Lecture 006

Threads

Multithread

• 3 sets of local variables

• 3 stack pointers

• 3 %eax and other registers

• 3 schedulable RAM mutators

Potential Solution:

• 1 Process per Player: share object via syscalls, expensive to make game object

• 1 Process per Game Object: expensive to communicate

• 1 Thread per Player: same memory space, just load and restore registers

Benifits:

• share data

• UI responsiveness, code separation

• Multiprocessing speedup

Kinds of Threads:

• User-space threads (N:1): not actually thread

  • content switch via register swap (1 set of registers)
  • multiple stacks in one address space
  • switch via yeild() by cooperative scheduling
  • no kernel code needed to be changed to support this
  • any systemcall will block all threads (they are done in process level)
  • no multiprocessing speedup
  • need: process control block, one kernel stack

• Kernel-space threads

  • multiple sets of registers
  • multiple stacks in one address space
  • OS know threads ownership
  • references to object is counted
  • scheduling by kernel
  • user code need to be thread-safe
  • require more kernel memory: thread control block, multiple child thread control block, more kernel stack

• Many-to-many threads: combination of above two methods

Cancelation

Asynchronous cancelation: NEVER DO THIS

• behavior:

  • run 0 more instructions
  • free stack, registers

• cannot garbage collect (free)

• stop in the middle of updating shared resource will corrupt invariants of data structures

Deferred cancelation: thread B checks if it should be canceled at certain points

• behavior:
  • stop a thread via communication
  • thread must check if it should be canceled or define self-cancelation points
  • POSIX: all library code must contain cancelation points

setuid()

When a program is executed, it can run and switch between two identities:

• invoker's identity (user who runs the program)

• program's identity (owner of the program file)

During name->file, file->privilege mapping, race condition can happen allowing attacker to execute program with different privilege.