Some file systems (including ext4, xfs, ramfs ...) have the following problem as I've described in the commit message of the 1/4 patch.
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 Note: Some file systems which indivisually implements llseek() and hold inode mutex lock in it are not affected. ex:) btrfs, ocfs2 Patch 1: re-implements inode lock for SEEK_END in generic_file_llseek() Patch 2 to 4: implement inode lock for SEEK_END in each file systems Eiichi Tsukata (4): vfs: fix race between llseek SEEK_END and write ext4: fix race between llseek SEEK_END and write f2fs: fix race between llseek SEEK_END and write overlayfs: fix race between llseek SEEK_END and write fs/btrfs/file.c | 2 +- fs/ext4/file.c | 10 ++++++++++ fs/f2fs/file.c | 6 +----- fs/fuse/file.c | 5 +++-- fs/gfs2/file.c | 3 ++- fs/overlayfs/file.c | 23 ++++++++++++++++++++--- fs/read_write.c | 37 ++++++++++++++++++++++++++++++++++--- include/linux/fs.h | 2 ++ 8 files changed, 73 insertions(+), 15 deletions(-) -- 2.19.1