Proper maintenance and care of multi-threading locks

Below are all of the locks that exist in the system and the mechanisms for using them that avoid the potential for deadlocks (no Ostrich algorithm allowed here):

The following are definitely leaf locks (level 1), and must not try to acquire any other lock:

• safepoint

  Note that this lock is acquired implicitly by and JL_UNLOCK. use the _NOGC variants to avoid that for level 1 locks.

  While holding this lock, the code must not do any allocation or hit any safepoints. Note that there are safepoints when doing allocation, enabling / disabling GC, entering / restoring exception frames, and taking / releasing locks.

• shared_map

• finalizers

• pagealloc

• gcpermlock

• flisp

  flisp itself is already threadsafe, this lock only protects the jl_ast_context_list_t pool

The following is a leaf lock (level 2), and only acquires level 1 locks (safepoint) internally:

• typecache

The following is a level 3 lock, which can only acquire level 1 or level 2 locks internally:

• Method->writelock

The following is a level 4 lock, which can only recurse to acquire level 1, 2, or 3 locks:

No Julia code may be called while holding a lock above this point.

The following are a level 6 lock, which can only recurse to acquire locks at lower levels:

• codegen
• jlmodulesmutex

The following is an almost root lock (level end-1), meaning only the root look may be held when trying to acquire it:

• this one is perhaps one of the most tricky ones, since type-inference can be invoked from many points

  currently the lock is merged with the codegen lock, since they call each other recursively

The following lock synchronizes IO operation. Be aware that doing any I/O (for example, printing warning messages or debug information) while holding any other lock listed above may result in pernicious and hard-to-find deadlocks. BE VERY CAREFUL!

• iolock

• Individual ThreadSynchronizers locks

  this may continue to be held after releasing the iolock, or acquired without it, but be very careful to never attempt to acquire the iolock while holding it

The following is the root lock, meaning no other lock shall be held when trying to acquire it:

The following locks are broken:

• toplevel

  doesn’t exist right now

  fix: create it

• Module->lock

  This is vulnerable to deadlocks since it can’t be certain it is acquired in sequence. Some operations (such as import_module) are missing a lock.

  fix: replace with ?

• This file potentially has numerous problems.

  fix: needs locks

These data structures each need locks due to being shared mutable global state. It is the inverse list for the above lock priority list. This list does not include level 1 leaf resources due to their simplicity.

MethodTable modifications (def, cache, kwsorter type) : MethodTable->writelock

Type declarations : toplevel lock

Type application : typecache lock

Global variable tables : Module->lock

Module serializer : toplevel lock

JIT & type-inference : codegen lock

MethodInstance/CodeInstance updates : Method->writelock, codegen lock

• These are set at construction and immutable:

  • specTypes
  • def

• These are set by jl_type_infer (while holding codegen lock):

  • cache
  • rettype
  • inferred

LLVMContext : codegen lock

Method : Method->writelock

• invoke / specializations / tfunc modifications