fsnotify: clear PARENT_WATCHED flags lazily

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: fsnotify: clear PARENT_WATCHED flags lazily In some setups directories can have many (usually negative) dentries. Hence __fsnotify_update_child_dentry_flags() function can take a significant amount of time. Since the bulk of this function happens under inode->i_lock this causes a significant contention on the lock when we remove the watch from the directory as the __fsnotify_update_child_dentry_flags() call from fsnotify_recalc_mask() races with __fsnotify_update_child_dentry_flags() calls from __fsnotify_parent() happening on children. This can lead upto softlockup reports reported by users. Fix the problem by calling fsnotify_update_children_dentry_flags() to set PARENT_WATCHED flags only when parent starts watching children. When parent stops watching children, clear false positive PARENT_WATCHED flags lazily in __fsnotify_parent() for each accessed child.

References

• https://git.kernel.org/stable/c/3f3ef1d9f66b93913ce2171120d9226b55acd41d
• https://git.kernel.org/stable/c/f9a48bc3dd9099935751458a5bbbea4b7c28abc8
• https://git.kernel.org/stable/c/d8c42405fc3507cc43ba7e4986a773c3fc633f6e
• https://git.kernel.org/stable/c/fc1b1e135c3f72382f792e6c319fc088d5523ad5
• https://git.kernel.org/stable/c/7ef1d2e240c32b1f337a37232d037b07e3919e1a
• https://git.kernel.org/stable/c/172e422ffea20a89bfdc672741c1aad6fbb5044e