On Thu, Mar 30, 2017 at 09:38:39AM +0200, Niklas Söderlund wrote: > Hi Geert, > > On 2017-03-29 15:30:42 +0200, Niklas Söderlund wrote: > > Hi Geert, > > > > On 2017-03-29 14:31:33 +0200, Geert Uytterhoeven wrote: > > > Hi Niklas, > > > > > > On Wed, Mar 29, 2017 at 12:40 AM, Niklas Söderlund > > > <niklas.soderlund+renesas@xxxxxxxxxxxx> wrote: > > > > This fixes a race condition where the channel resources could be freed > > > > before the ISR had finished running resulting in a NULL pointer > > > > reference from the ISR. > > > > > > > > [ 167.148934] Unable to handle kernel NULL pointer dereference at virtual address 00000000 > > > > [ 167.157051] pgd = ffff80003c641000 > > > > [ 167.160449] [00000000] *pgd=000000007c507003, *pud=000000007c4ff003, *pmd=0000000000000000 > > > > [ 167.168719] Internal error: Oops: 96000046 [#1] PREEMPT SMP > > > > [ 167.174289] Modules linked in: > > > > [ 167.177348] CPU: 3 PID: 10547 Comm: dma_ioctl Not tainted 4.11.0-rc1-00001-g8d92afddc2f6633a #73 > > > > [ 167.186131] Hardware name: Renesas Salvator-X board based on r8a7795 (DT) > > > > [ 167.192917] task: ffff80003a411a00 task.stack: ffff80003bcd4000 > > > > [ 167.198850] PC is at rcar_dmac_chan_prep_sg+0xe0/0x400 > > > > [ 167.203985] LR is at rcar_dmac_chan_prep_sg+0x48/0x400 > > > > > > Do you have a test case to trigger this? > > > > Yes I have a testcase, it's rather complex and involves both a kernel > > module and a userspaces application to stress the rcar-dmac. I'm > > checking if I can share this publicly or not, please hold :-) > > I have now received feedback that I'm unfortunately not allowed to share > the test case :-( > > The big picture in how to trigger this problem is that you start a DMA > transfer like this: > > struct dma_async_tx_descriptor *tx = ...; > > ... > > tx->tx_submit(tx); > > And then you directly call dma_release_channel() on this channel without > making sure the completion callback ran or anything. Now if you are > unlucky the ISR have not finished running for the DMA when > dma_release_channel() starts to clean up resources. The synchronisation > point in the dma_release_channel() call path fixes this. Well the API expectation would be you abort the txn before calling release. So the expected order should be: dmaengine_terminate_all(); dma_release_channel(); Terminate should then stop the channel, ie abort the pending descriptors.. -- ~Vinod
