From: Christian Brauner brauner@kernel.org
[ Upstream commit 391b59b045004d5b985d033263ccba3e941a7740 ]
Jan reported that 'cd ..' may take a long time in deep directory hierarchies under a bind-mount. If concurrent renames happen it is possible to livelock in is_subdir() because it will keep retrying.
Change is_subdir() from simply retrying over and over to retry once and then acquire the rename lock to handle deep ancestor chains better. The list of alternatives to this approach were less then pleasant. Change the scope of rcu lock to cover the whole walk while at it.
A big thanks to Jan and Linus. Both Jan and Linus had proposed effectively the same thing just that one version ended up being slightly more elegant.
Reported-by: Jan Kara jack@suse.cz Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/dcache.c | 31 ++++++++++++++----------------- 1 file changed, 14 insertions(+), 17 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c index 4d96eb591f5d9..93671238abce1 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -3003,28 +3003,25 @@ EXPORT_SYMBOL(d_splice_alias);
bool is_subdir(struct dentry *new_dentry, struct dentry *old_dentry) { - bool result; + bool subdir; unsigned seq;
if (new_dentry == old_dentry) return true;
- do { - /* for restarting inner loop in case of seq retry */ - seq = read_seqbegin(&rename_lock); - /* - * Need rcu_readlock to protect against the d_parent trashing - * due to d_move - */ - rcu_read_lock(); - if (d_ancestor(old_dentry, new_dentry)) - result = true; - else - result = false; - rcu_read_unlock(); - } while (read_seqretry(&rename_lock, seq)); - - return result; + /* Access d_parent under rcu as d_move() may change it. */ + rcu_read_lock(); + seq = read_seqbegin(&rename_lock); + subdir = d_ancestor(old_dentry, new_dentry); + /* Try lockless once... */ + if (read_seqretry(&rename_lock, seq)) { + /* ...else acquire lock for progress even on deep chains. */ + read_seqlock_excl(&rename_lock); + subdir = d_ancestor(old_dentry, new_dentry); + read_sequnlock_excl(&rename_lock); + } + rcu_read_unlock(); + return subdir; } EXPORT_SYMBOL(is_subdir);