Re: [PATCH 1/5] ipc/sem: do not call wake_sem_queue_do() prematurely

From: Manfred Spraul
Date: Tue Sep 13 2016 - 00:17:56 EST

Hi Davidlohr,

On 09/12/2016 01:53 PM, Davidlohr Bueso wrote:
... as this call should obviously be paired with its _prepare()
counterpart. At least whenever possible, as there is no harm in
calling it bogusly as we do now in a few places.
I would define the interface differently:
WAKE_Q creates an initialized wake queue. There is no need to track if any tasks were added to the wake queue, it is safe to call wake_up_q().
So especially for error paths, there is no need to optimize out calls to wake_up_q()
Immediate error
semop(2) paths that are far from ever having the task block can
be simplified and avoid a few unnecessary loads on their way out
of the call as it is not deeply nested.
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
ipc/sem.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 5e318c5f749d..a4e8bb2fae38 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1887,16 +1887,22 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
error = -EFBIG;
- if (max >= sma->sem_nsems)
- goto out_rcu_wakeup;
+ if (max >= sma->sem_nsems) {
+ rcu_read_unlock();
+ goto out_free;
+ }
error = -EACCES;
- if (ipcperms(ns, &sma->sem_perm, alter ? S_IWUGO : S_IRUGO))
- goto out_rcu_wakeup;
+ if (ipcperms(ns, &sma->sem_perm, alter ? S_IWUGO : S_IRUGO)) {
+ rcu_read_unlock();
+ goto out_free;
+ }
Is this really better/simpler?
You replace "if (error) goto cleanup" with "if (error) {cleanup_1(); goto cleanup_2()}".

From my point of view, this just increases the risks that some cleanup steps are forgotten.