Mutexes taken when spinlocks held

[Jakub Jermář]

In the current revision, changeset:mainline,402, the stats code attempts to lock the address space mutexes while holding task locks:

get_stats_tasks()
   spinlock_lock(&tasks_lock)
   task_serialize_walker()
      spinlock__lock(&task->lock)
      produce_stats_task()
         get_task_virtmem()
            mutex_lock(&as->lock)
            mutex_lock(&as->area->lock)

Note that this is actually a bug.

[Jiri Svoboda]

Because you cannot take a coarser lock type while holding a finer lock type, as I tried to explain at page 16 of my ​master thesis. I guess this is not immediately obvious because most systems do not seem to use several different locks with varying coarseness. At least I was somewhat surprised by this when I was new to HelenOS. Maybe we should have some article with introduction to synchronization in HelenOS.

[Jakub Jermář]

Fixed in revision changeset:head,409.

Note that it may be infinitely better to let the address space management code compute these statistics as it proceeds instead of having them calculated here over and over again here.