On 18/01/2018 10:55, Vladimir Sementsov-Ogievskiy wrote: >> >> Most functions that looks at the list are "called with BQL taken". >> Functions that write to the list are "called with BQL taken" and call >> bdrv_dirty_bitmaps_lock/bdrv_dirty_bitmaps_unlock themselves. > > Paolo, could you please explain about bitmap locking in more details? > Why do we need mutexes?
We have three cases: 1) monitor creates and destroy bitmaps. 2) monitor also has to read the list. We know it operates with BQL. 3) users such as mirror.c create a dirty bitmap in the monitor command (under BQL), but they can operate without BQL in a separate iothread so we create a separate lock (bitmap->mutex). While in the second and third case, bitmaps cannot disappear. So in the first case you operate with BQL+dirty bitmap mutex. The result is that you lock out both the second and the third case while creating and destroying bitmaps. > Why do we do not need them > on read from the bitmap, only on write? Indeed, reading the bitmap also requires taking the lock. So s/Modifying/Accessing/ in that comment. Paolo
