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Date:   Wed, 25 Oct 2017 15:07:49 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Xiong Zhou <xzhou@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/7] fsnotify: clean up fsnotify_prepare/finish_user_wait()

On Wed, Oct 25, 2017 at 3:06 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Wed, Oct 25, 2017 at 11:41 AM, Miklos Szeredi <mszeredi@...hat.com> wrote:
>> This patch doesn't actually fix any bug, just paves the way for fixing mark
>> and group pinning.
>>
>> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
>> Cc: <stable@...r.kernel.org> # v4.12
>> ---
>>  fs/notify/mark.c | 89 ++++++++++++++++++++++++++------------------------------
>>  1 file changed, 41 insertions(+), 48 deletions(-)
>>
>> diff --git a/fs/notify/mark.c b/fs/notify/mark.c
>> index 9991f8826734..982527ce6a58 100644
>> --- a/fs/notify/mark.c
>> +++ b/fs/notify/mark.c
>> @@ -109,16 +109,6 @@ void fsnotify_get_mark(struct fsnotify_mark *mark)
>>         atomic_inc(&mark->refcnt);
>>  }
>>
>> -/*
>> - * Get mark reference when we found the mark via lockless traversal of object
>> - * list. Mark can be already removed from the list by now and on its way to be
>> - * destroyed once SRCU period ends.
>> - */
>> -static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)
>> -{
>> -       return atomic_inc_not_zero(&mark->refcnt);
>> -}
>> -
>>  static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
>>  {
>>         u32 new_mask = 0;
>> @@ -256,32 +246,56 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
>>                            FSNOTIFY_REAPER_DELAY);
>>  }
>>
>> -bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info)
>> +/*
>> + * Get mark reference when we found the mark via lockless traversal of object
>> + * list. Mark can be already removed from the list by now and on its way to be
>> + * destroyed once SRCU period ends.
>> + */
>> +static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)

Sorry, hit send too soon...
This helper is now more than "get mark safe"
how about fsnotify_get_mark_wait() to pair with fsnotify_put_mark_wait()?

>>  {
>>         struct fsnotify_group *group;
>>
>> -       if (WARN_ON_ONCE(!iter_info->inode_mark && !iter_info->vfsmount_mark))
>> -               return false;
>> -
>> -       if (iter_info->inode_mark)
>> -               group = iter_info->inode_mark->group;
>> -       else
>> -               group = iter_info->vfsmount_mark->group;
>> +       if (!mark)
>> +               return true;
>>
>> +       group = mark->group;
>>         /*
>>          * Since acquisition of mark reference is an atomic op as well, we can
>>          * be sure this inc is seen before any effect of refcount increment.
>>          */
>>         atomic_inc(&group->user_waits);
>> +       if (atomic_inc_not_zero(&mark->refcnt))
>> +               return true;
>> +
>
> Please replicate the "We abuse notification_waitq on group shutdown" comment
> here as well to clarify why this wakeup is needed. Although comment says
> "for waiting for all marks pinned" and this code path did not change
> mark refcount,
> so is the wakeup really needed here?
>
>> +       if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
>> +               wake_up(&group->notification_waitq);
>>
>> -       if (iter_info->inode_mark) {
>> -               /* This can fail if mark is being removed */
>> -               if (!fsnotify_get_mark_safe(iter_info->inode_mark))
>> -                       goto out_wait;
>> +       return false;
>> +}
>> +
>> +static void fsnotify_put_mark_wait(struct fsnotify_mark *mark)
>> +{
>> +       if (mark) {
>> +               struct fsnotify_group *group = mark->group;
>> +
>> +               fsnotify_put_mark(mark);
>> +               /*
>> +                * We abuse notification_waitq on group shutdown for waiting for
>> +                * all marks pinned when waiting for userspace.
>> +                */
>> +               if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
>> +                       wake_up(&group->notification_waitq);
>>         }
>> -       if (iter_info->vfsmount_mark) {
>> -               if (!fsnotify_get_mark_safe(iter_info->vfsmount_mark))
>> -                       goto out_inode;
>> +}
>> +
>> +bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info)
>> +{
>> +       /* This can fail if mark is being removed */
>> +       if (!fsnotify_get_mark_safe(iter_info->inode_mark))
>> +               return false;
>> +       if (!fsnotify_get_mark_safe(iter_info->vfsmount_mark)) {
>> +               fsnotify_put_mark_wait(iter_info->inode_mark);
>> +               return false;
>>         }
>>
>>         /*
>> @@ -292,34 +306,13 @@ bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info)
>>         srcu_read_unlock(&fsnotify_mark_srcu, iter_info->srcu_idx);
>>
>>         return true;
>> -out_inode:
>> -       if (iter_info->inode_mark)
>> -               fsnotify_put_mark(iter_info->inode_mark);
>> -out_wait:
>> -       if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
>> -               wake_up(&group->notification_waitq);
>> -       return false;
>>  }
>>
>>  void fsnotify_finish_user_wait(struct fsnotify_iter_info *iter_info)
>>  {
>> -       struct fsnotify_group *group = NULL;
>> -
>>         iter_info->srcu_idx = srcu_read_lock(&fsnotify_mark_srcu);
>> -       if (iter_info->inode_mark) {
>> -               group = iter_info->inode_mark->group;
>> -               fsnotify_put_mark(iter_info->inode_mark);
>> -       }
>> -       if (iter_info->vfsmount_mark) {
>> -               group = iter_info->vfsmount_mark->group;
>> -               fsnotify_put_mark(iter_info->vfsmount_mark);
>> -       }
>> -       /*
>> -        * We abuse notification_waitq on group shutdown for waiting for all
>> -        * marks pinned when waiting for userspace.
>> -        */
>> -       if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
>> -               wake_up(&group->notification_waitq);
>> +       fsnotify_put_mark_wait(iter_info->inode_mark);
>> +       fsnotify_put_mark_wait(iter_info->vfsmount_mark);
>>  }
>>
>>  /*
>> --
>> 2.5.5
>>

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