ID: 16986
Updated by: [EMAIL PROTECTED]
Reported By: [EMAIL PROTECTED]
-Status: Open
+Status: Closed
Bug Type: Semaphore related
Operating System: Linux 2.4.18
PHP Version: 4.1.2
New Comment:
The new proto for sem_get is:
proto int sem_get(int key [, int max_acquire [, int perm [, int
auto_release]])
auto_release defaults to true so original behaviour is preserved; you
may specify false to override.
Previous Comments:
------------------------------------------------------------------------
[2002-05-07 10:58:07] [EMAIL PROTECTED]
Some people write me for explanations.....
ok I'll be more clear I hope:
READER:
p(mutex);
readcount++;
if (readcount is 1) then p(db);
v(mutex);
CRITICAL REGION where we read
p(mutex);
readcount--;
if (readcount is 0) then v(db);
v(mutex);
WRITER:
p(db);
CRITICAL REGION where we write
v(db);
this is the simplest r/w problem, ok ?!?
let we analize it now.
we have to acquire some mutex to protect owr access to variables
readcount ok
??
This is possible in PHP! .. ok !
now, I'm the 1st reader .. I acquire sem DB becouse readcount is equal
to 1,
I release mutex and begin to READ
NOW IT IS possible that an other reader arrive and it is faster than
me; it
DON'T acquire DB becouse readcont is now 2, it READS and DOESN'T HAVE
to
release sem DB becouse readcount IS NOT 1 (reader 1 is still reading
!!)...
PHP AUTORELEASE DB ... damn !!!
------------------------------------------------------------------------
[2002-05-06 10:44:30] [EMAIL PROTECTED]
Hi, I'm the coordinator of Italian Linux Portal and just last saturday
(04/05/2002 in european date) I have talked about PHP in a Linux Event.
I think that the rigth "position" is the position of AlberT: If a
script die and it "has" the semaphore is corret that another scripts
can get that semaphore.
my 0,02euro
------------------------------------------------------------------------
[2002-05-06 10:23:00] [EMAIL PROTECTED]
It cause deadlock, but only on the script... if you test your script
and have a deadlock, you fix it and release the sem you forgot.
I think the auto release may be a good DEFAULT, but it is important to
be able to modify this behaviour.
If you want to fulfill any classical IPC problem (see Tanenbuam!)
You have to be able to acquire a sem, and not to realease it.
Here you ca nfind an explanation of the problems one can have and that
auto release make impossible to solve:
http://joda.cis.temple.edu/~ingargio/old/cis307f95/readings/readwriters.html
------------------------------------------------------------------------
[2002-05-03 06:47:10] [EMAIL PROTECTED]
But a process that acquires a semaphore and then exits will
case deadlocks, because no one else can acquire the semaphore??
Can you give a good example of where leaving the semaphore acquired is
actually useful?
------------------------------------------------------------------------
[2002-05-03 05:37:33] [EMAIL PROTECTED]
sem_acquire() function has this behavior:
After processing a request, any semaphores acquired by the process but
not explicitly released will be released automatically and a warning
will be generated.
This is not a great thing, infact in this way the readers/writers
classical IPC problem, with writers precedence is impossible to
implement !!!
I think an option to disable autorelease is usefull.
(I sent a feature/change request, but I had no feadback :-( )
------------------------------------------------------------------------
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Edit this bug report at http://bugs.php.net/?id=16986&edit=1
