• Josh Boyer's avatar
    lockdep: Add helper function for dir vs file i_mutex annotation · e096d0c7
    Josh Boyer authored
    Purely in-memory filesystems do not use the inode hash as the dcache
    tells us if an entry already exists.  As a result, they do not call
    unlock_new_inode, and thus directory inodes do not get put into a
    different lockdep class for i_sem.
    
    We need the different lockdep classes, because the locking order for
    i_mutex is different for directory inodes and regular inodes.  Directory
    inodes can do "readdir()", which takes i_mutex *before* possibly taking
    mm->mmap_sem (due to a page fault while copying the directory entry to
    user space).
    
    In contrast, regular inodes can be mmap'ed, which takes mm->mmap_sem
    before accessing i_mutex.
    
    The two cases can never happen for the same inode, so no real deadlock
    can occur, but without the different lockdep classes, lockdep cannot
    understand that.  As a result, if CONFIG_DEBUG_LOCK_ALLOC is set, this
    can lead to false positives from lockdep like below:
    
        find/645 is trying to acquire lock:
         (&mm->mmap_sem){++++++}, at: [<ffffffff81109514>] might_fault+0x5c/0xac
    
        but task is already holding lock:
         (&sb->s_type->i_mutex_key#15){+.+.+.}, at: [<ffffffff81149f34>]
        vfs_readdir+0x5b/0xb4
    
        which lock already depends on the new lock.
    
        the existing dependency chain (in reverse order) is:
    
        -> #1 (&sb->s_type->i_mutex_key#15){+.+.+.}:
              [<ffffffff8108ac26>] lock_acquire+0xbf/0x103
              [<ffffffff814db822>] __mutex_lock_common+0x4c/0x361
              [<ffffffff814dbc46>] mutex_lock_nested+0x40/0x45
              [<ffffffff811daa87>] hugetlbfs_file_mmap+0x82/0x110
              [<ffffffff81111557>] mmap_region+0x258/0x432
              [<ffffffff811119dd>] do_mmap_pgoff+0x2ac/0x306
              [<ffffffff81111b4f>] sys_mmap_pgoff+0x118/0x16a
              [<ffffffff8100c858>] sys_mmap+0x22/0x24
              [<ffffffff814e3ec2>] system_call_fastpath+0x16/0x1b
    
        -> #0 (&mm->mmap_sem){++++++}:
              [<ffffffff8108a4bc>] __lock_acquire+0xa1a/0xcf7
              [<ffffffff8108ac26>] lock_acquire+0xbf/0x103
              [<ffffffff81109541>] might_fault+0x89/0xac
              [<ffffffff81149cff>] filldir+0x6f/0xc7
              [<ffffffff811586ea>] dcache_readdir+0x67/0x205
              [<ffffffff81149f54>] vfs_readdir+0x7b/0xb4
              [<ffffffff8114a073>] sys_getdents+0x7e/0xd1
              [<ffffffff814e3ec2>] system_call_fastpath+0x16/0x1b
    
    This patch moves the directory vs file lockdep annotation into a helper
    function that can be called by in-memory filesystems and has hugetlbfs
    call it.
    Signed-off-by: default avatarJosh Boyer <jwboyer@redhat.com>
    Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    e096d0c7
fs.h 89.5 KB