@@ -3256,7 +3256,7 @@ static loff_t btrfs_file_llseek(struct file *file, loff_t offset, int whence)
switch (whence) {
case SEEK_END:
case SEEK_CUR:
- offset = generic_file_llseek(file, offset, whence);
+ offset = generic_file_llseek_unlocked(file, offset, whence);
goto out;
case SEEK_DATA:
case SEEK_HOLE:
@@ -2336,13 +2336,14 @@ static loff_t fuse_file_llseek(struct file *file, loff_t offset, int whence)
case SEEK_SET:
case SEEK_CUR:
/* No i_mutex protection necessary for SEEK_CUR and SEEK_SET */
- retval = generic_file_llseek(file, offset, whence);
+ retval = generic_file_llseek_unlocked(file, offset, whence);
break;
case SEEK_END:
inode_lock(inode);
retval = fuse_update_attributes(inode, file);
if (!retval)
- retval = generic_file_llseek(file, offset, whence);
+ retval = generic_file_llseek_unlocked(file, offset,
+ whence);
inode_unlock(inode);
break;
case SEEK_HOLE:
@@ -66,7 +66,8 @@ static loff_t gfs2_llseek(struct file *file, loff_t offset, int whence)
error = gfs2_glock_nq_init(ip->i_gl, LM_ST_SHARED, LM_FLAG_ANY,
&i_gh);
if (!error) {
- error = generic_file_llseek(file, offset, whence);
+ error = generic_file_llseek_unlocked(file, offset,
+ whence);
gfs2_glock_dq_uninit(&i_gh);
}
break;
@@ -131,6 +131,24 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
}
EXPORT_SYMBOL(generic_file_llseek_size);
+/**
+ * generic_file_llseek_unlocked - lockless generic llseek implementation
+ * @file: file structure to seek on
+ * @offset: file offset to seek to
+ * @whence: type of seek
+ *
+ */
+loff_t generic_file_llseek_unlocked(struct file *file, loff_t offset,
+ int whence)
+{
+ struct inode *inode = file->f_mapping->host;
+
+ return generic_file_llseek_size(file, offset, whence,
+ inode->i_sb->s_maxbytes,
+ i_size_read(inode));
+}
+EXPORT_SYMBOL(generic_file_llseek_unlocked);
+
/**
* generic_file_llseek - generic llseek implementation for regular files
* @file: file structure to seek on
@@ -144,10 +162,23 @@ EXPORT_SYMBOL(generic_file_llseek_size);
loff_t generic_file_llseek(struct file *file, loff_t offset, int whence)
{
struct inode *inode = file->f_mapping->host;
+ loff_t retval;
- return generic_file_llseek_size(file, offset, whence,
- inode->i_sb->s_maxbytes,
- i_size_read(inode));
+ switch (whence) {
+ default:
+ return generic_file_llseek_unlocked(file, offset, whence);
+ case SEEK_END:
+ case SEEK_DATA:
+ case SEEK_HOLE:
+ /*
+ * protects against inode size race with write so that llseek
+ * doesn't see inode size being updated in write.
+ */
+ inode_lock_shared(inode);
+ retval = generic_file_llseek_unlocked(file, offset, whence);
+ inode_unlock_shared(inode);
+ return retval;
+ }
}
EXPORT_SYMBOL(generic_file_llseek);
@@ -3054,6 +3054,8 @@ extern loff_t noop_llseek(struct file *file, loff_t offset, int whence);
extern loff_t no_llseek(struct file *file, loff_t offset, int whence);
extern loff_t vfs_setpos(struct file *file, loff_t offset, loff_t maxsize);
extern loff_t generic_file_llseek(struct file *file, loff_t offset, int whence);
+extern loff_t generic_file_llseek_unlocked(struct file *file, loff_t offset,
+ int whence);
extern loff_t generic_file_llseek_size(struct file *file, loff_t offset,
int whence, loff_t maxsize, loff_t eof);
extern loff_t fixed_size_llseek(struct file *file, loff_t offset,
The commit ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek") removed almost all locks in llseek() including SEEK_END. It based on the idea that write() updates size atomically. But in fact, write() can be divided into two or more parts in generic_perform_write() when pos straddles over the PAGE_SIZE, which results in updating size multiple times in one write(). It means that llseek() can see the size being updated during write(). This race changes behavior of some applications. 'tail' is one of those applications. It reads range [pos, pos_end] where pos_end is obtained via llseek() SEEK_END. Sometimes, a read line could be broken. reproducer: $ while true; do echo 123456 >> out; done $ while true; do tail out | grep -v 123456 ; done example output(take 30 secs): 12345 1 1234 1 12 1234 This patch re-introduces generic_file_llseek_unlocked() and implements a lock for SEEK_END/DATA/HOLE in generic_file_llseek(). I replaced all generic_file_llseek() callers with _unlocked() if they are called with a inode lock. All file systems which call generic_file_llseek_size() directly are fixed in the later commits. Fixes: ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek") Signed-off-by: Eiichi Tsukata <devel@etsukata.com> --- fs/btrfs/file.c | 2 +- fs/fuse/file.c | 5 +++-- fs/gfs2/file.c | 3 ++- fs/read_write.c | 37 ++++++++++++++++++++++++++++++++++--- include/linux/fs.h | 2 ++ 5 files changed, 42 insertions(+), 7 deletions(-)