TODO revision 1.8 
1$NetBSD: TODO,v 1.8 2007/03/02 18:53:51 ad Exp $
2
3Bugs to fix:
4
5- Add locking to ld.elf_so so that multiple threads doing lazy binding
6  doesn't trash things. XXX Still the case?
7- Verify the cancel stub symbol trickery.
8
9Interfaces/features to implement:
10
11- priority scheduling
12- libc integration: 
13   - foo_r interfaces
14- system integration
15   - some macros and prototypes belong in headers other than pthread.h
16
17Features that need more/better regression tests:
18
19 - pthread_cond_broadcast()
20 - pthread_once()
21 - pthread_get/setspecific()
22 - signals
23
24Ideas to play with:
25
26- Explore the trapcontext vs. usercontext distinction in ucontext_t.
27
28- Get rid of thread structures when too many accumulate (is this
29  actually a good idea?)
30
31- Currently, each thread uses two real pages of memory: one at the top
32  of the stack for actual stack data, and one at the bottom for the
33  pthread_st. If we can get suitable space above the initial stack for
34  main(), we can cut this to one page per thread. Perhaps crt0 should
35  do something different (give us more space) if libpthread is linked
36  in?
37
38- Figure out whether/how to expose the inline version of
39  pthread_self().
40
41- Along the same lines, figure out whether/how to use registers reserved
42  in the ABI for thread-specific-data to implement pthread_self().
43
44- Figure out what to do with changing stack sizes.
45
46- Stress testing, particularly with multiple CPUs.
47
48- A race between pthread_exit() and pthread_create() for detached LWPs,
49  where the stack (and pthread structure) could be reclaimed before the
50  thread has a chance to call _lwp_exit(), is currently prevented by
51  checking the return of _lwp_kill(target, 0).  It could be done more
52  efficiently.  (See shared page item.)
53
54- Adaptive mutexes and spinlocks (see shared page item). These need
55  to implement exponential backoff to reduce bus contention. On x86 we
56  need to issue the 'pause' instruction while spinning, perhaps on other
57  SMT processors too.
58
59- Have a shared page that:
60
61  o Allows an LWP to request it not be preempted by the kernel. This would
62    be used over critical sections like pthread_cond_wait(), where we can
63    acquire a bunch of spin locks: being preempted while holding them would
64    suck. _lwp_park() would reset the flag once in kernel mode, and there
65    would need to be an equivalent way to do this from user mode. The user
66    path would probably need to notice deferred preemption and call
67    sched_yield() on exit from the critical section.
68
69  o Perhaps has some kind of hint mechanism that gives us a clue about
70    whether an LWP is currently running on another CPU. This could be used
71    for adaptive locks, but would need to be cheap to do in-kernel.
72
73  o Perhaps has a flag value that's reset when a detached LWP is into the
74    kernel and lwp_exit1(), meaning that its stack can be reclaimed. Again,
75    may or may not be worth it.
76
77- Keep a pool of dead LWPs so that we do not have take the full hit of
78  _lwp_create() every time pthread_create() is called. If nothing else
79  this is important for benchmarks.. There are a few different ways this
80  could be implemented, but it needs to be clear if the advantages are
81  real. Lots of thought and benchmarking required.
82
83- LWPs that are parked or that have called nanosleep() (common) burn up
84  kernel resources. "struct lwp" itself isn't a big deal, but the VA space
85  and swap used by kernel stacks is. _lwp_park() takes a ucontext_t pointer
86  in expectation that at some point we may be able to recycle the kernel
87  stack and re-start the LWP at the correct point, using pageable user
88  memory to hold state. It might also be useful to have a nanosleep call
89  that does something similar. Again, lots of thought and benchmarking
90  required. (Original idea from matt@)
91
92- Need to give consideration to the order in which threads enter and exit
93  synchronisation objects, both in the pthread library and in the kernel.
94  Commonly locks are acquired/released in order (a, b, c -> c, b, a).
95
96- The kernel scheduler needs improving to handle LWPs and processor affinity
97  better, and user space tools like top(1) and ps(1) need to be changed to
98  report correctly.  Tied into that is the need for a mechanism to impose
99  limits on various aspects of LWPs.
100
101- Streamlining of the park/unpark path.
102
103- Priority inheritance and similar nasties.
104