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SubjectRe: [BUG] mm: bdi: export BDI attributes in sysfs
Hi Miklos,  ...

On Thu, May 15, 2008 at 11:53:02AM -0700, Miklos Szeredi wrote:
> > About 1 in 4 times, when turning the power back
> > on for a drive, I get the BUG reported below. The
> > drives are not mounted, just sitting there getting
> > power cycled (the test looks to make sure they are
> > being seen by the scsi subsystem and udev).
>
> > > BUG: unable to handle kernel NULL pointer dereference at
> > > 0000000000000000
> > > IP: [<ffffffff802694e3>] read_ahead_kb_show+0x1b/0x2d
>
> I suspect this is because there's a race between the show/store
> functions and dev_set_drvdata() in bdi_register().
>
> Could you try this patch to verify that this is indeed the problem?

Well, the patch had more conflicts than Obama at an
RNC fundraiser (spaces/tabs?) and didn't compile at first
-- but w/ some minor tweaks it finally succumbed and after
many power cycles I no longer see the problem.

Thanks for your help...

Arthur

> Index: linux-2.6/mm/backing-dev.c
> ===================================================================
> --- linux-2.6.orig/mm/backing-dev.c 2008-05-14 12:49:33.000000000 +0200
> +++ linux-2.6/mm/backing-dev.c 2008-05-15 20:37:55.000000000 +0200
> @@ -15,6 +15,7 @@ static struct class *bdi_class;
> #include <linux/seq_file.h>
>
> static struct dentry *bdi_debug_root;
> +static DEFINE_MUTEX(bdi_dev_mutex);
>
> static void bdi_debug_init(void)
> {
> @@ -84,11 +85,19 @@ static inline void bdi_debug_unregister(
> }
> #endif
>
> +static struct backing_dev_info *dev_get_bdi(struct device *dev)
> +{
> + mutex_lock(&bdi_dev_mutex);
> + mutex_unlock(&bdi_dev_mutex);
> +
> + return dev_get_drvdata(dev);
> +}
> +
> static ssize_t read_ahead_kb_store(struct device *dev,
> struct device_attribute *attr,
> const char *buf, size_t count)
> {
> - struct backing_dev_info *bdi = dev_get_drvdata(dev);
> + struct backing_dev_info *bdi = dev_get_bdi(dev);
> char *end;
> unsigned long read_ahead_kb;
> ssize_t ret = -EINVAL;
> @@ -107,7 +116,7 @@ static ssize_t read_ahead_kb_store(struc
> static ssize_t name##_show(struct device *dev, \
> struct device_attribute *attr, char *page) \
> { \
> - struct backing_dev_info *bdi = dev_get_drvdata(dev); \
> + struct backing_dev_info *bdi = dev_get_bdi(dev); \
> \
> return snprintf(page, PAGE_SIZE-1, "%lld\n", (long long)expr); \
> }
> @@ -117,7 +126,7 @@ BDI_SHOW(read_ahead_kb, K(bdi->ra_pages)
> static ssize_t min_ratio_store(struct device *dev,
> struct device_attribute *attr, const char *buf, size_t count)
> {
> - struct backing_dev_info *bdi = dev_get_drvdata(dev);
> + struct backing_dev_info *bdi = dev_get_bdi(dev);
> char *end;
> unsigned int ratio;
> ssize_t ret = -EINVAL;
> @@ -135,7 +144,7 @@ BDI_SHOW(min_ratio, bdi->min_ratio)
> static ssize_t max_ratio_store(struct device *dev,
> struct device_attribute *attr, const char *buf, size_t count)
> {
> - struct backing_dev_info *bdi = dev_get_drvdata(dev);
> + struct backing_dev_info *bdi = dev_get_bdi(dev);
> char *end;
> unsigned int ratio;
> ssize_t ret = -EINVAL;
> @@ -184,6 +193,7 @@ int bdi_register(struct backing_dev_info
> if (!name)
> return -ENOMEM;
>
> + mutex_lock(&bdi_dev_mutex);
> dev = device_create(bdi_class, parent, MKDEV(0, 0), name);
> if (IS_ERR(dev)) {
> ret = PTR_ERR(dev);
> @@ -195,6 +205,7 @@ int bdi_register(struct backing_dev_info
> bdi_debug_register(bdi, name);
>
> exit:
> + mutex_unlock(&bdi_dev_mutex);
> kfree(name);
> return ret;
> }
>


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