[ace-bugs] Implementation of POSIX CVs: spur.wakeups/lost signals/deadlocks
TERE...@de.ibm.com
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ACE VERSION:
5.1.12 (pthread-win32 snapshot 2000-12-29)
HOST MACHINE and OPERATING SYSTEM:
IBM IntelliStation Z Pro, 2 x XEON 1GHz, Win2K
TARGET MACHINE and OPERATING SYSTEM, if different from HOST:
COMPILER NAME AND VERSION (AND PATCHLEVEL):
Microsoft Visual C++ 6.0
AREA/CLASS/EXAMPLE AFFECTED:
Implementation of POSIX condition variables - OS.cpp/.h
DOES THE PROBLEM AFFECT:
EXECUTION? YES!
SYNOPSIS:
a) spurious wakeups (minor problem)
b) lost signals
c) broadcast deadlock
d) unfairness (minor problem)
DESCRIPTION:
Please see attached copy of discussion thread
from comp.programming.threads for more details on
some reported problems. (i've also posted a "fyi"
message to ace-users a week or two ago but
unfortunately did not get any response so far).
It seems that current implementation suffers from
two essential problems:
1) cond.waiters_count does not accurately reflect
number of waiters blocked on semaphore - w/o
proper synchronisation that could result (in the
time window when counter is not accurate)
in spurious wakeups organised by subsequent
_signals and _broadcasts.
2) Always having (with no e.g. copy_and_clear/..)
the same queue in use (semaphore+counter)
neither signal nor broadcast provide 'atomic'
behaviour with respect to other threads/subsequent
calls to signal/broadcast/wait.
Each problem and combination of both could produce
various nasty things:
a) spurious wakeups (minor problem)
it is possible that waiter(s) which was already
unblocked even so is still counted as blocked
waiter. signal and broadcast will release
semaphore which will produce a spurious wakeup
for a 'real' waiter coming later.
b) lost signals
signalling thread ends up consuming its own
signal. please see demo/discussion below.
c) broadcast deadlock
last_waiter processing code does not correctly
handle the case with multiple threads
waiting for the end of broadcast.
please see demo/discussion below.
d) unfairness (minor problem)
without SignalObjectAndWait some waiter(s)
may end up consuming broadcasted signals
multiple times (spurious wakeups) because waiter
thread(s) can be preempted before they call
semaphore wait (but after count++ and mtx.unlock).
REPEAT BY:
See below... run problem demos programs (tennis.cpp and
tennisb.cpp) number of times concurrently (on multiprocessor)
and in multiple sessions or just add a couple of "Sleep"s
as described in the attached copy of discussion thread
from comp.programming.threads
SAMPLE FIX/WORKAROUND:
See attached patch to pthread-win32.. well, I can not
claim that it is completely bug free but at least my
test and tests provided by pthreads-win32 seem to work.
Perhaps that will help.
regards,
alexander.
>> Forum: comp.programming.threads
>> Thread: pthread_cond_* implementation questions
.
.
.
David Schwartz <dav...@webmaster.com> wrote:
> tere...@my-deja.com wrote:
>
>> BTW, could you please also share your view on other perceived
>> "problems" such as nested broadcast deadlock, spurious wakeups
>> and (the latest one) lost signals??
>
>I'm not sure what you mean. The standard allows an implementation
>to do almost whatever it likes. In fact, you could implement
>pthread_cond_wait by releasing the mutex, sleeping a random
>amount of time, and then reacquiring the mutex. Of course,
>this would be a pretty poor implementation, but any code that
>didn't work under that implementation wouldn't be strictly
>compliant.
The implementation you suggested is indeed correct
one (yes, now I see it :). However it requires from
signal/broadcast nothing more than to "{ return 0; }"
That is not the case for pthread-win32 and ACE
implementations. I do think that these implementations
(basically the same implementation) have some serious
problems with wait/signal/broadcast calls. I am looking
for help to clarify whether these problems are real
or not. I think that I can demonstrate what I mean
using one or two small sample programs.
.
.
.
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
tennis.cpp
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
#include "ace/Synch.h"
#include "ace/Thread.h"
enum GAME_STATE {
START_GAME,
PLAYER_A, // Player A playes the ball
PLAYER_B, // Player B playes the ball
GAME_OVER,
ONE_PLAYER_GONE,
BOTH_PLAYERS_GONE
};
enum GAME_STATE eGameState;
ACE_Mutex* pmtxGameStateLock;
ACE_Condition< ACE_Mutex >* pcndGameStateChange;
void*
playerA(
void* pParm
)
{
// For access to game state variable
pmtxGameStateLock->acquire();
// Play loop
while ( eGameState < GAME_OVER ) {
// Play the ball
cout << endl << "PLAYER-A" << endl;
// Now its PLAYER-B's turn
eGameState =3D PLAYER_B;
// Signal to PLAYER-B that now it is his turn
pcndGameStateChange->signal();
// Wait until PLAYER-B finishes playing the ball
do {
pcndGameStateChange->wait();
if ( PLAYER_B =3D=3D eGameState )
cout << endl << "----PLAYER-A: SPURIOUS WAKEUP!!!" << endl;
} while ( PLAYER_B =3D=3D eGameState );
}
// PLAYER-A gone
eGameState =3D (GAME_STATE)(eGameState+1);
cout << endl << "PLAYER-A GONE" << endl;
// No more access to state variable needed
pmtxGameStateLock->release();
// Signal PLAYER-A gone event
pcndGameStateChange->broadcast();
return 0;
}
void*
playerB(
void* pParm
)
{
// For access to game state variable
pmtxGameStateLock->acquire();
// Play loop
while ( eGameState < GAME_OVER ) {
// Play the ball
cout << endl << "PLAYER-B" << endl;
// Now its PLAYER-A's turn
eGameState =3D PLAYER_A;
// Signal to PLAYER-A that now it is his turn
pcndGameStateChange->signal();
// Wait until PLAYER-A finishes playing the ball
do {
pcndGameStateChange->wait();
if ( PLAYER_A =3D=3D eGameState )
cout << endl << "----PLAYER-B: SPURIOUS WAKEUP!!!" << endl;
} while ( PLAYER_A =3D=3D eGameState );
}
// PLAYER-B gone
eGameState =3D (GAME_STATE)(eGameState+1);
cout << endl << "PLAYER-B GONE" << endl;
// No more access to state variable needed
pmtxGameStateLock->release();
// Signal PLAYER-B gone event
pcndGameStateChange->broadcast();
return 0;
}
int
main (int, ACE_TCHAR *[])
{
pmtxGameStateLock =3D new ACE_Mutex();
pcndGameStateChange =3D new ACE_Condition< ACE_Mutex >( *pmtxGameStat=
eLock
);
// Set initial state
eGameState =3D START_GAME;
// Create players
ACE_Thread::spawn( playerA );
ACE_Thread::spawn( playerB );
// Give them 5 sec. to play
Sleep( 5000 );//sleep( 5 );
// Set game over state
pmtxGameStateLock->acquire();
eGameState =3D GAME_OVER;
// Let them know
pcndGameStateChange->broadcast();
// Wait for players to stop
do {
pcndGameStateChange->wait();
} while ( eGameState < BOTH_PLAYERS_GONE );
// Cleanup
cout << endl << "GAME OVER" << endl;
pmtxGameStateLock->release();
delete pcndGameStateChange;
delete pmtxGameStateLock;
return 0;
}
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
tennisb.cpp
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
#include "ace/Synch.h"
#include "ace/Thread.h"
enum GAME_STATE {
START_GAME,
PLAYER_A, // Player A playes the ball
PLAYER_B, // Player B playes the ball
GAME_OVER,
ONE_PLAYER_GONE,
BOTH_PLAYERS_GONE
};
enum GAME_STATE eGameState;
ACE_Mutex* pmtxGameStateLock;
ACE_Condition< ACE_Mutex >* pcndGameStateChange;
void*
playerA(
void* pParm
)
{
// For access to game state variable
pmtxGameStateLock->acquire();
// Play loop
while ( eGameState < GAME_OVER ) {
// Play the ball
cout << endl << "PLAYER-A" << endl;
// Now its PLAYER-B's turn
eGameState =3D PLAYER_B;
// Signal to PLAYER-B that now it is his turn
pcndGameStateChange->broadcast();
// Wait until PLAYER-B finishes playing the ball
do {
pcndGameStateChange->wait();
if ( PLAYER_B =3D=3D eGameState )
cout << endl << "----PLAYER-A: SPURIOUS WAKEUP!!!" << endl;
} while ( PLAYER_B =3D=3D eGameState );
}
// PLAYER-A gone
eGameState =3D (GAME_STATE)(eGameState+1);
cout << endl << "PLAYER-A GONE" << endl;
// No more access to state variable needed
pmtxGameStateLock->release();
// Signal PLAYER-A gone event
pcndGameStateChange->broadcast();
return 0;
}
void*
playerB(
void* pParm
)
{
// For access to game state variable
pmtxGameStateLock->acquire();
// Play loop
while ( eGameState < GAME_OVER ) {
// Play the ball
cout << endl << "PLAYER-B" << endl;
// Now its PLAYER-A's turn
eGameState =3D PLAYER_A;
// Signal to PLAYER-A that now it is his turn
pcndGameStateChange->broadcast();
// Wait until PLAYER-A finishes playing the ball
do {
pcndGameStateChange->wait();
if ( PLAYER_A =3D=3D eGameState )
cout << endl << "----PLAYER-B: SPURIOUS WAKEUP!!!" << endl;
} while ( PLAYER_A =3D=3D eGameState );
}
// PLAYER-B gone
eGameState =3D (GAME_STATE)(eGameState+1);
cout << endl << "PLAYER-B GONE" << endl;
// No more access to state variable needed
pmtxGameStateLock->release();
// Signal PLAYER-B gone event
pcndGameStateChange->broadcast();
return 0;
}
int
main (int, ACE_TCHAR *[])
{
pmtxGameStateLock =3D new ACE_Mutex();
pcndGameStateChange =3D new ACE_Condition< ACE_Mutex >( *pmtxGameStat=
eLock
);
// Set initial state
eGameState =3D START_GAME;
// Create players
ACE_Thread::spawn( playerA );
ACE_Thread::spawn( playerB );
// Give them 5 sec. to play
Sleep( 5000 );//sleep( 5 );
// Make some noise
pmtxGameStateLock->acquire();
cout << endl << "---Noise ON..." << endl;
pmtxGameStateLock->release();
for ( int i =3D 0; i < 100000; i++ )
pcndGameStateChange->broadcast();
cout << endl << "---Noise OFF" << endl;
// Set game over state
pmtxGameStateLock->acquire();
eGameState =3D GAME_OVER;
cout << endl << "---Stopping the game..." << endl;
// Let them know
pcndGameStateChange->broadcast();
// Wait for players to stop
do {
pcndGameStateChange->wait();
} while ( eGameState < BOTH_PLAYERS_GONE );
// Cleanup
cout << endl << "GAME OVER" << endl;
pmtxGameStateLock->release();
delete pcndGameStateChange;
delete pmtxGameStateLock;
return 0;
}
.
.
.
David Schwartz <dav...@webmaster.com> wrote:
>> > It's compliant
>>
>> That is really good.
>
>> Tomorrow (I have to go urgently now) I will try to
>> demonstrate the lost-signal "problem" of current
>> pthread-win32 and ACE-(variant w/o SingleObjectAndWait)
>> implementations: players start suddenly drop their balls :-)
>> (with no change in source code).
>
>Signals aren't lost, they're going to the main thread,
>which isn't coded correctly to handle them. Try this:
>
> // Wait for players to stop
> do {
>
> pthread_cond_wait( &cndGameStateChange,&mtxGameStateLock );
>printf("Main thread stole a signal\n");
>
> } while ( eGameState < BOTH_PLAYERS_GONE );
>
>I bet everytime you thing a signal is lost, you'll see that printf.
>The signal isn't lost, it was stolen by another thread.
well, you can probably loose your bet.. it was indeed stolen
by "another" thread but not the one you seem to think of.
I think that what actually happens is the following:
H:\SA\UXX\pt\PTHREADS\TESTS>tennis3.exe
PLAYER-A
PLAYER-B
----PLAYER-B: SPURIOUS WAKEUP!!!
PLAYER-A GONE
PLAYER-B GONE
GAME OVER
H:\SA\UXX\pt\PTHREADS\TESTS>
here you can see that PLAYER-B after playing his first
ball (which came via signal from PLAYER-A) just dropped
it down. What happened is that his signal to player A
was consumed as spurious wakeup by himself (player B).
The implementation has a problem:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
waiting threads:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
{ /** Critical Section
inc cond.waiters_count
}
/*
/* Atomic only if using Win32 SignalObjectAndWait
/*
cond.mtx.release
/*** ^^-- A THREAD WHICH DID SIGNAL MAY ACQUIRE THE MUTEX,
/*** GO INTO WAIT ON THE SAME CONDITION AND OVERTAKE
/*** ORIGINAL WAITER(S) CONSUMING ITS OWN SIGNAL!
cond.sem.wait
Player-A after playing game's initial ball went into
wait (called _wait) but was pre-empted before reaching
wait semaphore. He was counted as waiter but was not
actually waiting/blocked yet.
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
signal threads:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
{ /** Critical Section
waiters_count =3D cond.waiters_count
}
if ( waiters_count !=3D 0 )
sem.post 1
endif
Player-B after he received signal/ball from Player A
called _signal. The _signal did see that there was
one waiter blocked on the condition (Player-A) and
released the semaphore.. (but it did not unblock
Player-A because he was not actually blocked).
Player-B thread continued its execution, called _wait,
was counted as second waiter BUT was allowed to slip
through opened semaphore gate (which was opened for
Player-B) and received his own signal. Player B remained
blocked followed by Player A. Deadlock happened which
lasted until main thread came in and said game over.
It seems to me that the implementation fails to
correctly implement the following statement
from specification:
http://www.opengroup.org/
onlinepubs/007908799/xsh/pthread_cond_wait.html
"These functions atomically release mutex and cause
the calling thread to block on the condition variable
cond; atomically here means "atomically with respect
to access by another thread to the mutex and then the
condition variable". That is, if another thread is
able to acquire the mutex after the about-to-block
thread has released it, then a subsequent call to
pthread_cond_signal() or pthread_cond_broadcast()
in that thread behaves as if it were issued after
the about-to-block thread has blocked."
Question: Am I right?
(I produced the program output above by simply
adding ?Sleep( 1 )?:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
waiting threads:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
{ /** Critical Section
inc cond.waiters_count
}
/*
/* Atomic only if using Win32 SignalObjectAndWait
/*
cond.mtx.release
Sleep( 1 ); // Win32
/*** ^^-- A THREAD WHICH DID SIGNAL MAY ACQUIRE THE MUTEX,
/*** GO INTO WAIT ON THE SAME CONDITION AND OVERTAKE
/*** ORIGINAL WAITER(S) CONSUMING ITS OWN SIGNAL!
cond.sem.wait
to the source code of pthread-win32 implementation:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/pthreads/
condvar.c?rev=3D1.36&content-type=3Dtext/
x-cvsweb-markup&cvsroot=3Dpthreads-win32
/*
* We keep the lock held just long enough to increment the count of
* waiters by one (above).
* Note that we can't keep it held across the
* call to sem_wait since that will deadlock other calls
* to pthread_cond_signal
*/
cleanup_args.mutexPtr =3D mutex;
cleanup_args.cv =3D cv;
cleanup_args.resultPtr =3D &result;
pthread_cleanup_push (ptw32_cond_wait_cleanup, (void *)
&cleanup_args);
if ((result =3D pthread_mutex_unlock (mutex)) =3D=3D 0)
{((result
Sleep( 1 ); // @AT
/*
* Wait to be awakened by
* pthread_cond_signal, or
* pthread_cond_broadcast, or
* a timeout
*
* Note:
* ptw32_sem_timedwait is a cancelation point,
* hence providing the
* mechanism for making pthread_cond_wait a cancelation
* point. We use the cleanup mechanism to ensure we
=
* re-lock the mutex and decrement the waiters count
* if we are canceled.
*/
if (ptw32_sem_timedwait (&(cv->sema), abstime) =3D=3D -1) =
{
result =3D errno;
}
}
pthread_cleanup_pop (1); /* Always cleanup */
BTW, on my system (2 CPUs) I can manage to get
signals lost even without any source code modification
if I run the tennis program many times in different
shell sessions.
.
.
.
David Schwartz <dav...@webmaster.com> wrote:
>tere...@my-deja.com wrote:
>
>> well, it might be that the program is in fact buggy.
>> but you did not show me any bug.
>
>You're right. I was close but not dead on. I was correct, however,
>that the code is buggy because it uses 'pthread_cond_signal' even
>though not any thread waiting on the condition variable can do the
>job. I was wrong in which thread could be waiting on the cv but
>unable to do the job.
Okay, lets change 'pthread_cond_signal' to 'pthread_cond_broadcast'
but also add some noise from main() right before declaring the game
to be over (I need it in order to demonstrate another problem of
pthread-win32/ACE implementations - broadcast deadlock)...
.
.
.
It is my understanding of POSIX conditions,
that on correct implementation added noise
in form of unnecessary broadcasts from main,
should not break the tennis program. The
only 'side effect' of added noise on correct
implementation would be 'spurious wakeups' of
players (in fact they are not spurious,
players just see them as spurious) unblocked,
not by another player but by main before
another player had a chance to acquire the
mutex and change the game state variable:
.
.
.
PLAYER-B
PLAYER-A
---Noise ON...
PLAYER-B
PLAYER-A
.
.
.
PLAYER-B
PLAYER-A
----PLAYER-A: SPURIOUS WAKEUP!!!
PLAYER-B
PLAYER-A
---Noise OFF
PLAYER-B
---Stopping the game...
PLAYER-A GONE
PLAYER-B GONE
GAME OVER
H:\SA\UXX\pt\PTHREADS\TESTS>
On pthread-win32/ACE implementations the
program could stall:
.
.
.
PLAYER-A
PLAYER-B
PLAYER-A
PLAYER-B
PLAYER-A
PLAYER-B
PLAYER-A
PLAYER-B
---Noise ON...
PLAYER-A
---Noise OFF
^C
H:\SA\UXX\pt\PTHREADS\TESTS>
The implementation has problems:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
waiting threads:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
{ /** Critical Section
inc cond.waiters_count
}
/*
/* Atomic only if using Win32 SignalObjectAndWait
/*
cond.mtx.release
cond.sem.wait
/*** ^^-- WAITER CAN BE PREEMPTED AFTER BEING UNBLOCKED...
{ /** Critical Section
dec cond.waiters_count
/*** ^^- ...AND BEFORE DECREMENTING THE COUNT (1)
last_waiter =3D ( cond.was_broadcast &&
cond.waiters_count =3D=3D 0 )
if ( last_waiter )
cond.was_broadcast =3D FALSE
endif
}
if ( last_waiter )
/*
/* Atomic only if using Win32 SignalObjectAndWait
/*
cond.auto_reset_event_or_sem.post /* Event for Win32
cond.mtx.acquire
/*** ^^-- ...AND BEFORE CALL TO mtx.acquire (2)
/*** ^^-- NESTED BROADCASTS RESULT IN A DEADLOCK
else
cond.mtx.acquire
/*** ^^-- ...AND BEFORE CALL TO mtx.acquire (3)
endif
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
broadcast threads:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
{ /** Critical Section
waiters_count =3D cond.waiters_count
if ( waiters_count !=3D 0 )
cond.was_broadcast =3D TRUE
endif
}
if ( waiters_count !=3D 0 )
cond.sem.post waiters_count
/*** ^^^^^--- SPURIOUS WAKEUPS DUE TO (1)
cond.auto_reset_event_or_sem.wait /* Event for Win32
/*** ^^^^^--- DEADLOCK FOR FURTHER BROADCASTS IF THEY
HAPPEN TO GO INTO WAIT WHILE PREVIOUS
BROADCAST IS STILL IN PROGRESS/WAITING
endif
a) cond.waiters_count does not accurately reflect
number of waiters blocked on semaphore - that could
result (in the time window when counter is not accurate)
in spurios wakeups organised by subsequent _signals
and _broadcasts. From standard compliance point of view
that is OK but that could be a real problem from
performance/efficiency point of view.
b) If subsequent broadcast happen to go into wait on
cond.auto_reset_event_or_sem before previous
broadcast was unblocked from cond.auto_reset_event_or_sem
by its last waiter, one of two blocked threads will
remain blocked because last_waiter processing code
fails to unblock both threads.
In the situation with tennisb.c the Player-B was put
in a deadlock by noise (broadcast) coming from main
thread. And since Player-B holds the game state
mutex when it calls broadcast, the whole program
stalled: Player-A was deadlocked on mutex and
main thread after finishing with producing the noise
was deadlocked on mutex too (needed to declare the
game over)
(I produced the program output above by simply
adding ?Sleep( 1 )?:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
broadcast threads:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
{ /** Critical Section
waiters_count =3D cond.waiters_count
if ( waiters_count !=3D 0 )
cond.was_broadcast =3D TRUE
endif
}
if ( waiters_count !=3D 0 )
Sleep( 1 ); //Win32
cond.sem.post waiters_count
/*** ^^^^^--- SPURIOUS WAKEUPS DUE TO (1)
cond.auto_reset_event_or_sem.wait /* Event for Win32
/*** ^^^^^--- DEADLOCK FOR FURTHER BROADCASTS IF THEY
HAPPEN TO GO INTO WAIT WHILE PREVIOUS
BROADCAST IS STILL IN PROGRESS/WAITING
endif
to the source code of pthread-win32 implementation:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/pthreads/
condvar.c?rev=3D1.36&content-type=3Dtext/
x-cvsweb-markup&cvsroot=3Dpthreads-win32
if (wereWaiters)
{(wereWaiters)sroot=3Dpthreads-win32eb.cgi/pthreads/Yem...m
/*
* Wake up all waiters
*/
Sleep( 1 ); //@AT
#ifdef NEED_SEM
result =3D (ptw32_increase_semaphore( &cv->sema, cv->waiters )
? 0
: EINVAL);
#else /* NEED_SEM */
result =3D (ReleaseSemaphore( cv->sema, cv->waiters, NULL )
? 0
: EINVAL);
#endif /* NEED_SEM */
}
(void) pthread_mutex_unlock(&(cv->waitersLock));
if (wereWaiters && result =3D=3D 0)
{(wereWaiters
/*
* Wait for all the awakened threads to acquire their part of
* the counting semaphore
*/
if (WaitForSingleObject (cv->waitersDone, INFINITE)
=3D=3D WAIT_OBJECT_0)
{
result =3D 0;
}
else
{
result =3D EINVAL;
}
}
return (result);
}
BTW, on my system (2 CPUs) I can manage to get
the program stalled even without any source code
modification if I run the tennisb program many
times in different shell sessions.
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
pthread-win32 patch
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
struct pthread_cond_t_ {
long nWaitersBlocked; /* Number of threads blocked
*/
long nWaitersUnblocked; /* Number of threads unblocked
*/
long nWaitersToUnblock; /* Number of threads to unblock
*/
sem_t semBlockQueue; /* Queue up threads waiting for th=
e
*/
/* condition to become signalled=
*/
sem_t semBlockLock; /* Semaphore that guards access to=
*/
/* | waiters blocked count/block q=
ueue
*/
/* +-> Mandatory Sync.LEVEL-1
*/
pthread_mutex_t mtxUnblockLock; /* Mutex that guards access to
*/
/* | waiters (to)unblock(ed) count=
s
*/
/* +-> Optional* Sync.LEVEL-2
*/
}; /* Opt*) for _timedwait and
cancellation*/
int
pthread_cond_init (pthread_cond_t * cond, const pthread_condattr_t * at=
tr)
int result =3D EAGAIN;
pthread_cond_t cv =3D NULL;
if (cond =3D=3D NULL)
{(cond
return EINVAL;
}
if ((attr !=3D NULL && *attr !=3D NULL) &&
((*attr)->pshared =3D=3D PTHREAD_PROCESS_SHARED))
{
/*
* Creating condition variable that can be shared between
* processes.
*/
=
result =3D ENOSYS;
goto FAIL0;
}
cv =3D (pthread_cond_t) calloc (1, sizeof (*cv));
if (cv =3D=3D NULL)
{(cv
result =3D ENOMEM;
goto FAIL0;
}
cv->nWaitersBlocked =3D 0;
cv->nWaitersUnblocked =3D 0;
cv->nWaitersToUnblock =3D 0;
if (sem_init (&(cv->semBlockLock), 0, 1) !=3D 0)
{(sem_init
goto FAIL0;
}
if (sem_init (&(cv->semBlockQueue), 0, 0) !=3D 0)
{(sem_init
goto FAIL1;
}
if (pthread_mutex_init (&(cv->mtxUnblockLock), 0) !=3D 0)
{(pthread_mutex_init
goto FAIL2;
}
result =3D 0;
goto DONE;
/*
* -------------
* Failed...
* -------------
*/
FAIL2:
(void) sem_destroy (&(cv->semBlockQueue));
FAIL1:
(void) sem_destroy (&(cv->semBlockLock));
FAIL0:
DONE:
*cond =3D cv;
return (result);
} /* pthread_cond_init */
int
pthread_cond_destroy (pthread_cond_t * cond)
{
int result =3D 0;
pthread_cond_t cv;
/*
* Assuming any race condition here is harmless.
*/
if (cond =3D=3D NULL
|| *cond =3D=3D NULL)
{
return EINVAL;
}
if (*cond !=3D (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
{(*cond
cv =3D *cond;
/*
* Synchronize access to waiters blocked count (LEVEL-1)
*/
if (sem_wait(&(cv->semBlockLock)) !=3D 0)
{(sem_wait(&(cv->semBlockLock))
return errno;
}
/*
* Synchronize access to waiters (to)unblock(ed) counts (LEVEL-2)=
*/
if ((result =3D pthread_mutex_lock(&(cv->mtxUnblockLock))) !=3D 0=
)
{((result
(void) sem_post(&(cv->semBlockLock));
return result;
}
/*
* Check whether cv is still busy (still has waiters blocked)
*/
if (cv->nWaitersBlocked - cv->nWaitersUnblocked > 0)
{(cv->nWaitersBlocked
(void) sem_post(&(cv->semBlockLock));
(void) pthread_mutex_unlock(&(cv->mtxUnblockLock));
return EBUSY;
}
/*
* Now it is safe to destroy
*/
(void) sem_destroy (&(cv->semBlockLock));
(void) sem_destroy (&(cv->semBlockQueue));
(void) pthread_mutex_unlock (&(cv->mtxUnblockLock));
(void) pthread_mutex_destroy (&(cv->mtxUnblockLock));
free(cv);
*cond =3D NULL;
}
else
{
/*
* See notes in ptw32_cond_check_need_init() above also.
*/
EnterCriticalSection(&ptw32_cond_test_init_lock);
/*
* Check again.
*/
if (*cond =3D=3D (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
{(*cond
/*
* This is all we need to do to destroy a statically
* initialised cond that has not yet been used (initialised).=
* If we get to here, another thread
* waiting to initialise this cond will get an EINVAL.
*/
*cond =3D NULL;
}
else
{
/*
* The cv has been initialised while we were waiting
* so assume it's in use.
*/
result =3D EBUSY;
}
LeaveCriticalSection(&ptw32_cond_test_init_lock);
}
return (result);
}
/*
* Arguments for cond_wait_cleanup, since we can only pass a
* single void * to it.
*/
typedef struct {
pthread_mutex_t * mutexPtr;
pthread_cond_t cv;
int * resultPtr;
} ptw32_cond_wait_cleanup_args_t;
static void
ptw32_cond_wait_cleanup(void * args)
{
ptw32_cond_wait_cleanup_args_t * cleanup_args =3D
(ptw32_cond_wait_cleanup_args_t *) args;
pthread_cond_t cv =3D cleanup_args->cv;
int * resultPtr =3D cleanup_args->resultPtr;
int eLastSignal; /* enum: 1=3Dyes 0=3Dno -1=3Dcancelled/timedout w/o =
signal(s)
*/
int result;
/*
* Whether we got here as a result of signal/broadcast or because of
* timeout on wait or thread cancellation we indicate that we are no
* longer waiting. The waiter is responsible for adjusting waiters
* (to)unblock(ed) counts (protected by unblock lock).
* Unblock lock/Sync.LEVEL-2 supports _timedwait and cancellation.
*/
if ((result =3D pthread_mutex_lock(&(cv->mtxUnblockLock))) !=3D 0)
{((result
*resultPtr =3D result;
return;
}
cv->nWaitersUnblocked++;
eLastSignal =3D (cv->nWaitersToUnblock =3D=3D 0) ?
-1 : (--cv->nWaitersToUnblock =3D=3D 0);
/*
* No more LEVEL-2 access to waiters (to)unblock(ed) counts needed
*/
if ((result =3D pthread_mutex_unlock(&(cv->mtxUnblockLock))) !=3D 0)
{((result
*resultPtr =3D result;
return;
}
/*
* If last signal...
*/
if (eLastSignal =3D=3D 1)
{(eLastSignal
/*
* ...it means that we have end of 'atomic' signal/broadcast
*/
if (sem_post(&(cv->semBlockLock)) !=3D 0)
{(sem_post(&(cv->semBlockLock))
*resultPtr =3D errno;
return;
}
}
/*
* If not last signal and not timed out/cancelled wait w/o signal...
*/
else if (eLastSignal =3D=3D 0)
{
/*
* ...it means that next waiter can go through semaphore
*/
if (sem_post(&(cv->semBlockQueue)) !=3D 0)
{(sem_post(&(cv->semBlockQueue))
*resultPtr =3D errno;
return;
}
}
/*
* XSH: Upon successful return, the mutex has been locked and is owne=
d
* by the calling thread
*/
if ((result =3D pthread_mutex_lock(cleanup_args->mutexPtr)) !=3D 0)
{((result
*resultPtr =3D result;
}
} /* ptw32_cond_wait_cleanup */
static int
ptw32_cond_timedwait (pthread_cond_t * cond,
pthread_mutex_t * mutex,
const struct timespec *abstime)
{
int result =3D 0;
pthread_cond_t cv;
ptw32_cond_wait_cleanup_args_t cleanup_args;
if (cond =3D=3D NULL || *cond =3D=3D NULL)
{(cond
return EINVAL;
}
/*
* We do a quick check to see if we need to do more work
* to initialise a static condition variable. We check
* again inside the guarded section of ptw32_cond_check_need_init()
* to avoid race conditions.
*/
if (*cond =3D=3D (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
{(*cond
result =3D ptw32_cond_check_need_init(cond);
}
if (result !=3D 0 && result !=3D EBUSY)
{(result
return result;
}
cv =3D *cond;
/*
* Synchronize access to waiters blocked count (LEVEL-1)
*/
if (sem_wait(&(cv->semBlockLock)) !=3D 0)
{(sem_wait(&(cv->semBlockLock))
return errno;
}
cv->nWaitersBlocked++;
/*
* Thats it. Counted means waiting, no more access needed
*/
if (sem_post(&(cv->semBlockLock)) !=3D 0)
{(sem_post(&(cv->semBlockLock))
return errno;
}
/*
* Setup this waiter cleanup handler
*/
cleanup_args.mutexPtr =3D mutex;
cleanup_args.cv =3D cv;
cleanup_args.resultPtr =3D &result;
pthread_cleanup_push (ptw32_cond_wait_cleanup, (void *) &cleanup_args=
);
/*
* Now we can release 'mutex' and...
*/
if ((result =3D pthread_mutex_unlock (mutex)) =3D=3D 0)
{((result
/*
* ...wait to be awakened by
* pthread_cond_signal, or
* pthread_cond_broadcast, or
* timeout, or
* thread cancellation
*
* Note:
*
* ptw32_sem_timedwait is a cancellation point,
* hence providing the mechanism for making
* pthread_cond_wait a cancellation point.
* We use the cleanup mechanism to ensure we
* re-lock the mutex and adjust (to)unblock(ed) waiters
* counts if we are cancelled, timed out or signalled.
*/
if (ptw32_sem_timedwait (&(cv->semBlockQueue), abstime) !=3D 0)
{(ptw32_sem_timedwait
result =3D errno;
}
}
/*
* Always cleanup
*/
pthread_cleanup_pop (1);
/*
* "result" can be modified by the cleanup handler.
*/
return (result);
=
--0__=hViAJp426bkopDa8gKTDr4QnOXHCN1b7pOAIXS0FVU4FW5KuObKEqqSH
Content-type: text/plain; charset=us-ascii
Content-Disposition: inline
} /* ptw32_cond_timedwait */
static int
ptw32_cond_unblock (pthread_cond_t * cond,
int unblockAll)
{
int result;
pthread_cond_t cv;
if (cond == NULL || *cond == NULL)
{(cond
return EINVAL;
}
cv = *cond;
/*
* No-op if the CV is static and hasn't been initialised yet.
* Assuming that any race condition is harmless.
*/
if (cv == (pthread_cond_t) PTW32_OBJECT_AUTO_INIT)
{(cv
return 0;
}
/*
* Synchronize access to waiters blocked count (LEVEL-1)
*/
if (sem_wait(&(cv->semBlockLock)) != 0)
{(sem_wait(&(cv->semBlockLock))
return errno;
}
/*
* Synchronize access to waiters (to)unblock(ed) counts (LEVEL-2)
* This sync.level supports _timedwait and cancellation
*/
if ((result = pthread_mutex_lock(&(cv->mtxUnblockLock))) != 0)
{((result
return result;
}
/*
* Adjust waiters blocked and unblocked counts (collect garbage)
*/
if (cv->nWaitersUnblocked != 0)
{(cv->nWaitersUnblocked
cv->nWaitersBlocked -= cv->nWaitersUnblocked;
cv->nWaitersUnblocked = 0;
}
/*
* If (after adjustment) there are still some waiters blocked counted...
*/
if ( cv->nWaitersBlocked > 0)
{(
/*
* We will unblock first waiter and leave semBlockLock/LEVEL-1 locked
* LEVEL-1 access is left disabled until last signal/unblock
completes
*/
cv->nWaitersToUnblock = (unblockAll) ? cv->nWaitersBlocked : 1;
/*
* No more LEVEL-2 access to waiters (to)unblock(ed) counts needed
* This sync.level supports _timedwait and cancellation
*/
if ((result = pthread_mutex_unlock(&(cv->mtxUnblockLock))) != 0)
{((result
return result;
}
/*
* Now, with LEVEL-2 lock released let first waiter go through
semaphore
*/
if (sem_post(&(cv->semBlockQueue)) != 0)
{(sem_post(&(cv->semBlockQueue))
return errno;
}
}
/*
* No waiter blocked - no more LEVEL-1 access to blocked count needed...
*/
else if (sem_post(&(cv->semBlockLock)) != 0)
{
return errno;
}
/*
* ...and no more LEVEL-2 access to waiters (to)unblock(ed) counts needed
too
* This sync.level supports _timedwait and cancellation
*/
else
{
result = pthread_mutex_unlock(&(cv->mtxUnblockLock));
}
return(result);
} /* ptw32_cond_unblock */
int
pthread_cond_wait (pthread_cond_t * cond,
pthread_mutex_t * mutex)
{
/* The NULL abstime arg means INFINITE waiting. */
return(ptw32_cond_timedwait(cond, mutex, NULL));
} /* pthread_cond_wait */
int
pthread_cond_timedwait (pthread_cond_t * cond,
pthread_mutex_t * mutex,
const struct timespec *abstime)
{
if (abstime == NULL)
{(abstime
return EINVAL;
}
return(ptw32_cond_timedwait(cond, mutex, abstime));
} /* pthread_cond_timedwait */
int
pthread_cond_signal (pthread_cond_t * cond)
{
/* The '0'(FALSE) unblockAll arg means unblock ONE waiter. */
return(ptw32_cond_unblock(cond, 0));
} /* pthread_cond_signal */
int
pthread_cond_broadcast (pthread_cond_t * cond)
{
/* The '1'(TRUE) unblockAll arg means unblock ALL waiters. */
return(ptw32_cond_unblock(cond, 1));
} /* pthread_cond_broadcast */
--0__=hViAJp426bkopDa8gKTDr4QnOXHCN1b7pOAIXS0FVU4FW5KuObKEqqSH--