Re: [PATCH/RFC] driver core: Postpone DMA tear-down until after devres release

From: Robin Murphy
Date: Fri Feb 08 2019 - 12:55:39 EST

On 08/02/2019 16:40, Joerg Roedel wrote:
Hi Geert,

On Thu, Feb 07, 2019 at 08:36:53PM +0100, Geert Uytterhoeven wrote:
diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 8ac10af17c0043a3..d62487d024559620 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -968,9 +968,9 @@ static void __device_release_driver(struct device *dev, struct device *parent)
- arch_teardown_dma_ops(dev);
+ arch_teardown_dma_ops(dev);
dev->driver = NULL;
dev_set_drvdata(dev, NULL);
if (dev->pm_domain && dev->pm_domain->dismiss)

Thanks for the fix! Should it also be tagged for stable and get a Fixes
tag? I know it only triggers with a fix in v5.0-rc, but still...

I think so:

Fixes: 09515ef5ddad ("of/acpi: Configure dma operations at probe time for platform/amba/pci bus devices")

There aren't many drivers using dmam_alloc_*(), let alone which would also find themselves behind an IOMMU on an Arm system, but it turns out I actually have another one which can reproduce the BUG() with 5.0-rc.

I've tried a 4.12 kernel with a bit of instrumentation[1] and sure enough the devres-managed buffer is freed with the wrong ops[2] even then. How it manages not to blow up more catastrophically I have no idea... I guess at best it just leaks the buffers and IOMMU mappings, and at worst quietly frees random other pages instead.



diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 4f3eecedca2d..f4dbaa5598e3 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -491,6 +491,7 @@ static inline void *dma_alloc_attrs(struct device *dev, size_t size,
return NULL;

cpu_addr = ops->alloc(dev, size, dma_handle, flag, attrs);
+ dev_info(dev, "alloc %lx %lx\n", (unsigned long)cpu_addr, (unsigned long)ops);
debug_dma_alloc_coherent(dev, size, *dma_handle, cpu_addr);
return cpu_addr;
@@ -512,6 +513,7 @@ static inline void dma_free_attrs(struct device *dev, size_t size,

debug_dma_free_coherent(dev, size, cpu_addr, dma_handle);
ops->free(dev, size, cpu_addr, dma_handle, attrs);
+ dev_info(dev, "free %lx %lx\n", (unsigned long)cpu_addr, (unsigned long)ops);

static inline void *dma_alloc_coherent(struct device *dev, size_t size,


/ # echo '0000:03:00.0' > /sys/bus/pci/drivers/sata_sil24/bind
[ 107.417252] sata_sil24 0000:03:00.0: alloc ffff00000a6f9000 ffff0000089b8090
[ 107.424397] sata_sil24 0000:03:00.0: alloc ffff00000a719000 ffff0000089b8090
[ 107.432216] scsi host0: sata_sil24
[ 107.436134] scsi host1: sata_sil24
[ 107.439853] ata7: SATA max UDMA/100 host m128@0x50084000 port 0x50080000 irq 51
[ 107.447228] ata8: SATA max UDMA/100 host m128@0x50084000 port 0x50082000 irq 51
/ # echo '0000:03:00.0' > /sys/bus/pci/drivers/sata_sil24/unbind
[ 112.048654] sata_sil24 0000:03:00.0: free ffff00000a719000 ffff0000089b8120
[ 112.055579] sata_sil24 0000:03:00.0: free ffff00000a6f9000 ffff0000089b8120