Re: [PATCH] ipc/sem.c: fix update sem_otime when calling sem_op in semaphore initialization

[Manfred Spraul <manfred@colorfullife.com>]

Hi all,

On 09/22/2013 10:26 AM, Mike Galbraith wrote:
> On Sun, 2013-09-22 at 10:17 +0200, Mike Galbraith wrote:
>> On Sun, 2013-09-22 at 10:11 +0800, Jia He wrote:
>>> In commit 0a2b9d4c,the update of semaphore's sem_otime(last semop time)
>>> was removed because he wanted to move setting sem->sem_otime to one
>>> place. But after that, the initial semop() will not set the otime
>>> because its sem_op value is 0(in semtimedop,will not change
>>> otime if alter == 1).
>>>
>>> the error case:
>>> process_a(server)       process_b(client)
>>> semget()
>>> semctl(SETVAL)
>>> semop()
>>>                          semget()
>>>                          setctl(IP_STAT)
>>>                          for(;;) {               <--not successful here
>>>                            check until sem_otime > 0
>>>                          }
Good catch:
Since commit 0a2b9d4c, wait-for-zero semops do not update sem_otime anymore.

Let's reverse that part of my commit and move the update of sem_otime 
back into perform_atomic_semop().

Jia: If perform_atomic_semop() updates sem_otime, then the update in 
do_smart_update() is not necessary anymore.
Thus the whole logic with passing arround "semop_completed" can be 
removed, too.
Are you interested in writing that patch?


>> Why not..
> (pokes evolution's don't-munge-me button)
>
> ipc,sem: Create semaphores with plausible sem_otime.
Mike: no, your patch makes it worse:
- wait-for-zero semops still don't update sem_otime
- sem_otime is initialized to sem_ctime. That's not mentioned in the 
sysv standard.

--
     Manfred