[cpp-threads] pthreads (was: RE: C++ Connections proposal)

[Alexander Terekhov]

On 4/25/05, Peter A. Buhr <pabuhr at plg.uwaterloo.ca> wrote:

[...] 

> pthreads solves the problem by throwing away performance to gain 
> correctness, which I have pointed out is the necessary trade off.

I agree that pthreads is broken. But I think that it can be fixed. 

Memory isolation aside for a moment, pls see

http://www.opengroup.org/austin/mailarchives/ag-review/msg01865.html
http://www.opengroup.org/austin/mailarchives/ag-review/msg01867.html
http://www.opengroup.org/austin/mailarchives/ag-review/msg01869.html
http://www.opengroup.org/austin/mailarchives/ag-review/msg01870.html
http://www.opengroup.org/austin/mailarchives/ag-review/msg01871.html
http://www.opengroup.org/austin/mailarchives/ag-review/msg01872.html
http://www.opengroup.org/austin/mailarchives/ag-review/msg01875.html
http://www.opengroup.org/austin/mailarchives/ag-review/msg01877.html

All but one DR referenced above are still hanging "open". The last
version of my XBD 4.10 edit is this:

-----
Applications shall ensure that access to any memory location by more 
than one thread is restricted such that no thread can read or modify 
a memory location while another thread of control may be modifying it. 
Applications shall use functions listed below that synchronize thread 
execution and ensure that modifications to locations in memory are 
ordered with respect to accesses to the same memory locations in other 
threads. There is a happens-before inter-thread ordering with respect 
to preceding (in program and inter-thread order) modifications of 
memory locations in

  a thread calling pthreads_create() and accesses to modified memory 
  locations in the created thread;

  a thread calling fork() and accesses to modified memory locations 
  in the initial thread of the created process;

  a terminated thread and accesses to modified memory locations in 
  another thread in the same process after that thread returns from 
  pthread_join() joining the terminated thread, or in another thread 
  in a parent process after that thread returns from wait() or 
  waitpid() observing terminated status of a child process;

  a thread calling sem_post() on a specified semaphore and accesses 
  to modified memory locations in another thread after that thread 
  calls sem_wait() or sem_trywait() or sem_timedwait() and 
  locks the same semaphore unlocked by the calling thread, or 
  another thread observes the incremented semaphore value of the 
  same semaphore by calling sem_getvalue();

  a thread calling sem_wait() or sem_trywait() or sem_timedwait() 
  decrementing semaphore value of a specified semaphore and accesses 
  to modified memory locations in another thread after that thread 
  observes the decremented semaphore value of the same semaphore 
  using sem_getvalue();

  < XSI IPC semas... >

  < realtime signals? sigvalue is a union with pointer... >

  a thread calling pthread_spin_unlock() on a specified spin lock 
  and accesses to modified memory locations in another thread 
  after that thread calls pthread_spin_lock() or 
  pthread_spin_trylock() and locks the same spin lock unlocked by 
  the calling thread;

  a thread calling pthread_rwlock_unlock() on a specified read-
  write lock releasing a write lock and read accesses to modified 
  memory locations in another thread after that thread acquires a 
  read lock using pthread_rwlock_rdlock() or 
  pthread_rwlock_timedrdlock() or pthread_rwlock_tryrdlock() on 
  the same read-write lock unlocked by the calling thread; 

  a thread calling pthread_rwlock_unlock() on a specified read-
  write lock releasing a write lock and read/write accesses to 
  modified memory locations in another thread after that thread 
  acquires a write lock using pthread_rwlock_timedwrlock() or 
  pthread_rwlock_trywrlock() or pthread_rwlock_wrlock() on 
  the same read-write lock unlocked by the calling thread; 

  a thread calling pthread_mutex_unlock() on a specified mutex 
  and accesses to modified memory locations in another thread 
  after that thread calls pthread_mutex_lock() or 
  pthread_mutex_timedlock() or pthread_mutex_trylock() and locks 
  the same mutex unlocked by the calling thread or when another
  thread locks the same mutex unlocked by the calling thread 
  and returns from a call to pthread_cond_wait() or 
  pthread_cond_timedwait();

  a thread calling pthread_cond_wait() or pthread_cond_timedwait() 
  and accesses to modified memory locations in another thread 
  after that thread calls pthread_mutex_lock() or 
  pthread_mutex_timedlock() or pthread_mutex_trylock() and locks 
  the same mutex unlocked by the calling thread or when another
  thread locks the same mutex unlocked by the calling thread 
  and returns from a call to pthread_cond_wait() or 
  pthread_cond_timedwait();

  a thread calling pthread_barrier_wait() on a specified barrier 
  and accesses to modified memory locations in another thread 
  after that thread returns from pthread_barrier_wait() call on 
  the same specified barrier on the same or later barrier cycle;

  a thread calling pthread_once() on a specified pthread_once_t 
  object returning from a specified init_routine and accesses to 
  modified memory locations in another thread after that thread 
  returns from pthread_once() on the same pthread_once_t object.

Functions listed above and also pthread_cond_signal() and 
pthread_cond_broadcast() are all side effects.

Conforming applications are said to be data race-free.
-----

In the past, POSIX folks refused to address the use of undefined
term "memory location". "Language issue," they say. Fixing that
problem and extending POSIX data-race free model with atomic<>
and msync reordering constraints/labels (all unidirectional
apart from bidirectional msync::{sl}fence() barrier functions) 
for operations on competing atomic<> calls (that's in addition 
to noncompeting and/or "exclusive" accesses) is the way to go. 
First step, so to speak. The next step is ADA-like higher level 
stuff along the lines of protected objects. But that's C++1X, 
I'm afraid. Just my 0.2 euros.

regards,
alexander.