[PATCH 3/4] ipc/sem.c: convert sem_array.sem_pending to struct list_head

Manfred Spraul
2008-05-24T17:06:25Z

sem_array.sem_pending is a double linked list, the attached
patch converts it to struct list_head.

Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>
---
include/linux/sem.h | 12 +++----
ipc/sem.c | 90 ++++++++++++++++++--------------------------------
2 files changed, 38 insertions(+), 64 deletions(-)

diff --git a/include/linux/sem.h b/include/linux/sem.h
index 87756ef..d425993 100644
--- a/include/linux/sem.h
+++ b/include/linux/sem.h
@@ -93,21 +93,19 @@ struct sem_array {
time_t sem_otime; /* last semop time */
time_t sem_ctime; /* last change time */
struct sem *sem_base; /* ptr to first semaphore in array */
- struct sem_queue *sem_pending; /* pending operations to be processed */
- struct sem_queue **sem_pending_last; /* last pending operation */
+ struct list_head sem_pending; /* pending operations to be processed */
struct list_head list_id; /* undo requests on this array */
unsigned long sem_nsems; /* no. of semaphores in array */
};

/* One queue for each sleeping process in the system. */
struct sem_queue {
- struct sem_queue * next; /* next entry in the queue */
- struct sem_queue ** prev; /* previous entry in the queue, *(q->prev) == q */
- struct task_struct* sleeper; /* this process */
- struct sem_undo * undo; /* undo structure */
+ struct list_head list; /* queue of pending operations */
+ struct task_struct *sleeper; /* this process */
+ struct sem_undo *undo; /* undo structure */
int pid; /* process id of requesting process */
int status; /* completion status of operation */
- struct sembuf * sops; /* array of pending operations */
+ struct sembuf *sops; /* array of pending operations */
int nsops; /* number of operations */
int alter; /* does the operation alter the array? */
};
diff --git a/ipc/sem.c b/ipc/sem.c
index 8cd96f1..38996c0 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -272,8 +272,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
ns->used_sems += nsems;

sma->sem_base = (struct sem *) &sma[1];
- /* sma->sem_pending = NULL; */
- sma->sem_pending_last = &sma->sem_pending;
+ INIT_LIST_HEAD(&sma->sem_pending);
INIT_LIST_HEAD(&sma->list_id);
sma->sem_nsems = nsems;
sma->sem_ctime = get_seconds();
@@ -331,38 +330,6 @@ asmlinkage long sys_semget(key_t key, int nsems, int semflg)
return ipcget(ns, &sem_ids(ns), &sem_ops, &sem_params);
}

-/* Manage the doubly linked list sma->sem_pending as a FIFO:
- * insert new queue elements at the tail sma->sem_pending_last.
- */
-static inline void append_to_queue (struct sem_array * sma,
- struct sem_queue * q)
-{
- *(q->prev = sma->sem_pending_last) = q;
- *(sma->sem_pending_last = &q->next) = NULL;
-}
-
-static inline void prepend_to_queue (struct sem_array * sma,
- struct sem_queue * q)
-{
- q->next = sma->sem_pending;
- *(q->prev = &sma->sem_pending) = q;
- if (q->next)
- q->next->prev = &q->next;
- else /* sma->sem_pending_last == &sma->sem_pending */
- sma->sem_pending_last = &q->next;
-}
-
-static inline void remove_from_queue (struct sem_array * sma,
- struct sem_queue * q)
-{
- *(q->prev) = q->next;
- if (q->next)
- q->next->prev = q->prev;
- else /* sma->sem_pending_last == &q->next */
- sma->sem_pending_last = q->prev;
- q->prev = NULL; /* mark as removed */
-}
-
/*
* Determine whether a sequence of semaphore operations would succeed
* all at once. Return 0 if yes, 1 if need to sleep, else return error code.
@@ -438,16 +405,15 @@ static void update_queue (struct sem_array * sma)
int error;
struct sem_queue * q;

- q = sma->sem_pending;
- while(q) {
+ q = list_entry(sma->sem_pending.next, struct sem_queue, list);
+ while(&q->list != &sma->sem_pending) {
error = try_atomic_semop(sma, q->sops, q->nsops,
q->undo, q->pid);

/* Does q->sleeper still need to sleep? */
if (error <= 0) {
struct sem_queue *n;
- remove_from_queue(sma,q);
- q->status = IN_WAKEUP;
+
/*
* Continue scanning. The next operation
* that must be checked depends on the type of the
@@ -458,11 +424,24 @@ static void update_queue (struct sem_array * sma)
* for semaphore values to become 0.
* - if the operation didn't modify the array,
* then just continue.
+ * The order of list_del() and reading ->next
+ * is crucial: In the former case, the list_del()
+ * must be done first [because we might be the
+ * first entry in ->sem_pending], in the latter
+ * case the list_del() must be done last
+ * [because the list is invalid after the list_del()]
*/
- if (q->alter)
- n = sma->sem_pending;
- else
- n = q->next;
+ if (q->alter) {
+ list_del(&q->list);
+ n = list_entry(sma->sem_pending.next, struct sem_queue, list);
+ } else {
+ n = list_entry(q->list.next, struct sem_queue, list);
+ list_del(&q->list);
+ }
+
+ /* wake up the waiting thread */
+ q->status = IN_WAKEUP;
+
wake_up_process(q->sleeper);
/* hands-off: q will disappear immediately after
* writing q->status.
@@ -471,7 +450,7 @@ static void update_queue (struct sem_array * sma)
q->status = error;
q = n;
} else {
- q = q->next;
+ q = list_entry(q->list.next, struct sem_queue, list);
}
}
}
@@ -491,7 +470,7 @@ static int count_semncnt (struct sem_array * sma, ushort semnum)
struct sem_queue * q;

semncnt = 0;
- for (q = sma->sem_pending; q; q = q->next) {
+ list_for_each_entry(q, &sma->sem_pending, list) {
struct sembuf * sops = q->sops;
int nsops = q->nsops;
int i;
@@ -503,13 +482,14 @@ static int count_semncnt (struct sem_array * sma, ushort semnum)
}
return semncnt;
}
+
static int count_semzcnt (struct sem_array * sma, ushort semnum)
{
int semzcnt;
struct sem_queue * q;

semzcnt = 0;
- for (q = sma->sem_pending; q; q = q->next) {
+ list_for_each_entry(q, &sma->sem_pending, list) {
struct sembuf * sops = q->sops;
int nsops = q->nsops;
int i;
@@ -529,7 +509,7 @@ static int count_semzcnt (struct sem_array * sma, ushort semnum)
static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
{
struct sem_undo *un;
- struct sem_queue *q;
+ struct sem_queue *q, *t;
struct sem_array *sma = container_of(ipcp, struct sem_array, sem_perm);

/* Invalidate the existing undo structures for this semaphore set.
@@ -541,17 +521,14 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
un->semid = -1;

/* Wake up all pending processes and let them fail with EIDRM. */
- q = sma->sem_pending;
- while(q) {
- struct sem_queue *n;
- /* lazy remove_from_queue: we are killing the whole queue */
- q->prev = NULL;
- n = q->next;
+
+ list_for_each_entry_safe(q, t, &sma->sem_pending, list) {
+ list_del(&q->list);
+
q->status = IN_WAKEUP;
wake_up_process(q->sleeper); /* doesn't sleep */
smp_wmb();
q->status = -EIDRM; /* hands-off q */
- q = n;
}

/* Remove the semaphore set from the IDR */
@@ -1166,9 +1143,9 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
queue.pid = task_tgid_vnr(current);
queue.alter = alter;
if (alter)
- append_to_queue(sma ,&queue);
+ list_add_tail(&queue.list, &sma->sem_pending);
else
- prepend_to_queue(sma ,&queue);
+ list_add(&queue.list, &sma->sem_pending);

queue.status = -EINTR;
queue.sleeper = current;
@@ -1194,7 +1171,6 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,

sma = sem_lock(ns, semid);
if (IS_ERR(sma)) {
- BUG_ON(queue.prev != NULL);
error = -EIDRM;
goto out_free;
}
@@ -1212,7 +1188,7 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
*/
if (timeout && jiffies_left == 0)
error = -EAGAIN;
- remove_from_queue(sma,&queue);
+ list_del(&queue.list);
goto out_unlock_free;

out_unlock_free:
--
1.5.4.1