[PATCH] VFS: make file->f_pos access atomic on 32bit arch

[PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Hisashi Hifumi]

Hi.

Currently reading or writing file->f_pos is not atomic on 32bit environment,
so two or more simultaneous access can corrupt file->f_pos value.
There are some past discussions about this issue, but this is not fixed yet.
http://marc.info/?l=linux-kernel&m=120764199819899&w=2
http://marc.info/?l=linux-kernel&m=114490379102476&w=2

Protecting file->f_pos with a lock adds some overhead but I think we should
fix this.
I used seqlock to serialize the access to file->f_pos. This approach is similar
to inode->i_size synchronization.

Comments?

Thanks.

Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>

diff -Nrup linux-2.6.27-rc6.org/fs/block_dev.c linux-2.6.27-rc6.fpos/fs/block_dev.c
--- linux-2.6.27-rc6.org/fs/block_dev.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/block_dev.c	2008-09-16 18:45:17.000000000 +0900
@@ -225,13 +225,12 @@ static loff_t block_llseek(struct file *
 			offset += size;
 			break;
 		case 1:
-			offset += file->f_pos;
+			offset += file_pos_read(file);
 	}
 	retval = -EINVAL;
 	if (offset >= 0 && offset <= size) {
-		if (offset != file->f_pos) {
-			file->f_pos = offset;
-		}
+		if (offset != file_pos_read(file))
+			file_pos_write(file, offset);
 		retval = offset;
 	}
 	mutex_unlock(&bd_inode->i_mutex);
diff -Nrup linux-2.6.27-rc6.org/fs/file_table.c linux-2.6.27-rc6.fpos/fs/file_table.c
--- linux-2.6.27-rc6.org/fs/file_table.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/file_table.c	2008-09-16 14:18:29.000000000 +0900
@@ -121,6 +121,7 @@ struct file *get_empty_filp(void)
 	tsk = current;
 	INIT_LIST_HEAD(&f->f_u.fu_list);
 	atomic_long_set(&f->f_count, 1);
+	f_pos_ordered_init(f);
 	rwlock_init(&f->f_owner.lock);
 	f->f_uid = tsk->fsuid;
 	f->f_gid = tsk->fsgid;
diff -Nrup linux-2.6.27-rc6.org/fs/locks.c linux-2.6.27-rc6.fpos/fs/locks.c
--- linux-2.6.27-rc6.org/fs/locks.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/locks.c	2008-09-16 18:35:14.000000000 +0900
@@ -317,7 +317,7 @@ static int flock_to_posix_lock(struct fi
 		start = 0;
 		break;
 	case SEEK_CUR:
-		start = filp->f_pos;
+		start = file_pos_read(filp);
 		break;
 	case SEEK_END:
 		start = i_size_read(filp->f_path.dentry->d_inode);
@@ -367,7 +367,7 @@ static int flock64_to_posix_lock(struct 
 		start = 0;
 		break;
 	case SEEK_CUR:
-		start = filp->f_pos;
+		start = file_pos_read(filp);
 		break;
 	case SEEK_END:
 		start = i_size_read(filp->f_path.dentry->d_inode);
diff -Nrup linux-2.6.27-rc6.org/fs/read_write.c linux-2.6.27-rc6.fpos/fs/read_write.c
--- linux-2.6.27-rc6.org/fs/read_write.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/read_write.c	2008-09-16 18:40:26.000000000 +0900
@@ -42,13 +42,13 @@ generic_file_llseek_unlocked(struct file
 			offset += inode->i_size;
 			break;
 		case SEEK_CUR:
-			offset += file->f_pos;
+			offset += file_pos_read(file);
 	}
 	retval = -EINVAL;
 	if (offset>=0 && offset<=inode->i_sb->s_maxbytes) {
 		/* Special lock needed here? */
-		if (offset != file->f_pos) {
-			file->f_pos = offset;
+		if (offset != file_pos_read(file)) {
+			file_pos_write(file, offset);
 			file->f_version = 0;
 		}
 		retval = offset;
@@ -83,12 +83,12 @@ loff_t default_llseek(struct file *file,
 			offset += i_size_read(file->f_path.dentry->d_inode);
 			break;
 		case SEEK_CUR:
-			offset += file->f_pos;
+			offset += file_pos_read(file);
 	}
 	retval = -EINVAL;
 	if (offset >= 0) {
-		if (offset != file->f_pos) {
-			file->f_pos = offset;
+		if (offset != file_pos_read(file)) {
+			file_pos_write(file, offset);
 			file->f_version = 0;
 		}
 		retval = offset;
@@ -324,16 +324,6 @@ ssize_t vfs_write(struct file *file, con
 
 EXPORT_SYMBOL(vfs_write);
 
-static inline loff_t file_pos_read(struct file *file)
-{
-	return file->f_pos;
-}
-
-static inline void file_pos_write(struct file *file, loff_t pos)
-{
-	file->f_pos = pos;
-}
-
 asmlinkage ssize_t sys_read(unsigned int fd, char __user * buf, size_t count)
 {
 	struct file *file;
diff -Nrup linux-2.6.27-rc6.org/include/linux/fs.h linux-2.6.27-rc6.fpos/include/linux/fs.h
--- linux-2.6.27-rc6.org/include/linux/fs.h	2008-09-10 10:30:10.000000000 +0900
+++ linux-2.6.27-rc6.fpos/include/linux/fs.h	2008-09-16 18:34:14.000000000 +0900
@@ -805,6 +805,16 @@ static inline int ra_has_index(struct fi
 #define FILE_MNT_WRITE_TAKEN	1
 #define FILE_MNT_WRITE_RELEASED	2
 
+/*
+ * Use sequence lock to get consistent f_pos on 32-bit processors.
+ */
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+#define __NEED_F_POS_ORDERED
+#define f_pos_ordered_init(file) seqlock_init(&file->f_pos_seqlock)
+#else
+#define f_pos_ordered_init(file) do { } while (0)
+#endif
+
 struct file {
 	/*
 	 * fu_list becomes invalid after file_free is called and queued via
@@ -822,6 +832,9 @@ struct file {
 	unsigned int 		f_flags;
 	mode_t			f_mode;
 	loff_t			f_pos;
+#ifdef __NEED_F_POS_ORDERED
+	seqlock_t		f_pos_seqlock;
+#endif
 	struct fown_struct	f_owner;
 	unsigned int		f_uid, f_gid;
 	struct file_ra_state	f_ra;
@@ -850,6 +863,47 @@ extern spinlock_t files_lock;
 #define get_file(x)	atomic_long_inc(&(x)->f_count)
 #define file_count(x)	atomic_long_read(&(x)->f_count)
 
+/*
+ * file_pos_read/write is atomic by using sequence lock on 32bit arch.
+ */
+static inline loff_t file_pos_read(struct file *file)
+{
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	loff_t pos;
+	unsigned int seq;
+
+	do {
+		seq = read_seqbegin(&file->f_pos_seqlock);
+		pos = file->f_pos;
+	} while (read_seqretry(&file->f_pos_seqlock, seq));
+	return pos;
+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
+	loff_t pos;
+
+	preempt_disable();
+	pos = file->f_pos;
+	preempt_enable();
+	return pos;
+#else
+	return file->f_pos;
+#endif
+}
+
+static inline void file_pos_write(struct file *file, loff_t pos)
+{
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	write_seqlock(&file->f_pos_seqlock);
+	file->f_pos = pos;
+	write_sequnlock(&file->f_pos_seqlock);
+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
+	preempt_disable();
+	file->f_pos = pos;
+	preempt_enable();
+#else
+	file->f_pos = pos;
+#endif
+}
+
 #ifdef CONFIG_DEBUG_WRITECOUNT
 static inline void file_take_write(struct file *f)
 {

Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Michael Trimarchi]

Hi,




> +        if (offset != file_pos_read(file))
> +            file_pos_write(file, offset);
>         retval = offset;
>     }

Is not more efficient here a compare xchg operation?

Regards Michael

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Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Nick Piggin]

On Tuesday 16 September 2008 21:11, Michael Trimarchi wrote:
> Hi,
>
> > +        if (offset != file_pos_read(file))
> > +            file_pos_write(file, offset);
> >         retval = offset;
> >     }
>
> Is not more efficient here a compare xchg operation?

The compiler should do it if it was.

Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Michael Trimarchi]

Hi,



----- Messaggio originale -----
> Da: Nick Piggin <nickpiggin@yahoo.com.au>
> A: Michael Trimarchi <trimarchimichael@yahoo.it>
> Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>; akpm@linux-foundation.org; linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org
> Inviato: Martedì 16 settembre 2008, 14:54:21
> Oggetto: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
> 
> On Tuesday 16 September 2008 21:11, Michael Trimarchi wrote:
> > Hi,
> >
> > > +        if (offset != file_pos_read(file))
> > > +            file_pos_write(file, offset);
> > >         retval = offset;
> > >     }
> >
> > Is not more efficient here a compare xchg operation?
> 
> The compiler should do it if it was.
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

 i was thinking about introducing a file_pos_update() not implemented using 
file_pos_read()/file_pos_write() and taking the seq_lock or 
disabling preemption only one time

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Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Nick Piggin]

On Tuesday 16 September 2008 23:57, Michael Trimarchi wrote:
> Hi,
>
>
>
> ----- Messaggio originale -----
>
> > Da: Nick Piggin <nickpiggin@yahoo.com.au>
> > A: Michael Trimarchi <trimarchimichael@yahoo.it>
> > Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>;
> > akpm@linux-foundation.org; linux-kernel@vger.kernel.org;
> > linux-fsdevel@vger.kernel.org Inviato: Martedì 16 settembre 2008,
> > 14:54:21
> > Oggetto: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
> >
> > On Tuesday 16 September 2008 21:11, Michael Trimarchi wrote:
> > > Hi,
> > >
> > > > +        if (offset != file_pos_read(file))
> > > > +            file_pos_write(file, offset);
> > > >         retval = offset;
> > > >     }
> > >
> > > Is not more efficient here a compare xchg operation?
> >
> > The compiler should do it if it was.
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel"
> > in the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
>
>  i was thinking about introducing a file_pos_update() not implemented using
> file_pos_read()/file_pos_write() and taking the seq_lock or
> disabling preemption only one time

AFAIK, the only way to do an atomic 64 bit store on 32-bit x86 is
"cmpxchg8b" (with lock prefix on SMP). That would be far slower
I'm sure.

Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Michael Trimarchi]

Hi,

----- Messaggio originale -----
> Da: Nick Piggin <nickpiggin@yahoo.com.au>
> A: Michael Trimarchi <trimarchimichael@yahoo.it>
>
Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>;
akpm@linux-foundation.org; linux-kernel@vger.kernel.org;
linux-fsdevel@vger.kernel.org
> Inviato: Martedì 16 settembre 2008, 16:11:21
> Oggetto: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
> 
...
> >  i was thinking about introducing a file_pos_update() not implemented using
> > file_pos_read()/file_pos_write() and taking the seq_lock or
> > disabling preemption only one time
> 
> AFAIK, the only way to do an atomic 64 bit store on 32-bit x86 is
> "cmpxchg8b" (with lock prefix on SMP). That would be far slower
> I'm sure.

Maybe it is clear with an example:
 
+/*
+ * file_pos_read/write is atomic by using sequence lock on 32bit arch.
+ */
+static inline loff_t file_pos_read(struct file *file)
+{
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	loff_t pos;
+	unsigned int seq;
+
+	do {
+		seq = read_seqbegin(&file->f_pos_seqlock);
+		pos = file->f_pos;
+	} while (read_seqretry(&file->f_pos_seqlock, seq));
+	return pos;
+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
+	loff_t pos;
+
+	preempt_disable();
+	pos = file->f_pos;
+	preempt_enable();
+	return pos;
+#else
+	return file->f_pos;
+#endif
+}
+
+static inline void file_pos_write(struct file *file, loff_t pos)
+{
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	write_seqlock(&file->f_pos_seqlock);
+	file->f_pos = pos;
+	write_sequnlock(&file->f_pos_seqlock);
+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
+	preempt_disable();
+	file->f_pos = pos;
+	preempt_enable();
+#else
+	file->f_pos = pos;
+#endif
+}
+
if (offset != file_pos_read(file))
                 file_pos_write(file, offset);

Compile with:
BITS_PER_LONG=32
CONFIG_PREEMPT

The code does:

file_pos_read(...)
preempt_disable
....
preempt_enable

file_pos_write(...)
preempt_disable
....
preempt_enable

with the file_pos_update() the code does:

preempt_disable
...
...
preempt_enable

Regards Michael

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Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Bodo Eggert]

Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp> wrote:

> Protecting file->f_pos with a lock adds some overhead but I think we should
> fix this.
> I used seqlock to serialize the access to file->f_pos. This approach is
> similar to inode->i_size synchronization.
> 
> Comments?

> +             if (offset != file_pos_read(file))
> +                     file_pos_write(file, offset);

Maybe file_pos_write should do this check?

Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Michael Trimarchi]

hi,



----- Messaggio originale -----
> Da: Bodo Eggert <7eggert@gmx.de>
> A: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>; akpm@linux-foundation.org; linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org
> Inviato: Martedì 16 settembre 2008, 17:32:08
> Oggetto: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
> 
> Hisashi Hifumi wrote:
> 
> > Protecting file->f_pos with a lock adds some overhead but I think we should
> > fix this.
> > I used seqlock to serialize the access to file->f_pos. This approach is
> > similar to inode->i_size synchronization.
> > 
> > Comments?
> 
> > +             if (offset != file_pos_read(file))
> > +                     file_pos_write(file, offset);
> 
> Maybe file_pos_write should do this check?
> 
I agree :)

Regards Michael

> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
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> Please read the FAQ at  http://www.tux.org/lkml/


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Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Nick Piggin]

On Wednesday 17 September 2008 00:57, Michael Trimarchi wrote:
> Hi,
>
> ----- Messaggio originale -----
>
> > Da: Nick Piggin <nickpiggin@yahoo.com.au>
> > A: Michael Trimarchi <trimarchimichael@yahoo.it>
>
> Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>;
> akpm@linux-foundation.org; linux-kernel@vger.kernel.org;
> linux-fsdevel@vger.kernel.org
>
> > Inviato: Martedì 16 settembre 2008, 16:11:21
> > Oggetto: Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch
>
> ...
>
> > >  i was thinking about introducing a file_pos_update() not implemented
> > > using file_pos_read()/file_pos_write() and taking the seq_lock or
> > > disabling preemption only one time
> >
> > AFAIK, the only way to do an atomic 64 bit store on 32-bit x86 is
> > "cmpxchg8b" (with lock prefix on SMP). That would be far slower
> > I'm sure.
>
> Maybe it is clear with an example:

I understand and agree.
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Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Hisashi Hifumi]

>
>Maybe it is clear with an example:
> 
>+/*
>+ * file_pos_read/write is atomic by using sequence lock on 32bit arch.
>+ */
>+static inline loff_t file_pos_read(struct file *file)
>+{
>+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
>+	loff_t pos;
>+	unsigned int seq;
>+
>+	do {
>+		seq = read_seqbegin(&file->f_pos_seqlock);
>+		pos = file->f_pos;
>+	} while (read_seqretry(&file->f_pos_seqlock, seq));
>+	return pos;
>+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
>+	loff_t pos;
>+
>+	preempt_disable();
>+	pos = file->f_pos;
>+	preempt_enable();
>+	return pos;
>+#else
>+	return file->f_pos;
>+#endif
>+}
>+
>+static inline void file_pos_write(struct file *file, loff_t pos)
>+{
>+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
>+	write_seqlock(&file->f_pos_seqlock);
>+	file->f_pos = pos;
>+	write_sequnlock(&file->f_pos_seqlock);
>+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
>+	preempt_disable();
>+	file->f_pos = pos;
>+	preempt_enable();
>+#else
>+	file->f_pos = pos;
>+#endif
>+}
>+
>if (offset != file_pos_read(file))
>                 file_pos_write(file, offset);
>
>Compile with:
>BITS_PER_LONG=32
>CONFIG_PREEMPT
>
>The code does:
>
>file_pos_read(...)
>preempt_disable
>....
>preempt_enable
>
>file_pos_write(...)
>preempt_disable
>....
>preempt_enable
>
>with the file_pos_update() the code does:
>
>preempt_disable
>...
>...
>preempt_enable
>
>Regards Michael
>

I think file_pos_update() with BITS_PER_LONG=32 && CONFIG_PREEMPT is easy,
this is like this.

static inline void file_pos_update(struct file *file, loff_t pos)
{
	preempt_disable();
	if (pos != file->f_pos)
		file->f_pos = pos;
	preempt_enable();
}


But I think BITS_PER_LONG=32 && CONFIG_SMP version has a problem, this is like this.

static inline void file_pos_update(struct file *file, loff_t pos)
{
	write_seqlock(&file->f_pos_seqlock);
	if (pos != file->f_pos)
		file->f_pos = pos;
	write_sequnlock(&file->f_pos_seqlock);
}

file_pos_update acquires seqlock only once, but write_seqlock is called whether file->f_pos
is overwritten or not.

Re: [PATCH] VFS: make file->f_pos access atomic on 32bit arch

[Hisashi Hifumi]

At 10:24 08/09/17, Nick Piggin wrote:
>> > >  i was thinking about introducing a file_pos_update() not implemented
>> > > using file_pos_read()/file_pos_write() and taking the seq_lock or
>> > > disabling preemption only one time
>> >
>> > AFAIK, the only way to do an atomic 64 bit store on 32-bit x86 is
>> > "cmpxchg8b" (with lock prefix on SMP). That would be far slower
>> > I'm sure.
>>
>> Maybe it is clear with an example:
>
>I understand and agree.

I modified my patch by introducing file_pos_update().

Comments?

Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>

diff -Nrup linux-2.6.27-rc6.org/fs/block_dev.c linux-2.6.27-rc6.fpos/fs/block_dev.c
--- linux-2.6.27-rc6.org/fs/block_dev.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/block_dev.c	2008-09-18 18:24:42.000000000 +0900
@@ -225,13 +225,11 @@ static loff_t block_llseek(struct file *
 			offset += size;
 			break;
 		case 1:
-			offset += file->f_pos;
+			offset += file_pos_read(file);
 	}
 	retval = -EINVAL;
 	if (offset >= 0 && offset <= size) {
-		if (offset != file->f_pos) {
-			file->f_pos = offset;
-		}
+		file_pos_update(file, offset);
 		retval = offset;
 	}
 	mutex_unlock(&bd_inode->i_mutex);
diff -Nrup linux-2.6.27-rc6.org/fs/file_table.c linux-2.6.27-rc6.fpos/fs/file_table.c
--- linux-2.6.27-rc6.org/fs/file_table.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/file_table.c	2008-09-16 14:18:29.000000000 +0900
@@ -121,6 +121,7 @@ struct file *get_empty_filp(void)
 	tsk = current;
 	INIT_LIST_HEAD(&f->f_u.fu_list);
 	atomic_long_set(&f->f_count, 1);
+	f_pos_ordered_init(f);
 	rwlock_init(&f->f_owner.lock);
 	f->f_uid = tsk->fsuid;
 	f->f_gid = tsk->fsgid;
diff -Nrup linux-2.6.27-rc6.org/fs/locks.c linux-2.6.27-rc6.fpos/fs/locks.c
--- linux-2.6.27-rc6.org/fs/locks.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/locks.c	2008-09-16 18:35:14.000000000 +0900
@@ -317,7 +317,7 @@ static int flock_to_posix_lock(struct fi
 		start = 0;
 		break;
 	case SEEK_CUR:
-		start = filp->f_pos;
+		start = file_pos_read(filp);
 		break;
 	case SEEK_END:
 		start = i_size_read(filp->f_path.dentry->d_inode);
@@ -367,7 +367,7 @@ static int flock64_to_posix_lock(struct 
 		start = 0;
 		break;
 	case SEEK_CUR:
-		start = filp->f_pos;
+		start = file_pos_read(filp);
 		break;
 	case SEEK_END:
 		start = i_size_read(filp->f_path.dentry->d_inode);
diff -Nrup linux-2.6.27-rc6.org/fs/read_write.c linux-2.6.27-rc6.fpos/fs/read_write.c
--- linux-2.6.27-rc6.org/fs/read_write.c	2008-09-10 10:30:08.000000000 +0900
+++ linux-2.6.27-rc6.fpos/fs/read_write.c	2008-09-18 18:27:53.000000000 +0900
@@ -42,15 +42,13 @@ generic_file_llseek_unlocked(struct file
 			offset += inode->i_size;
 			break;
 		case SEEK_CUR:
-			offset += file->f_pos;
+			offset += file_pos_read(file);
 	}
 	retval = -EINVAL;
 	if (offset>=0 && offset<=inode->i_sb->s_maxbytes) {
 		/* Special lock needed here? */
-		if (offset != file->f_pos) {
-			file->f_pos = offset;
+		if (file_pos_update(file, offset))
 			file->f_version = 0;
-		}
 		retval = offset;
 	}
 	return retval;
@@ -83,14 +81,12 @@ loff_t default_llseek(struct file *file,
 			offset += i_size_read(file->f_path.dentry->d_inode);
 			break;
 		case SEEK_CUR:
-			offset += file->f_pos;
+			offset += file_pos_read(file);
 	}
 	retval = -EINVAL;
 	if (offset >= 0) {
-		if (offset != file->f_pos) {
-			file->f_pos = offset;
+		if (file_pos_update(file, offset))
 			file->f_version = 0;
-		}
 		retval = offset;
 	}
 	unlock_kernel();
@@ -324,16 +320,6 @@ ssize_t vfs_write(struct file *file, con
 
 EXPORT_SYMBOL(vfs_write);
 
-static inline loff_t file_pos_read(struct file *file)
-{
-	return file->f_pos;
-}
-
-static inline void file_pos_write(struct file *file, loff_t pos)
-{
-	file->f_pos = pos;
-}
-
 asmlinkage ssize_t sys_read(unsigned int fd, char __user * buf, size_t count)
 {
 	struct file *file;
diff -Nrup linux-2.6.27-rc6.org/include/linux/fs.h linux-2.6.27-rc6.fpos/include/linux/fs.h
--- linux-2.6.27-rc6.org/include/linux/fs.h	2008-09-10 10:30:10.000000000 +0900
+++ linux-2.6.27-rc6.fpos/include/linux/fs.h	2008-09-18 18:29:09.000000000 +0900
@@ -805,6 +805,16 @@ static inline int ra_has_index(struct fi
 #define FILE_MNT_WRITE_TAKEN	1
 #define FILE_MNT_WRITE_RELEASED	2
 
+/*
+ * Use sequence lock to get consistent f_pos on 32-bit processors.
+ */
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+#define __NEED_F_POS_ORDERED
+#define f_pos_ordered_init(file) seqlock_init(&file->f_pos_seqlock)
+#else
+#define f_pos_ordered_init(file) do { } while (0)
+#endif
+
 struct file {
 	/*
 	 * fu_list becomes invalid after file_free is called and queued via
@@ -822,6 +832,9 @@ struct file {
 	unsigned int 		f_flags;
 	mode_t			f_mode;
 	loff_t			f_pos;
+#ifdef __NEED_F_POS_ORDERED
+	seqlock_t		f_pos_seqlock;
+#endif
 	struct fown_struct	f_owner;
 	unsigned int		f_uid, f_gid;
 	struct file_ra_state	f_ra;
@@ -850,6 +863,73 @@ extern spinlock_t files_lock;
 #define get_file(x)	atomic_long_inc(&(x)->f_count)
 #define file_count(x)	atomic_long_read(&(x)->f_count)
 
+/*
+ * file_pos_read/write is atomic by using sequence lock on 32bit arch.
+ */
+static inline loff_t file_pos_read(struct file *file)
+{
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	loff_t pos;
+	unsigned int seq;
+
+	do {
+		seq = read_seqbegin(&file->f_pos_seqlock);
+		pos = file->f_pos;
+	} while (read_seqretry(&file->f_pos_seqlock, seq));
+	return pos;
+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
+	loff_t pos;
+
+	preempt_disable();
+	pos = file->f_pos;
+	preempt_enable();
+	return pos;
+#else
+	return file->f_pos;
+#endif
+}
+
+static inline void file_pos_write(struct file *file, loff_t pos)
+{
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	write_seqlock(&file->f_pos_seqlock);
+	file->f_pos = pos;
+	write_sequnlock(&file->f_pos_seqlock);
+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
+	preempt_disable();
+	file->f_pos = pos;
+	preempt_enable();
+#else
+	file->f_pos = pos;
+#endif
+}
+
+static inline int file_pos_update(struct file *file, loff_t offset)
+{
+	int ret = 0;
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	write_seqlock(&file->f_pos_seqlock);
+	if (offset != file->f_pos) {
+		file->f_pos = offset;
+		ret = 1;
+	}
+	write_sequnlock(&file->f_pos_seqlock);
+#elif BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT)
+	preempt_disable();
+	if (offset != file->f_pos) {
+		file->f_pos = offset;
+		ret = 1;
+	}
+	preempt_enable();
+#else
+	if (offset != file->f_pos) {
+		file->f_pos = offset;
+		ret = 1;
+	}
+#endif
+	return ret;
+}
+
 #ifdef CONFIG_DEBUG_WRITECOUNT
 static inline void file_take_write(struct file *f)
 {