[PATCH v4 4/5] fsnotify: allow sleepable child flag update

Stephen Brennan
2022-11-11T22:07:42Z

With very large d_subdirs lists, iteration can take a long time. Since
iteration needs to hold alias->d_lock, this can trigger soft lockups.
It would be best to make this iteration sleepable. We can drop
alias->d_lock and sleep, by taking a reference to the current child.
However, we need to be careful, since it's possible for the child's
list pointers to be modified once we drop alias->d_lock. The following
are the only cases where the list pointers are modified:

1. dentry_unlist() in fs/dcache.c

This is a helper of dentry_kill(). This function is quite careful to
check the reference count of the dentry once it has taken the
requisite locks, and bail out if a new reference was taken. So we
can be safe that, assuming we found the dentry and took a reference
before dropping alias->d_lock, any concurrent dentry_kill() should
bail out and leave our list pointers untouched.

2. __d_move() in fs/dcache.c

If the child was moved to a new parent, then we can detect this by
testing d_parent and retrying the iteration.

3. Initialization code in d_alloc() family

We are safe from this code, since we cannot encounter a dentry until
it has been initialized.

4. Cursor code in fs/libfs.c for dcache_readdir()

Dentries with DCACHE_DENTRY_CURSOR should be skipped before sleeping,
since we could awaken to find they have skipped over a portion of the
child list.

Given these considerations, we can carefully write a loop that iterates
over d_subdirs and is capable of going to sleep periodically.

Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---

Notes:
v4:
- I've updated this patch so it should be safe even without the
inode locked, by handling cursors and d_move() races.
- Moved comments to lower indentation
- I didn't keep Amir's R-b since this was a fair bit of change.
v3:
- removed if statements around dput()
v2:
- added a check for child->d_parent != alias and retry logic

fs/notify/fsnotify.c | 46 ++++++++++++++++++++++++++++++++++++++++----
1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 409d479cbbc6..0ba61211456c 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -105,7 +105,7 @@ void fsnotify_sb_delete(struct super_block *sb)
*/
void fsnotify_update_children_dentry_flags(struct inode *inode, bool watched)
{
- struct dentry *alias, *child;
+ struct dentry *alias, *child, *last_ref = NULL;

if (!S_ISDIR(inode->i_mode))
return;
@@ -116,12 +116,49 @@ void fsnotify_update_children_dentry_flags(struct inode *inode, bool watched)
return;

/*
- * run all of the children of the original inode and fix their
- * d_flags to indicate parental interest (their parent is the
- * original inode)
+ * These lists can get very long, so we may need to sleep during
+ * iteration. Normally this would be impossible without a cursor,
+ * but since we have the inode locked exclusive, we're guaranteed
+ * that the directory won't be modified, so whichever dentry we
+ * pick to sleep on won't get moved. So, start a manual iteration
+ * over d_subdirs which will allow us to sleep.
*/
spin_lock(&alias->d_lock);
+retry:
list_for_each_entry(child, &alias->d_subdirs, d_child) {
+ /*
+ * We need to hold a reference while we sleep. But we cannot
+ * sleep holding a reference to a cursor, or we risk skipping
+ * through the list.
+ *
+ * When we wake, dput() could free the dentry, invalidating the
+ * list pointers. We can't look at the list pointers until we
+ * re-lock the parent, and we can't dput() once we have the
+ * parent locked. So the solution is to hold onto our reference
+ * and free it the *next* time we drop alias->d_lock: either at
+ * the end of the function, or at the time of the next sleep.
+ */
+ if (child->d_flags & DCACHE_DENTRY_CURSOR)
+ continue;
+ if (need_resched()) {
+ dget(child);
+ spin_unlock(&alias->d_lock);
+ dput(last_ref);
+ last_ref = child;
+ cond_resched();
+ spin_lock(&alias->d_lock);
+ /* Check for races with __d_move() */
+ if (child->d_parent != alias)
+ goto retry;
+ }
+
+ /*
+ * This check is after the cond_resched() for a reason: it is
+ * safe to sleep holding a negative dentry reference. If the
+ * directory contained many negative dentries, and we skipped
+ * them checking need_resched(), we may never end up sleeping
+ * where necessary, and could trigger a soft lockup.
+ */
if (!child->d_inode)
continue;

@@ -133,6 +170,7 @@ void fsnotify_update_children_dentry_flags(struct inode *inode, bool watched)
spin_unlock(&child->d_lock);
}
spin_unlock(&alias->d_lock);
+ dput(last_ref);
dput(alias);
}

--
2.34.1