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fs: better handle deep ancestor chains in is_subdir()

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[ Upstream commit 391b59b045 ]

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>
This commit is contained in:
Christian Brauner 2024-07-02 21:03:26 +02:00 committed by Greg Kroah-Hartman
parent a2b201f839
commit 5f569a2301
1 changed files with 14 additions and 17 deletions
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31
fs/dcache.c
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@ -3127,28 +3127,25 @@ 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);