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SubjectRe: [PATCH 1/4] ipc/sem.c: convert undo structures to struct list_head
Manfred Spraul wrote:
> The undo structures contain two linked lists, the
> attached patch replaces them with generic struct list_head lists.

If I'm not wrong the undo list is a singly-linked list.
So here we are moving from a set of 4 pointers to a set of 8 pointers.
It's true that this makes the code much much more readable and clear,
but I was wondering if it's worth?

+ 2 small comments embedded.


>
> Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>
> ---
> include/linux/sem.h | 12 ++--
> ipc/sem.c | 165 ++++++++++++++++++++++++++++-----------------------
> 2 files changed, 96 insertions(+), 81 deletions(-)
>
> diff --git a/include/linux/sem.h b/include/linux/sem.h
> index c8eaad9..6a1af1b 100644
> --- a/include/linux/sem.h
> +++ b/include/linux/sem.h
> @@ -95,7 +95,7 @@ struct sem_array {
> 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 sem_undo *undo; /* undo requests on this array */
> + struct list_head list_id; /* undo requests on this array */
> unsigned long sem_nsems; /* no. of semaphores in array */
> };
>
> @@ -118,8 +118,8 @@ struct sem_queue {
> * when the process exits.
> */
> struct sem_undo {
> - struct sem_undo * proc_next; /* next entry on this process */
> - struct sem_undo * id_next; /* next entry on this semaphore set */
> + struct list_head list_proc; /* per-process list: all undos from one process */
> + struct list_head list_id; /* per semaphore array list: all undos for one array */
> int semid; /* semaphore set identifier */
> short * semadj; /* array of adjustments, one per semaphore */
> };
> @@ -128,9 +128,9 @@ struct sem_undo {
> * that may be shared among all a CLONE_SYSVSEM task group.
> */
> struct sem_undo_list {
> - atomic_t refcnt;
> - spinlock_t lock;
> - struct sem_undo *proc_list;
> + atomic_t refcnt;
> + spinlock_t lock;
> + struct list_head list_proc;
> };
>
> struct sysv_sem {
> diff --git a/ipc/sem.c b/ipc/sem.c
> index e9418df..211632e 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -274,7 +274,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
> sma->sem_base = (struct sem *) &sma[1];
> /* sma->sem_pending = NULL; */
> sma->sem_pending_last = &sma->sem_pending;
> - /* sma->undo = NULL; */
> + INIT_LIST_HEAD(&sma->list_id);
> sma->sem_nsems = nsems;
> sma->sem_ctime = get_seconds();
> sem_unlock(sma);
> @@ -536,7 +536,8 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
> * (They will be freed without any further action in exit_sem()
> * or during the next semop.)
> */
> - for (un = sma->undo; un; un = un->id_next)
> + assert_spin_locked(&sma->sem_perm.lock);
> + list_for_each_entry(un, &sma->list_id, list_id)
> un->semid = -1;
>
> /* Wake up all pending processes and let them fail with EIDRM. */
> @@ -763,9 +764,12 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
>
> for (i = 0; i < nsems; i++)
> sma->sem_base[i].semval = sem_io[i];
> - for (un = sma->undo; un; un = un->id_next)
> +
> + assert_spin_locked(&sma->sem_perm.lock);

This assert() comes a couple of lines after actually locking the sma: do
you think it is really necessary to leave it here?

> + list_for_each_entry(un, &sma->list_id, list_id) {
> for (i = 0; i < nsems; i++)
> un->semadj[i] = 0;
> + }
> sma->sem_ctime = get_seconds();
> /* maybe some queued-up processes were waiting for this */
> update_queue(sma);
> @@ -797,12 +801,15 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
> {
> int val = arg.val;
> struct sem_undo *un;
> +
> err = -ERANGE;
> if (val > SEMVMX || val < 0)
> goto out_unlock;
>
> - for (un = sma->undo; un; un = un->id_next)
> + assert_spin_locked(&sma->sem_perm.lock);
> + list_for_each_entry(un, &sma->list_id, list_id)
> un->semadj[semnum] = 0;
> +
> curr->semval = val;
> curr->sempid = task_tgid_vnr(current);
> sma->sem_ctime = get_seconds();
> @@ -952,6 +959,8 @@ static inline int get_undo_list(struct sem_undo_list **undo_listp)
> return -ENOMEM;
> spin_lock_init(&undo_list->lock);
> atomic_set(&undo_list->refcnt, 1);
> + INIT_LIST_HEAD(&undo_list->list_proc);
> +
> current->sysvsem.undo_list = undo_list;
> }
> *undo_listp = undo_list;
> @@ -960,25 +969,30 @@ static inline int get_undo_list(struct sem_undo_list **undo_listp)
>
> static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid)
> {
> - struct sem_undo **last, *un;
> -
> - last = &ulp->proc_list;
> - un = *last;
> - while(un != NULL) {
> - if(un->semid==semid)
> - break;
> - if(un->semid==-1) {
> - *last=un->proc_next;
> - kfree(un);
> - } else {
> - last=&un->proc_next;
> + struct sem_undo *walk, *tmp;
> +
> + assert_spin_locked(&ulp->lock);
> + list_for_each_entry_safe(walk, tmp, &ulp->list_proc, list_proc) {
> + if(walk->semid==semid)
> + return walk;
> + if(walk->semid==-1) {
> + list_del(&walk->list_proc);
> + kfree(walk);
> }
> - un=*last;
> }
> - return un;
> + return NULL;
> }
>
> -static struct sem_undo *find_undo(struct ipc_namespace *ns, int semid)
> +/**
> + * find_alloc_undo - Lookup (and if not present create) undo array
> + * @ns: namespace
> + * @semid: semaphore array id
> + *
> + * The function looks up (and if not present creates) the undo structure.
> + * The size of the undo structure depends on the size of the semaphore
> + * array, thus the alloc path is not that straightforward.
> + */
> +static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
> {
> struct sem_array *sma;
> struct sem_undo_list *ulp;
> @@ -997,6 +1011,7 @@ static struct sem_undo *find_undo(struct ipc_namespace *ns, int semid)
> goto out;
>
> /* no undo structure around - allocate one. */
> + /* step 1: figure out the size of the semaphore array */
> sma = sem_lock_check(ns, semid);
> if (IS_ERR(sma))
> return ERR_PTR(PTR_ERR(sma));
> @@ -1004,15 +1019,19 @@ static struct sem_undo *find_undo(struct ipc_namespace *ns, int semid)
> nsems = sma->sem_nsems;
> sem_getref_and_unlock(sma);
>
> + /* step 2: allocate new undo structure */
> new = kzalloc(sizeof(struct sem_undo) + sizeof(short)*nsems, GFP_KERNEL);
> if (!new) {
> sem_putref(sma);
> return ERR_PTR(-ENOMEM);
> }
> - new->semadj = (short *) &new[1];
> - new->semid = semid;
>
> + /* step 3: Acquire the lock on the undo list pointer */
> spin_lock(&ulp->lock);
> +
> + /* step 4: check for races: someone else allocated the undo struct,
> + * semaphore array was destroyed.
> + */
> un = lookup_undo(ulp, semid);
> if (un) {
> spin_unlock(&ulp->lock);
> @@ -1028,13 +1047,17 @@ static struct sem_undo *find_undo(struct ipc_namespace *ns, int semid)
> un = ERR_PTR(-EIDRM);
> goto out;
> }
> - new->proc_next = ulp->proc_list;
> - ulp->proc_list = new;
> - new->id_next = sma->undo;
> - sma->undo = new;
> + /* step 5: initialize & link new undo structure */
> + new->semadj = (short *) &new[1];
> + new->semid = semid;
> + assert_spin_locked(&ulp->lock);
> + list_add(&new->list_proc, &ulp->list_proc);
> + assert_spin_locked(&sma->sem_perm.lock);
> + list_add(&new->list_id, &sma->list_id);
> +
> sem_unlock(sma);
> - un = new;
> spin_unlock(&ulp->lock);
> + un = new;
> out:
> return un;
> }
> @@ -1090,9 +1113,8 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
> alter = 1;
> }
>
> -retry_undos:
> if (undos) {
> - un = find_undo(ns, semid);
> + un = find_alloc_undo(ns, semid);
> if (IS_ERR(un)) {
> error = PTR_ERR(un);
> goto out_free;
> @@ -1107,14 +1129,14 @@ retry_undos:
> }
>
> /*
> - * semid identifiers are not unique - find_undo may have
> + * semid identifiers are not unique - find_alloc_undo may have
> * allocated an undo structure, it was invalidated by an RMID
> - * and now a new array with received the same id. Check and retry.
> + * and now a new array with received the same id. Check and fail.
> */
> - if (un && un->semid == -1) {
> - sem_unlock(sma);
> - goto retry_undos;
> - }
> + error = -EIDRM;
> + if (un && un->semid == -1)
> + goto out_unlock_free;
> +
> error = -EFBIG;
> if (max >= sma->sem_nsems)
> goto out_unlock_free;
> @@ -1243,56 +1265,44 @@ int copy_semundo(unsigned long clone_flags, struct task_struct *tsk)
> */
> void exit_sem(struct task_struct *tsk)
> {
> - struct sem_undo_list *undo_list;
> - struct sem_undo *u, **up;
> - struct ipc_namespace *ns;
> + struct sem_undo_list *ulp;
> + struct sem_undo *un, *tmp;
>
> - undo_list = tsk->sysvsem.undo_list;
> - if (!undo_list)
> + ulp= tsk->sysvsem.undo_list;
> + if (!ulp)
> return;
> tsk->sysvsem.undo_list = NULL;
>
> - if (!atomic_dec_and_test(&undo_list->refcnt))
> + if (!atomic_dec_and_test(&ulp->refcnt))
> return;
>
> - ns = tsk->nsproxy->ipc_ns;
> - /* There's no need to hold the semundo list lock, as current
> - * is the last task exiting for this undo list.
> - */
> - for (up = &undo_list->proc_list; (u = *up); *up = u->proc_next, kfree(u)) {
> - struct sem_array *sma;
> - int nsems, i;
> - struct sem_undo *un, **unp;
> - int semid;
> -
> - semid = u->semid;
> -
> - if(semid == -1)
> - continue;
> - sma = sem_lock(ns, semid);
> + spin_lock(&ulp->lock);
> +
> + list_for_each_entry_safe(un, tmp, &ulp->list_proc, list_proc) {
> + struct sem_array *sma;
> + int i;
> +
> + if(un->semid == -1)
> + goto free;
> +
> + sma = sem_lock(tsk->nsproxy->ipc_ns, un->semid);
> if (IS_ERR(sma))
> - continue;
> + goto free;
>
> - if (u->semid == -1)
> - goto next_entry;
> + if (un->semid == -1)
> + goto unlock_free;
>
> - BUG_ON(sem_checkid(sma, u->semid));
> + BUG_ON(sem_checkid(sma, un->semid));
>
> - /* remove u from the sma->undo list */
> - for (unp = &sma->undo; (un = *unp); unp = &un->id_next) {
> - if (u == un)
> - goto found;
> - }
> - printk ("exit_sem undo list error id=%d\n", u->semid);
> - goto next_entry;
> -found:
> - *unp = un->id_next;
> - /* perform adjustments registered in u */
> - nsems = sma->sem_nsems;
> - for (i = 0; i < nsems; i++) {
> + /* remove un from sma->list_id */
> + assert_spin_locked(&sma->sem_perm.lock);

Once the patch applied, the assert comes a couple of lines after the
lock has actually been taken. Is it really needed here?

> + list_del(&un->list_id);
> +
> + /* perform adjustments registered in un */
> + for (i = 0; i < sma->sem_nsems; i++) {
> struct sem * semaphore = &sma->sem_base[i];
> - if (u->semadj[i]) {
> - semaphore->semval += u->semadj[i];
> + if (un->semadj[i]) {
> + semaphore->semval += un->semadj[i];
> /*
> * Range checks of the new semaphore value,
> * not defined by sus:
> @@ -1316,10 +1326,15 @@ found:
> sma->sem_otime = get_seconds();
> /* maybe some queued-up processes were waiting for this */
> update_queue(sma);
> -next_entry:
> +unlock_free:
> sem_unlock(sma);
> +free:
> + assert_spin_locked(&ulp->lock);
> + list_del(&un->list_proc);
> + kfree(un);
> }
> - kfree(undo_list);
> + spin_unlock(&ulp->lock);
> + kfree(ulp);
> }
>
> #ifdef CONFIG_PROC_FS


Regards,
Nadia


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