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SubjectRe: Problems with read() on /proc/devices with x86_64 system
On Wed, Feb 22, 2006 at 09:07:45PM +0200, Martin Schlemmer wrote:

> Not sure when it started, but 2.6.16-rc[1234] at least have problems
> with unbuffered read() and /proc/devices on my x86_64 box. I first
> picked it up with dmsetup that did not want to work properly built
> against klibc (glibc with fread() worked fine though, as it mmap()'d the
> file).
>
> Following code (from HPA and klibc mailing lists), when compiled and run
> with /proc/devices only reads the first two lines and then exits
> normally, where with any other file works as expected.

Hi Martin,
I case you haven't found out yet, this is fixed in 2.6.17 by:

[PATCH] Simplify proc/devices and fix early termination regression

Attached is the version for 2.6.16.

Regards,
Joe


Repair /proc/devices early-termination regression.

2.6.16 broke /proc/devices. An application often gets an
EOF before the end of data is reached, if that application
uses a series of short read(2)s to access the data. I have
used read buffers of varying sizes with varying degrees
of unsuccess (larger sizes get further into the data than
smaller sizes, following a simple pattern). It appears
that the only safe way to get the data is to use a single
read buffer larger than all the data in /proc/devices.

The following example demonstates the problem:

# dd if=/proc/devices bs=1
Character devices:
1 mem
27+0 records in
27+0 records out

This patch is a backport of the fix recently accepted to
Linus's tree:

commit 68eef3b4791572ecb70249c7fb145bb3742dd899
[PATCH] Simplify proc/devices and fix early termination regression

It replaces the complex, state-machine algorithm introduced
in 2.6.16 with a simple algorithm, modeled on the implementation
of /proc/interrupts.

[akpm@osdl.org: cleanups, simplifications]

Signed-off-by: Joe Korty <joe.korty@ccur.com>


2.6.16.1-jak/block/genhd.c | 103 ++----------------------
2.6.16.1-jak/fs/char_dev.c | 87 ++-------------------
2.6.16.1-jak/fs/proc/proc_misc.c | 161 ++++++++-------------------------------
2.6.16.1-jak/include/linux/fs.h | 15 ---
4 files changed, 61 insertions(+), 305 deletions(-)
diff -puNa block/genhd.c~patch block/genhd.c
--- 2.6.16.1/block/genhd.c~patch 2006-03-31 11:09:50.000000000 -0500
+++ 2.6.16.1-jak/block/genhd.c 2006-03-31 11:49:10.000000000 -0500
@@ -16,8 +16,6 @@
#include <linux/kobj_map.h>
#include <linux/buffer_head.h>

-#define MAX_PROBE_HASH 255 /* random */
-
static struct subsystem block_subsys;

static DECLARE_MUTEX(block_subsys_sem);
@@ -30,108 +28,29 @@ static struct blk_major_name {
struct blk_major_name *next;
int major;
char name[16];
-} *major_names[MAX_PROBE_HASH];
+} *major_names[BLKDEV_MAJOR_HASH_SIZE];

/* index in the above - for now: assume no multimajor ranges */
static inline int major_to_index(int major)
{
- return major % MAX_PROBE_HASH;
-}
-
-struct blkdev_info {
- int index;
- struct blk_major_name *bd;
-};
-
-/*
- * iterate over a list of blkdev_info structures. allows
- * the major_names array to be iterated over from outside this file
- * must be called with the block_subsys_sem held
- */
-void *get_next_blkdev(void *dev)
-{
- struct blkdev_info *info;
-
- if (dev == NULL) {
- info = kmalloc(sizeof(*info), GFP_KERNEL);
- if (!info)
- goto out;
- info->index=0;
- info->bd = major_names[info->index];
- if (info->bd)
- goto out;
- } else {
- info = dev;
- }
-
- while (info->index < ARRAY_SIZE(major_names)) {
- if (info->bd)
- info->bd = info->bd->next;
- if (info->bd)
- goto out;
- /*
- * No devices on this chain, move to the next
- */
- info->index++;
- info->bd = (info->index < ARRAY_SIZE(major_names)) ?
- major_names[info->index] : NULL;
- if (info->bd)
- goto out;
- }
-
-out:
- return info;
-}
-
-void *acquire_blkdev_list(void)
-{
- down(&block_subsys_sem);
- return get_next_blkdev(NULL);
-}
-
-void release_blkdev_list(void *dev)
-{
- up(&block_subsys_sem);
- kfree(dev);
+ return major % BLKDEV_MAJOR_HASH_SIZE;
}

+#ifdef CONFIG_PROC_FS

-/*
- * Count the number of records in the blkdev_list.
- * must be called with the block_subsys_sem held
- */
-int count_blkdev_list(void)
+void blkdev_show(struct seq_file *f, off_t offset)
{
- struct blk_major_name *n;
- int i, count;
-
- count = 0;
+ struct blk_major_name *dp;

- for (i = 0; i < ARRAY_SIZE(major_names); i++) {
- for (n = major_names[i]; n; n = n->next)
- count++;
+ if (offset < BLKDEV_MAJOR_HASH_SIZE) {
+ down(&block_subsys_sem);
+ for (dp = major_names[offset]; dp; dp = dp->next)
+ seq_printf(f, "%3d %s\n", dp->major, dp->name);
+ up(&block_subsys_sem);
}
-
- return count;
-}
-
-/*
- * extract the major and name values from a blkdev_info struct
- * passed in as a void to *dev. Must be called with
- * block_subsys_sem held
- */
-int get_blkdev_info(void *dev, int *major, char **name)
-{
- struct blkdev_info *info = dev;
-
- if (info->bd == NULL)
- return 1;
-
- *major = info->bd->major;
- *name = info->bd->name;
- return 0;
}

+#endif /* CONFIG_PROC_FS */

int register_blkdev(unsigned int major, const char *name)
{
diff -puNa fs/char_dev.c~patch fs/char_dev.c
--- 2.6.16.1/fs/char_dev.c~patch 2006-03-31 11:09:50.000000000 -0500
+++ 2.6.16.1-jak/fs/char_dev.c 2006-03-31 11:49:10.000000000 -0500
@@ -15,6 +15,7 @@
#include <linux/module.h>
#include <linux/smp_lock.h>
#include <linux/devfs_fs_kernel.h>
+#include <linux/seq_file.h>

#include <linux/kobject.h>
#include <linux/kobj_map.h>
@@ -26,8 +27,6 @@

static struct kobj_map *cdev_map;

-#define MAX_PROBE_HASH 255 /* random */
-
static DECLARE_MUTEX(chrdevs_lock);

static struct char_device_struct {
@@ -38,93 +37,29 @@ static struct char_device_struct {
char name[64];
struct file_operations *fops;
struct cdev *cdev; /* will die */
-} *chrdevs[MAX_PROBE_HASH];
+} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];

/* index in the above */
static inline int major_to_index(int major)
{
- return major % MAX_PROBE_HASH;
-}
-
-struct chrdev_info {
- int index;
- struct char_device_struct *cd;
-};
-
-void *get_next_chrdev(void *dev)
-{
- struct chrdev_info *info;
-
- if (dev == NULL) {
- info = kmalloc(sizeof(*info), GFP_KERNEL);
- if (!info)
- goto out;
- info->index=0;
- info->cd = chrdevs[info->index];
- if (info->cd)
- goto out;
- } else {
- info = dev;
- }
-
- while (info->index < ARRAY_SIZE(chrdevs)) {
- if (info->cd)
- info->cd = info->cd->next;
- if (info->cd)
- goto out;
- /*
- * No devices on this chain, move to the next
- */
- info->index++;
- info->cd = (info->index < ARRAY_SIZE(chrdevs)) ?
- chrdevs[info->index] : NULL;
- if (info->cd)
- goto out;
- }
-
-out:
- return info;
-}
-
-void *acquire_chrdev_list(void)
-{
- down(&chrdevs_lock);
- return get_next_chrdev(NULL);
-}
-
-void release_chrdev_list(void *dev)
-{
- up(&chrdevs_lock);
- kfree(dev);
+ return major % CHRDEV_MAJOR_HASH_SIZE;
}

+#ifdef CONFIG_PROC_FS

-int count_chrdev_list(void)
+void chrdev_show(struct seq_file *f, off_t offset)
{
struct char_device_struct *cd;
- int i, count;
-
- count = 0;

- for (i = 0; i < ARRAY_SIZE(chrdevs) ; i++) {
- for (cd = chrdevs[i]; cd; cd = cd->next)
- count++;
+ if (offset < CHRDEV_MAJOR_HASH_SIZE) {
+ down(&chrdevs_lock);
+ for (cd = chrdevs[offset]; cd; cd = cd->next)
+ seq_printf(f, "%3d %s\n", cd->major, cd->name);
+ up(&chrdevs_lock);
}
-
- return count;
}

-int get_chrdev_info(void *dev, int *major, char **name)
-{
- struct chrdev_info *info = dev;
-
- if (info->cd == NULL)
- return 1;
-
- *major = info->cd->major;
- *name = info->cd->name;
- return 0;
-}
+#endif /* CONFIG_PROC_FS */

/*
* Register a single major with a specified minor range.
diff -puNa fs/proc/proc_misc.c~patch fs/proc/proc_misc.c
--- 2.6.16.1/fs/proc/proc_misc.c~patch 2006-03-31 11:09:50.000000000 -0500
+++ 2.6.16.1-jak/fs/proc/proc_misc.c 2006-03-31 11:49:10.000000000 -0500
@@ -249,144 +249,60 @@ static int cpuinfo_open(struct inode *in
return seq_open(file, &cpuinfo_op);
}

-enum devinfo_states {
- CHR_HDR,
- CHR_LIST,
- BLK_HDR,
- BLK_LIST,
- DEVINFO_DONE
-};
-
-struct devinfo_state {
- void *chrdev;
- void *blkdev;
- unsigned int num_records;
- unsigned int cur_record;
- enum devinfo_states state;
+static struct file_operations proc_cpuinfo_operations = {
+ .open = cpuinfo_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = seq_release,
};

-static void *devinfo_start(struct seq_file *f, loff_t *pos)
+static int devinfo_show(struct seq_file *f, void *v)
{
- struct devinfo_state *info = f->private;
+ int i = *(loff_t *) v;

- if (*pos) {
- if ((info) && (*pos <= info->num_records))
- return info;
- return NULL;
+ if (i < CHRDEV_MAJOR_HASH_SIZE) {
+ if (i == 0)
+ seq_printf(f, "Character devices:\n");
+ chrdev_show(f, i);
+ } else {
+ i -= CHRDEV_MAJOR_HASH_SIZE;
+ if (i == 0)
+ seq_printf(f, "\nBlock devices:\n");
+ blkdev_show(f, i);
}
- info = kmalloc(sizeof(*info), GFP_KERNEL);
- f->private = info;
- info->chrdev = acquire_chrdev_list();
- info->blkdev = acquire_blkdev_list();
- info->state = CHR_HDR;
- info->num_records = count_chrdev_list();
- info->num_records += count_blkdev_list();
- info->num_records += 2; /* Character and Block headers */
- *pos = 1;
- info->cur_record = *pos;
- return info;
+ return 0;
}

-static void *devinfo_next(struct seq_file *f, void *v, loff_t *pos)
+static void *devinfo_start(struct seq_file *f, loff_t *pos)
{
- int idummy;
- char *ndummy;
- struct devinfo_state *info = f->private;
-
- switch (info->state) {
- case CHR_HDR:
- info->state = CHR_LIST;
- (*pos)++;
- /*fallthrough*/
- case CHR_LIST:
- if (get_chrdev_info(info->chrdev,&idummy,&ndummy)) {
- /*
- * The character dev list is complete
- */
- info->state = BLK_HDR;
- } else {
- info->chrdev = get_next_chrdev(info->chrdev);
- }
- (*pos)++;
- break;
- case BLK_HDR:
- info->state = BLK_LIST;
- (*pos)++;
- /*fallthrough*/
- case BLK_LIST:
- if (get_blkdev_info(info->blkdev,&idummy,&ndummy)) {
- /*
- * The block dev list is complete
- */
- info->state = DEVINFO_DONE;
- } else {
- info->blkdev = get_next_blkdev(info->blkdev);
- }
- (*pos)++;
- break;
- case DEVINFO_DONE:
- (*pos)++;
- info->cur_record = *pos;
- info = NULL;
- break;
- default:
- break;
- }
- if (info)
- info->cur_record = *pos;
- return info;
+ if (*pos < (BLKDEV_MAJOR_HASH_SIZE + CHRDEV_MAJOR_HASH_SIZE))
+ return pos;
+ return NULL;
}

-static void devinfo_stop(struct seq_file *f, void *v)
+static void *devinfo_next(struct seq_file *f, void *v, loff_t *pos)
{
- struct devinfo_state *info = f->private;
-
- if (info) {
- release_chrdev_list(info->chrdev);
- release_blkdev_list(info->blkdev);
- f->private = NULL;
- kfree(info);
- }
+ (*pos)++;
+ if (*pos >= (BLKDEV_MAJOR_HASH_SIZE + CHRDEV_MAJOR_HASH_SIZE))
+ return NULL;
+ return pos;
}

-static int devinfo_show(struct seq_file *f, void *arg)
+static void devinfo_stop(struct seq_file *f, void *v)
{
- int major;
- char *name;
- struct devinfo_state *info = f->private;
-
- switch(info->state) {
- case CHR_HDR:
- seq_printf(f,"Character devices:\n");
- /* fallthrough */
- case CHR_LIST:
- if (!get_chrdev_info(info->chrdev,&major,&name))
- seq_printf(f,"%3d %s\n",major,name);
- break;
- case BLK_HDR:
- seq_printf(f,"\nBlock devices:\n");
- /* fallthrough */
- case BLK_LIST:
- if (!get_blkdev_info(info->blkdev,&major,&name))
- seq_printf(f,"%3d %s\n",major,name);
- break;
- default:
- break;
- }
-
- return 0;
+ /* Nothing to do */
}

-static struct seq_operations devinfo_op = {
- .start = devinfo_start,
- .next = devinfo_next,
- .stop = devinfo_stop,
- .show = devinfo_show,
+static struct seq_operations devinfo_ops = {
+ .start = devinfo_start,
+ .next = devinfo_next,
+ .stop = devinfo_stop,
+ .show = devinfo_show
};

-static int devinfo_open(struct inode *inode, struct file *file)
+static int devinfo_open(struct inode *inode, struct file *filp)
{
- return seq_open(file, &devinfo_op);
+ return seq_open(filp, &devinfo_ops);
}

static struct file_operations proc_devinfo_operations = {
@@ -396,13 +312,6 @@ static struct file_operations proc_devin
.release = seq_release,
};

-static struct file_operations proc_cpuinfo_operations = {
- .open = cpuinfo_open,
- .read = seq_read,
- .llseek = seq_lseek,
- .release = seq_release,
-};
-
extern struct seq_operations vmstat_op;
static int vmstat_open(struct inode *inode, struct file *file)
{
diff -puNa include/linux/fs.h~patch include/linux/fs.h
--- 2.6.16.1/include/linux/fs.h~patch 2006-03-31 11:09:50.000000000 -0500
+++ 2.6.16.1-jak/include/linux/fs.h 2006-03-31 11:09:50.000000000 -0500
@@ -1383,6 +1383,7 @@ extern int bd_claim(struct block_device
extern void bd_release(struct block_device *);

/* fs/char_dev.c */
+#define CHRDEV_MAJOR_HASH_SIZE 255
extern int alloc_chrdev_region(dev_t *, unsigned, unsigned, const char *);
extern int register_chrdev_region(dev_t, unsigned, const char *);
extern int register_chrdev(unsigned int, const char *,
@@ -1390,25 +1391,17 @@ extern int register_chrdev(unsigned int,
extern int unregister_chrdev(unsigned int, const char *);
extern void unregister_chrdev_region(dev_t, unsigned);
extern int chrdev_open(struct inode *, struct file *);
-extern int get_chrdev_list(char *);
-extern void *acquire_chrdev_list(void);
-extern int count_chrdev_list(void);
-extern void *get_next_chrdev(void *);
-extern int get_chrdev_info(void *, int *, char **);
-extern void release_chrdev_list(void *);
+extern void chrdev_show(struct seq_file *,off_t);

/* fs/block_dev.c */
+#define BLKDEV_MAJOR_HASH_SIZE 255
#define BDEVNAME_SIZE 32 /* Largest string for a blockdev identifier */
extern const char *__bdevname(dev_t, char *buffer);
extern const char *bdevname(struct block_device *bdev, char *buffer);
extern struct block_device *lookup_bdev(const char *);
extern struct block_device *open_bdev_excl(const char *, int, void *);
extern void close_bdev_excl(struct block_device *);
-extern void *acquire_blkdev_list(void);
-extern int count_blkdev_list(void);
-extern void *get_next_blkdev(void *);
-extern int get_blkdev_info(void *, int *, char **);
-extern void release_blkdev_list(void *);
+extern void blkdev_show(struct seq_file *,off_t);

extern void init_special_inode(struct inode *, umode_t, dev_t);

_

-
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