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

[Jia He]

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
                        }

provide test codes here:
$cat server.c

int semid;
int main()
{
        int key;
        struct semid64_ds sem_info;
        union semun arg;
        struct sembuf sop;

        key = ftok(SEM_PATH,'a');
        semid = semget(key,1,IPC_CREAT|IPC_EXCL|00666);
        if(semid < 0)
                perror("server:semget");

        arg.val = 0;
        if(semctl(semid,0,SETVAL,arg) == -1)
                perror("semctl setval error");

        sop.sem_num = 0;
        sop.sem_op = 0;
        sop.sem_flg = 0;
        if (semop(semid, &sop, 1) == -1)
                perror("semop error");

        sleep(30);

        if(semctl(semid, 0, IPC_RMID) == -1)
                perror("semctl IPC_RMID");
        else printf("remove sem ok\n");
}

$cat client.c
int semid;
int main()
{
        int i;
        int key;
        union semun arg;
        struct semid64_ds sem_info;

        key = ftok(SEM_PATH,'a');
        semid = semget(key,1,IPC_CREAT|00666);
        if(semid < 0)
                perror("client:semget");
        for (i = 0; i < MAX_TRIES; i++)
        {
                arg.buf = &sem_info;
                semctl(semid, 0, IPC_STAT, arg);
                if (sem_info.sem_otime != 0) break;
                sleep(1);
        }

        if(MAX_TRIES == i)
                printf("error in opening a existed sem\n");
        else
                printf("open exsited sem sucessfully\n");
        return 0;
}

the steps to test:
touch /tmp/my_sem
./server &
sleep 1
./client &

With the patch
1.test output:
error in opening a existed sem
2.cat /proc/sysvipc/sem
the field sem_otime is always zero

Without this patch
1.test output:
open exsited sem sucessfully
2.cat /proc/sysvipc/sem
the field sem_otime is not zero

Signed-off-by: Jia He <jiaker...@gmail.com>
---
 ipc/sem.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/ipc/sem.c b/ipc/sem.c
index 69b6a21..8e01e76 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -590,6 +590,7 @@ static int perform_atomic_semop(struct sem_array *sma, 
struct sembuf *sops,
                sop--;
        }
        
+       sma->sem_base[sops[0].sem_num].sem_otime = get_seconds();
        return 0;
 
 out_of_range:
-- 
1.8.1.2

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