[rw]mb': it appears several times. > As far as I understood he was referring to the dma descriptor. If the wmb() between *info = ... and *txbd_curr = ... is replaced by a dma_wmb(), the device will see a consistent descriptor when if performs a DMA read after the CPU wrote into the mailbox (arc_reg_set(..., TXPL_MASK)). Ok, I agree on this one. However, it doesn't help with the (SMP) requirement that no CPU sees the txbd_curr write before *info is written by the local CPU. Afaiui dma_wmb() is too weak for this part. If we don't want wmb() here, it will have to be dma_wmb() + smp_wmb(). -- Ueimor
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