UBIFS: FileSystem Background Thread

[辉少]

Hi,MTD lists
I've got some questions about ubi file system background thread recently listed below:
1. If I mout ubifs again when it's already mounted, will the background thread be created again?
2. Is file written to the write-buffer then synchronized to flash media by background thread in both sync mode and async mode? 
3. When I monut ubifs in sync mode and write files to it, it will take several minutes to finish a writting operation at some point as garbage collection, at that point, other threads are blocked by ubifs backgroud thread.But in async mode, that issue never appears. How can background thread block other threads in sync mode? Doesn't context switch happen?