xref: /src/lib/libpthread/TODO

$NetBSD: TODO,v 1.5 2006/12/25 11:36:36 ad Exp $

Bugs to fix, mostly with SA:

- some blocking routines (like sem_wait()) don't work if SA's aren't
  running yet, because the alarm system isn't up and running or there is no
  thread context to switch to. It would be weird to use them that
  way, but it's perfectly legal.
- There is a race between pthread_cancel() and
  pthread_cond_broadcast() or pthread_exit() about removing an item
  from the sleep queue. The locking protocols there need a little
  adjustment.
- pthread_sig.c: pthread__kill_self() passes a bogus ucontext to the handler.
  This is probably not very important.
- pthread_sig.c: Come up with a signal trampoline naming convention like
  libc's, so that GDB will have an easier time with things.
- Consider moving pthread__signal_tramp() to its own file, and building
  it with -fasync-unwind-tables, so that DWARF2 EH unwinding works through
  it.  (This is required for e.g. GCC's libjava.)
- Add locking to ld.elf_so so that multiple threads doing lazy binding
  doesn't trash things.
- Verify the cancel stub symbol trickery.


Interfaces/features to implement:
- pthread_atfork()
- priority scheduling
- libc integration: 
   - foo_r interfaces
- system integration
   - some macros and prototypes belong in headers other than pthread.h


Features that need more/better regression tests:
 - pthread_cond_broadcast()
 - pthread_once()
 - pthread_get/setspecific()
 - signals


Things that need fixing:
- Recycle dead threads for new threads.

Ideas to play with:
- Explore the trapcontext vs. usercontext distinction in ucontext_t.
- Get rid of thread structures when too many accumulate (is this
  actually a good idea?)
- Adaptive spin/sleep locks for mutexes.
- Currently, each thread uses two real pages of memory: one at the top
  of the stack for actual stack data, and one at the bottom for the
  pthread_st. If we can get suitable space above the initial stack for
  main(), we can cut this to one page per thread. Perhaps crt0 should
  do something different (give us more space) if libpthread is linked
  in?
- Figure out whether/how to expose the inline version of
  pthread_self().
- Along the same lines, figure out whether/how to use registers reserved
  in the ABI for thread-specific-data to implement pthread_self().
- Figure out what to do with changing stack sizes.

Future work for 1:1 threads:

- Stress testing, particularly with multiple CPUs.

- Verify that gdb still works well (basic functionality seems to be OK).

- There is a race between pthread_exit() and pthread_create() for
  detached LWPs, where the stack (and pthread structure) could be reclaimed
  before the thread has a chance to call _lwp_exit().  Checking the return
  of _lwp_kill(target, 0) could be used to fix this but that seems a bit
  heavyweight. (See shared page item.)

- Adaptive mutexes and spinlocks (see shared page item). These need
  to implement exponential backoff to reduce bus contention. On x86 we
  need to issue the 'pause' instruction while spinning, perhaps on other
  SMT processors too.

- Have a shared page that:

  o Allows an LWP to request it not be preempted by the kernel. This would
    be used over critical sections like pthread_cond_wait(), where we can
    acquire a bunch of spin locks: being preempted while holding them would
    suck. _lwp_park() would reset the flag once in kernel mode, and there
    would need to be an equivalent way to do this from user mode. The user
    path would probably need to notice deferred preemption and call
    sched_yield() on exit from the critical section.

  o Perhaps has some kind of hint mechanism that gives us a clue about
    whether an LWP is currently running on another CPU. This could be used
    for adaptive locks, but would need to be cheap to do in-kernel.

  o Perhaps has a flag value that's reset when a detached LWP is into the
    kernel and lwp_exit1(), meaning that its stack can be reclaimed. Again,
    may or may not be worth it.

- Keep a pool of dead LWPs so that we do not have take the full hit of
  _lwp_create() every time pthread_create() is called. If nothing else
  this is important for benchmarks.. There are a few different ways this
  could be implemented, but it needs to be clear if the advantages are
  real. Lots of thought and benchmarking required.

- LWPs that are parked or that have called nanosleep() (common) burn up
  kernel resources. "struct lwp" itself isn't a big deal, but the VA space
  and swap used by kernel stacks is. _lwp_park() takes a ucontext_t pointer
  in expectation that at some point we may be able to recycle the kernel
  stack and re-start the LWP at the correct point, using pageable user
  memory to hold state. It might also be useful to have a nanosleep call
  that does something similar. Again, lots of thought and benchmarking
  required. (Original idea from matt@)

- It's possible that we don't need to take so many spinlocks around
  cancellation points like pthread_cond_wait() given that _lwp_wakeup()
  and _lwp_unpark() need to synchronise anyway.

- Need to give consideration to the order in which threads enter and exit
  synchronisation objects, both in the pthread library and in the kernel.
  Commonly locks are acquired/released in order (a, b, c -> c, b, a). The
  pthread spec probably has something to say about this.

- The kernel scheduler needs improving to handle LWPs and processor affinity
  better, and user space tools like top(1) and ps(1) need to be changed to
  report correctly.  Tied into that is the need for a mechanism to impose
  limits on various aspects of LWPs.

- Streamlining of the park/unpark path.

- Priority inheritance and similar nasties.