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SubjectRe: [PATCH 1/2] dmaengine: shdma: fix locking
On Fri, Apr 29, 2011 at 07:09:21PM +0200, Guennadi Liakhovetski wrote:
> static int sh_dmae_rst(struct sh_dmae_device *shdev)
> {
..
> + dmaor_write(shdev, dmaor | shdev->pdata->dmaor_init);

On Fri, Apr 29, 2011 at 07:09:25PM +0200, Guennadi Liakhovetski wrote:
> +static int sh_dmae_runtime_resume(struct device *dev)
> +{
> + struct sh_dmae_device *shdev = dev_get_drvdata(dev);
> +
> + return sh_dmae_rst(shdev);
..

Yet in sh_dmae_probe() we have:

shdev->pdata = pdata;

pm_runtime_enable(&pdev->dev);
pm_runtime_get_sync(&pdev->dev);

..

/* reset dma controller - only needed as a test */
err = sh_dmae_rst(shdev);
if (err)
goto rst_err;
..

pm_runtime_put(&pdev->dev);

platform_set_drvdata(pdev, shdev);
dma_async_device_register(&shdev->common);

return err;
..

So I'm wondering how this was ever actually tested. The original sh_dmae_rst()
call is safe due to passing along the shdev pointer with pdata initialized
explicitly, while the runtime PM bits fetch the pointer via dev_get_drvdata()
at a time where drvdata hasn't even been initialized yet, resulting in a rather
predictable oops:

Unable to handle kernel NULL pointer dereference at virtual address 000000c4
pc = 8025adee
*pde = 00000000
Oops: 0000 [#1]
Modules linked in:
Pid : 1, Comm: swapper
CPU : 0 Not tainted (3.0.0-rc1-00012-g9436b4a-dirty #1456)

PC is at sh_dmae_rst+0x28/0x86
PR is at sh_dmae_rst+0x22/0x86
PC : 8025adee SP : 9e803d10 SR : 400080f1 TEA : 000000c4
R0 : 000000c4 R1 : 0000fff8 R2 : 00000000 R3 : 00000040
R4 : 000000f0 R5 : 00000000 R6 : 00000000 R7 : 804f184c
R8 : 00000000 R9 : 804dd0e8 R10 : 80283204 R11 : ffffffda
R12 : 000000a0 R13 : 804dd18c R14 : 9e803d10
MACH: 00000000 MACL: 00008f20 GBR : 00000000 PR : 8025ade8
Call trace:
[<8025ae70>] sh_dmae_runtime_resume+0x24/0x34
[<80283238>] pm_generic_runtime_resume+0x34/0x3c
[<80283370>] rpm_callback+0x4a/0x7e
[<80283efc>] rpm_resume+0x240/0x384
[<80283f54>] rpm_resume+0x298/0x384
[<8028428c>] __pm_runtime_resume+0x44/0x7c
[<8038a358>] __ioremap_caller+0x0/0xec
[<80284296>] __pm_runtime_resume+0x4e/0x7c
[<8038a358>] __ioremap_caller+0x0/0xec
[<80666254>] sh_dmae_probe+0x180/0x6a0
[<802803ae>] platform_drv_probe+0x26/0x2e

I've fixed this up now, but I am growing rather weary of applying anything with
runtime PM in the subject that alleges to have been tested.

The next runtime PM patch that doesn't even boot will be immediately reverted
and have a kernel version or two to sit things out in order to try to get
things in demonstrable functional order.


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