From: Stephen Brennan <stephen.s.brennan@oracle.com>
To: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Al Viro <viro@zeniv.linux.org.uk>,
Stephen Brennan <stephen.s.brennan@oracle.com>,
Amir Goldstein <amir73il@gmail.com>
Subject: [PATCH v4 4/5] fsnotify: allow sleepable child flag update
Date: Fri, 11 Nov 2022 14:06:13 -0800 [thread overview]
Message-ID: <20221111220614.991928-5-stephen.s.brennan@oracle.com> (raw)
In-Reply-To: <20221111220614.991928-1-stephen.s.brennan@oracle.com>
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
next prev parent reply other threads:[~2022-11-11 22:07 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-13 22:27 [RFC] fsnotify: allow sleepable child dentry flag update Stephen Brennan
2022-10-13 23:51 ` Al Viro
2022-11-01 21:47 ` Stephen Brennan
2022-10-14 8:01 ` Amir Goldstein
2022-10-17 7:59 ` Stephen Brennan
2022-10-17 11:44 ` Amir Goldstein
2022-10-17 16:59 ` Stephen Brennan
2022-10-17 17:42 ` Amir Goldstein
2022-10-17 9:09 ` Jan Kara
2022-10-18 4:12 ` [PATCH 0/2] fsnotify: fix softlockups iterating over d_subdirs Stephen Brennan
2022-10-18 4:12 ` [PATCH 1/2] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem Stephen Brennan
2022-10-18 7:39 ` Amir Goldstein
2022-10-21 0:33 ` Stephen Brennan
2022-10-21 7:22 ` Amir Goldstein
2022-10-18 4:12 ` [PATCH 2/2] fsnotify: allow sleepable child flag update Stephen Brennan
2022-10-18 5:36 ` Amir Goldstein
2022-10-27 7:50 ` kernel test robot
2022-10-27 8:44 ` Yujie Liu
2022-10-27 22:12 ` Stephen Brennan
2022-10-18 8:07 ` [PATCH 0/2] fsnotify: fix softlockups iterating over d_subdirs Amir Goldstein
2022-10-18 23:52 ` Stephen Brennan
2022-10-19 5:33 ` Amir Goldstein
2022-10-27 22:06 ` Stephen Brennan
2022-10-28 8:58 ` Amir Goldstein
2022-10-21 1:03 ` [PATCH v2 0/3] " Stephen Brennan
2022-10-21 1:03 ` [PATCH v2 1/3] fsnotify: Use d_find_any_alias to get dentry associated with inode Stephen Brennan
2022-10-21 9:25 ` Amir Goldstein
2022-10-21 1:03 ` [PATCH v2 2/3] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem Stephen Brennan
2022-10-21 4:01 ` kernel test robot
2022-10-21 8:22 ` Amir Goldstein
2022-10-21 9:18 ` Amir Goldstein
2022-10-25 18:02 ` Stephen Brennan
2022-10-26 5:41 ` Amir Goldstein
2022-10-21 9:17 ` Christian Brauner
2022-10-21 9:21 ` Amir Goldstein
2022-10-21 1:03 ` [PATCH v2 3/3] fsnotify: allow sleepable child flag update Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 0/3] fsnotify: fix softlockups iterating over d_subdirs Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 1/3] fsnotify: Use d_find_any_alias to get dentry associated with inode Stephen Brennan
2022-11-10 1:12 ` Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 2/3] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem Stephen Brennan
2022-10-28 9:11 ` Amir Goldstein
2022-11-10 0:03 ` kernel test robot
2022-11-10 1:06 ` Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 3/3] fsnotify: allow sleepable child flag update Stephen Brennan
2022-10-28 9:32 ` Amir Goldstein
2022-11-01 21:25 ` Stephen Brennan
2022-11-01 17:51 ` [PATCH v3 0/3] fsnotify: fix softlockups iterating over d_subdirs Jan Kara
2022-11-01 20:48 ` Stephen Brennan
2022-11-02 8:55 ` Amir Goldstein
2022-11-10 20:04 ` Stephen Brennan
2022-11-02 17:52 ` Jan Kara
2022-11-04 23:33 ` Stephen Brennan
2022-11-07 11:56 ` Jan Kara
2022-11-11 22:06 ` [PATCH v4 0/5] " Stephen Brennan
2022-11-11 22:06 ` [PATCH v4 1/5] fsnotify: clear PARENT_WATCHED flags lazily Stephen Brennan
2022-11-11 22:06 ` [PATCH v4 2/5] fsnotify: Use d_find_any_alias to get dentry associated with inode Stephen Brennan
2022-11-12 8:53 ` Amir Goldstein
2022-11-11 22:06 ` [PATCH v4 3/5] dnotify: move fsnotify_recalc_mask() outside spinlock Stephen Brennan
2022-11-12 9:06 ` Amir Goldstein
2022-11-11 22:06 ` Stephen Brennan [this message]
2022-11-12 10:00 ` [PATCH v4 4/5] fsnotify: allow sleepable child flag update Amir Goldstein
2022-11-15 7:10 ` kernel test robot
2022-11-11 22:06 ` [PATCH v4 5/5] fsnotify: require inode lock held during " Stephen Brennan
2022-11-12 9:42 ` Amir Goldstein
2022-11-11 22:08 ` [PATCH v4 0/5] fsnotify: fix softlockups iterating over d_subdirs Stephen Brennan
2022-11-22 11:50 ` Jan Kara
2022-11-22 14:03 ` Amir Goldstein
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