Re: [PATCH 1/2] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem

[Stephen Brennan]

Amir Goldstein <amir73il@xxxxxxxxx> writes:

> On Tue, Oct 18, 2022 at 7:12 AM Stephen Brennan
> <stephen.s.brennan@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>
>> 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.