iOS7 - iPhone or iPad music out of sync with iTunes

Recently I have a problem with my iPhone 5 (iOS 7) (Music). The playlist is out of sync with my iTunes. Every time I sync with my iTunes on my Mac, the smart playlists are not updated on my iPhone. The symptoms are:

• I have a smart playlist that only stores all songs that are added within a month ago. However, the smart playlist on my iPhone (iOS 7) does not get the latest songs and refresh the playlist. It still keep the songs that I have added a few months ago. 
• All the new songs in my smart playlist still added into iPhone music, but the playlist is not refreshed.

I have tried various methods, such as

• Delete the smart playlist and re-sync again.
• Delete all songs and re-sync with iTunes again

None of them works because when I re-sync, the smart playlist "auto-magically" bring back the old songs in the list, as if the information is stored somewhere in iPhone and never get removed.

After several attempts and googling around, finally I found a solution that works for me. If you have something similar, here is a solution:

• Plug your iPhone into PC/Mac with iTunes.
• Click on the iPhone tab and select Music tab.
• Uncheck "Sync Music". It will warn you that all songs and playlists on your iPhone will be removed (this is OK because we would like to start from clean sync).
• Click "Apply" and wait until it is fully sync. 
• Then go to iPhone.
• Select Settings -> General -> Usage
• You will see the space that Music consumes, i.e.

• Next select Music

• Swipe to the left on "All Music". This will remove the indexes as well after the clean sync to ensure that iPhone does not store any old data.

• Select Delete.
• Then go back to iTunes and select iPhone -> Music, select "Sync Music" again. Complete the sync and chceck your iPhone.
• This works for me on iPhone 5/iOS 7 with iTunes on Mac.