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@xxxxxxxxxxxxx> 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) { -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
