From: Stephen Brennan <stephen.s.brennan@oracle.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem
Date: Thu, 20 Oct 2022 17:33:13 -0700 [thread overview]
Message-ID: <87y1taggmu.fsf@oracle.com> (raw)
In-Reply-To: <CAOQ4uxhi27ZZmXMV1JTR1+3-1MVMY3W_R=+7LbOHWXbKOk4hjg@mail.gmail.com>
Amir Goldstein <amir73il@gmail.com> writes:
> On Tue, Oct 18, 2022 at 7:12 AM Stephen Brennan
> <stephen.s.brennan@oracle.com> wrote:
>>
>> When an inode is interested in events on its children, it must set
>> DCACHE_FSNOTIFY_PARENT_WATCHED flag on all its children. Currently, when
>> the fsnotify connector is removed and i_fsnotify_mask becomes zero, we
>> lazily allow __fsnotify_parent() to do this the next time we see an
>> event on a child.
>>
>> However, if the list of children is very long (e.g., in the millions),
>> and lots of activity is occurring on the directory, then it's possible
>> for many CPUs to end up blocked on the inode spinlock in
>> __fsnotify_update_child_flags(). Each CPU will then redundantly iterate
>> over the very long list of children. This situation can cause soft
>> lockups.
>>
>> To avoid this, stop lazily updating child flags in __fsnotify_parent().
>> Protect the child flag update with i_rwsem held exclusive, to ensure
>> that we only iterate over the child list when it's absolutely necessary,
>> and even then, only once.
>>
>> Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
>> ---
>>
>> Notes:
>>
>> It seems that there are two implementation options for this, regarding
>> what i_rwsem protects:
>>
>> 1. Both updates to i_fsnotify_mask, and the child dentry flags, or
>> 2. Only updates to the child dentry flags
>>
>> I wanted to do #1, but it got really tricky with fsnotify_put_mark(). We
>> don't want to hold the inode lock whenever we decrement the refcount,
>> but if we don't, then we're stuck holding a spinlock when the refcount
>> goes to zero, and we need to grab the inode rwsem to synchronize the
>> update to the child flags. I'm sure there's a way around this, but I
>> didn't keep going with it.
>>
>> With #1, as currently implemented, we have the unfortunate effect of
>> that a mark can be added, can see that no update is required, and
>> return, despite the fact that the flag update is still in progress on a
>> different CPU/thread. From our discussion, that seems to be the current
>> status quo, but I wanted to explicitly point that out. If we want to
>> move to #1, it should be possible with some work.
>
> I think the solution may be to store the state of children in conn
> like you suggested.
>
> See fsnotify_update_iref() and conn flag
> FSNOTIFY_CONN_FLAG_HAS_IREF.
>
> You can add a conn flag
> FSNOTIFY_CONN_FLAG_WATCHES_CHILDREN
> that caches the result of the last invocation of update children flags.
>
> For example, fsnotify_update_iref() becomes
> fsnotify_update_inode_conn_flags() and
> returns inode if either inode ref should be dropped
> or if children flags need to be updated (or both)
> maybe use some out argument to differentiate the cases.
> Same for fsnotify_detach_connector_from_object().
>
> Then, where fsnotify_drop_object() is called, for the
> case that inode children need to be updated,
> take inode_lock(), take connector spin lock
> to check if another thread has already done the update
> if not release spin lock, perform the update under inode lock
> and at the end, take spin lock again and set the
> FSNOTIFY_CONN_FLAG_WATCHES_CHILDREN
> connector flag.
>
> Not sure if it all works out... maybe
I did this for v2 and I think it has worked well, all threads seem to
block until the flags are updated on all dentries.
>>
>> fs/notify/fsnotify.c | 12 ++++++++--
>> fs/notify/mark.c | 55 ++++++++++++++++++++++++++++++++++----------
>> 2 files changed, 53 insertions(+), 14 deletions(-)
>>
>> diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
>> index 7974e91ffe13..e887a195983b 100644
>> --- a/fs/notify/fsnotify.c
>> +++ b/fs/notify/fsnotify.c
>> @@ -207,8 +207,16 @@ int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
>> parent = dget_parent(dentry);
>> p_inode = parent->d_inode;
>> p_mask = fsnotify_inode_watches_children(p_inode);
>> - if (unlikely(parent_watched && !p_mask))
>> - __fsnotify_update_child_dentry_flags(p_inode);
>> + if (unlikely(parent_watched && !p_mask)) {
>> + /*
>> + * Flag would be cleared soon by
>> + * __fsnotify_update_child_dentry_flags(), but as an
>> + * optimization, clear it now.
>> + */
>
> I think that we need to also take p_inode spin_lock here and
> check fsnotify_inode_watches_children() under lock
> otherwise, we could be clearing the WATCHED flag
> *after* __fsnotify_update_child_dentry_flags() had
> already set it, because you we not observe the change to
> p_inode mask.
I'm not sure I follow. The i_fsnotify_mask field isn't protected by the
p_inode spinlock. It isn't really protected at all, though it mainly
gets modified with the conn->lock held.
Wouldn't it be sufficient to do in a new helper:
spin_lock(&dentry->d_lock);
if (!fsnotify_inode_watches_children(p_inode))
dentry->d_flags &= ~DCACHE_FSNOTIFY_PARENT_WATCHED;
spin_unlock(&dentry->d_lock);
I'm sure I'm missing something about your comment. For the moment I left
it as is in the second version of the patch, we can discuss it more and
I can update it for a v3.
>
> I would consider renaming __fsnotify_update_child_dentry_flags()
> to __fsnotify_update_children_dentry_flags(struct inode *dir)
>
> and creating another inline helper for this call site called:
> fsnotify_update_child_dentry_flags(struct inode *dir, struct dentry *child)
>
>
>> + spin_lock(&dentry->d_lock);
>> + dentry->d_flags &= ~DCACHE_FSNOTIFY_PARENT_WATCHED;
>> + spin_unlock(&dentry->d_lock);
>> + }
>>
>> /*
>> * Include parent/name in notification either if some notification
>> diff --git a/fs/notify/mark.c b/fs/notify/mark.c
>> index c74ef947447d..da9f944fcbbb 100644
>> --- a/fs/notify/mark.c
>> +++ b/fs/notify/mark.c
>> @@ -184,15 +184,36 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
>> */
>> void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
>> {
>> + struct inode *inode = NULL;
>> + int watched_before, watched_after;
>> +
>> if (!conn)
>> return;
>>
>> - spin_lock(&conn->lock);
>> - __fsnotify_recalc_mask(conn);
>> - spin_unlock(&conn->lock);
>> - if (conn->type == FSNOTIFY_OBJ_TYPE_INODE)
>> - __fsnotify_update_child_dentry_flags(
>> - fsnotify_conn_inode(conn));
>> + if (conn->type == FSNOTIFY_OBJ_TYPE_INODE) {
>> + /*
>> + * For inodes, we may need to update flags on the child
>> + * dentries. To ensure these updates occur exactly once,
>> + * synchronize the recalculation with the inode mutex.
>> + */
>> + inode = fsnotify_conn_inode(conn);
>> + spin_lock(&conn->lock);
>> + watched_before = fsnotify_inode_watches_children(inode);
>> + __fsnotify_recalc_mask(conn);
>> + watched_after = fsnotify_inode_watches_children(inode);
>> + spin_unlock(&conn->lock);
>> +
>> + inode_lock(inode);
>
> With the pattern that I suggested above, this if / else would
> be unified to code that looks something like this:
>
> spin_lock(&conn->lock);
> inode = __fsnotify_recalc_mask(conn);
> spin_unlock(&conn->lock);
>
> if (inode)
> fsnotify_update_children_dentry_flags(conn, inode);
>
> Where fsnotify_update_children_dentry_flags()
> takes inode lock around entire update and conn spin lock
> only around check and update of conn flags.
>
> FYI, at this time in the code, adding a mark or updating
> existing mark mask cannot result in the need to drop iref.
> That is the reason that return value of __fsnotify_recalc_mask()
> is not checked here.
For v3 I tried this with a new "flags" out variable and two flags - one
for requiring an iput(), and one for calling
fsnotify_update_children_dentry_flags(). As a result, I did stick a
WARN_ON_ONCE here, but it more or less looks just like this code :)
>> + if ((watched_before && !watched_after) ||
>> + (!watched_before && watched_after)) {
>> + __fsnotify_update_child_dentry_flags(inode);
>> + }
>> + inode_unlock(inode);
>> + } else {
>> + spin_lock(&conn->lock);
>> + __fsnotify_recalc_mask(conn);
>> + spin_unlock(&conn->lock);
>> + }
>> }
>>
>> /* Free all connectors queued for freeing once SRCU period ends */
>> @@ -295,6 +316,8 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
>> struct fsnotify_mark_connector *conn = READ_ONCE(mark->connector);
>> void *objp = NULL;
>> unsigned int type = FSNOTIFY_OBJ_TYPE_DETACHED;
>> + struct inode *inode = NULL;
>> + int watched_before, watched_after;
>> bool free_conn = false;
>>
>> /* Catch marks that were actually never attached to object */
>> @@ -311,17 +334,31 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
>> if (!refcount_dec_and_lock(&mark->refcnt, &conn->lock))
>> return;
>>
>> + if (conn->type == FSNOTIFY_OBJ_TYPE_INODE) {
>> + inode = fsnotify_conn_inode(conn);
>> + watched_before = fsnotify_inode_watches_children(inode);
>> + }
>> +
>> hlist_del_init_rcu(&mark->obj_list);
>> if (hlist_empty(&conn->list)) {
>> objp = fsnotify_detach_connector_from_object(conn, &type);
>> free_conn = true;
>> + watched_after = 0;
>> } else {
>> objp = __fsnotify_recalc_mask(conn);
>> type = conn->type;
>> + watched_after = fsnotify_inode_watches_children(inode);
>> }
>> WRITE_ONCE(mark->connector, NULL);
>> spin_unlock(&conn->lock);
>>
>> + if (inode) {
>> + inode_lock(inode);
>> + if (watched_before && !watched_after)
>> + __fsnotify_update_child_dentry_flags(inode);
>> + inode_unlock(inode);
>> + }
>> +
>> fsnotify_drop_object(type, objp);
>>
>
> Here as well something like:
> if (objp)
> fsnotify_update_children_dentry_flags(conn, obj);
>
> But need to distinguish when inode ref needs to be dropped
> children flags updates or both.
With a flags out-param, it works well. I actually was able to stuff this
into fsnotify_drop_object, which was good because I had missed a whole
other function that can detach a connector from an inode
(fsnotify_destroy_marks()).
> Hope that this suggestion direction turns out to be useful and not
> a complete waste of time...
>
> Thanks,
> Amir.
next prev parent reply other threads:[~2022-10-21 0:33 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 [this message]
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 ` [PATCH v4 4/5] fsnotify: allow sleepable child flag update Stephen Brennan
2022-11-12 10:00 ` 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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