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Subject[PATCH] ipc/sem.c: fix update sem_otime when calling sem_op in semaphore initialization
Date
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 <jiakernel@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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