From 0a2b9d4c79671b05956806ede5d054e03ae56280 Mon Sep 17 00:00:00 2001 From: Manfred Spraul Date: Wed, 26 May 2010 14:43:41 -0700 Subject: [PATCH] ipc/sem.c: move wake_up_process out of the spinlock section The wake-up part of semtimedop() consists out of two steps: - the right tasks must be identified. - they must be woken up. Right now, both steps run while the array spinlock is held. This patch reorders the code and moves the actual wake_up_process() behind the point where the spinlock is dropped. The code also moves setting sem->sem_otime to one place: It does not make sense to set the last modify time multiple times. [akpm@linux-foundation.org: repair kerneldoc] [akpm@linux-foundation.org: fix uninitialised retval] Signed-off-by: Manfred Spraul Cc: Chris Mason Cc: Zach Brown Cc: Jens Axboe Cc: Nick Piggin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- ipc/sem.c | 123 ++++++++++++++++++++++++++++++++++++++++++++++---------------- 1 file changed, 91 insertions(+), 32 deletions(-) diff --git a/ipc/sem.c b/ipc/sem.c index 81a9c74..a744eb5 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -381,7 +381,6 @@ static int try_atomic_semop (struct sem_array * sma, struct sembuf * sops, sop--; } - sma->sem_otime = get_seconds(); return 0; out_of_range: @@ -404,25 +403,51 @@ undo: return result; } -/* - * Wake up a process waiting on the sem queue with a given error. - * The queue is invalid (may not be accessed) after the function returns. +/** wake_up_sem_queue_prepare(q, error): Prepare wake-up + * @q: queue entry that must be signaled + * @error: Error value for the signal + * + * Prepare the wake-up of the queue entry q. */ -static void wake_up_sem_queue(struct sem_queue *q, int error) +static void wake_up_sem_queue_prepare(struct list_head *pt, + struct sem_queue *q, int error) { - /* - * Hold preempt off so that we don't get preempted and have the - * wakee busy-wait until we're scheduled back on. We're holding - * locks here so it may not strictly be needed, however if the - * locks become preemptible then this prevents such a problem. - */ - preempt_disable(); + if (list_empty(pt)) { + /* + * Hold preempt off so that we don't get preempted and have the + * wakee busy-wait until we're scheduled back on. + */ + preempt_disable(); + } q->status = IN_WAKEUP; - wake_up_process(q->sleeper); - /* hands-off: q can disappear immediately after writing q->status. */ - smp_wmb(); - q->status = error; - preempt_enable(); + q->pid = error; + + list_add_tail(&q->simple_list, pt); +} + +/** + * wake_up_sem_queue_do(pt) - do the actual wake-up + * @pt: list of tasks to be woken up + * + * Do the actual wake-up. + * The function is called without any locks held, thus the semaphore array + * could be destroyed already and the tasks can disappear as soon as the + * status is set to the actual return code. + */ +static void wake_up_sem_queue_do(struct list_head *pt) +{ + struct sem_queue *q, *t; + int did_something; + + did_something = !list_empty(pt); + list_for_each_entry_safe(q, t, pt, simple_list) { + wake_up_process(q->sleeper); + /* q can disappear immediately after writing q->status. */ + smp_wmb(); + q->status = q->pid; + } + if (did_something) + preempt_enable(); } static void unlink_queue(struct sem_array *sma, struct sem_queue *q) @@ -502,17 +527,22 @@ static int check_restart(struct sem_array *sma, struct sem_queue *q) * update_queue(sma, semnum): Look for tasks that can be completed. * @sma: semaphore array. * @semnum: semaphore that was modified. + * @pt: list head for the tasks that must be woken up. * * update_queue must be called after a semaphore in a semaphore array * was modified. If multiple semaphore were modified, then @semnum * must be set to -1. + * The tasks that must be woken up are added to @pt. The return code + * is stored in q->pid. + * The function return 1 if at least one semop was completed successfully. */ -static void update_queue(struct sem_array *sma, int semnum) +static int update_queue(struct sem_array *sma, int semnum, struct list_head *pt) { struct sem_queue *q; struct list_head *walk; struct list_head *pending_list; int offset; + int semop_completed = 0; /* if there are complex operations around, then knowing the semaphore * that was modified doesn't help us. Assume that multiple semaphores @@ -557,40 +587,55 @@ again: unlink_queue(sma, q); - if (error) + if (error) { restart = 0; - else + } else { + semop_completed = 1; restart = check_restart(sma, q); + } - wake_up_sem_queue(q, error); + wake_up_sem_queue_prepare(pt, q, error); if (restart) goto again; } + return semop_completed; } -/** do_smart_update(sma, sops, nsops): Optimized update_queue +/** + * do_smart_update(sma, sops, nsops, otime, pt) - optimized update_queue * @sma: semaphore array * @sops: operations that were performed * @nsops: number of operations + * @otime: force setting otime + * @pt: list head of the tasks that must be woken up. * * do_smart_update() does the required called to update_queue, based on the * actual changes that were performed on the semaphore array. + * Note that the function does not do the actual wake-up: the caller is + * responsible for calling wake_up_sem_queue_do(@pt). + * It is safe to perform this call after dropping all locks. */ -static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsops) +static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsops, + int otime, struct list_head *pt) { int i; if (sma->complex_count || sops == NULL) { - update_queue(sma, -1); - return; + if (update_queue(sma, -1, pt)) + otime = 1; + goto done; } for (i = 0; i < nsops; i++) { if (sops[i].sem_op > 0 || (sops[i].sem_op < 0 && sma->sem_base[sops[i].sem_num].semval == 0)) - update_queue(sma, sops[i].sem_num); + if (update_queue(sma, sops[i].sem_num, pt)) + otime = 1; } +done: + if (otime) + sma->sem_otime = get_seconds(); } @@ -656,6 +701,7 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp) struct sem_undo *un, *tu; struct sem_queue *q, *tq; struct sem_array *sma = container_of(ipcp, struct sem_array, sem_perm); + struct list_head tasks; /* Free the existing undo structures for this semaphore set. */ assert_spin_locked(&sma->sem_perm.lock); @@ -669,15 +715,17 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp) } /* Wake up all pending processes and let them fail with EIDRM. */ + INIT_LIST_HEAD(&tasks); list_for_each_entry_safe(q, tq, &sma->sem_pending, list) { unlink_queue(sma, q); - wake_up_sem_queue(q, -EIDRM); + wake_up_sem_queue_prepare(&tasks, q, -EIDRM); } /* Remove the semaphore set from the IDR */ sem_rmid(ns, sma); sem_unlock(sma); + wake_up_sem_queue_do(&tasks); ns->used_sems -= sma->sem_nsems; security_sem_free(sma); ipc_rcu_putref(sma); @@ -799,11 +847,13 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum, ushort fast_sem_io[SEMMSL_FAST]; ushort* sem_io = fast_sem_io; int nsems; + struct list_head tasks; sma = sem_lock_check(ns, semid); if (IS_ERR(sma)) return PTR_ERR(sma); + INIT_LIST_HEAD(&tasks); nsems = sma->sem_nsems; err = -EACCES; @@ -891,7 +941,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum, } sma->sem_ctime = get_seconds(); /* maybe some queued-up processes were waiting for this */ - update_queue(sma, -1); + do_smart_update(sma, NULL, 0, 0, &tasks); err = 0; goto out_unlock; } @@ -933,13 +983,15 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum, curr->sempid = task_tgid_vnr(current); sma->sem_ctime = get_seconds(); /* maybe some queued-up processes were waiting for this */ - update_queue(sma, semnum); + do_smart_update(sma, NULL, 0, 0, &tasks); err = 0; goto out_unlock; } } out_unlock: sem_unlock(sma); + wake_up_sem_queue_do(&tasks); + out_free: if(sem_io != fast_sem_io) ipc_free(sem_io, sizeof(ushort)*nsems); @@ -1213,6 +1265,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, struct sem_queue queue; unsigned long jiffies_left = 0; struct ipc_namespace *ns; + struct list_head tasks; ns = current->nsproxy->ipc_ns; @@ -1261,6 +1314,8 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, } else un = NULL; + INIT_LIST_HEAD(&tasks); + sma = sem_lock_check(ns, semid); if (IS_ERR(sma)) { if (un) @@ -1309,7 +1364,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, error = try_atomic_semop (sma, sops, nsops, un, task_tgid_vnr(current)); if (error <= 0) { if (alter && error == 0) - do_smart_update(sma, sops, nsops); + do_smart_update(sma, sops, nsops, 1, &tasks); goto out_unlock_free; } @@ -1386,6 +1441,8 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, out_unlock_free: sem_unlock(sma); + + wake_up_sem_queue_do(&tasks); out_free: if(sops != fast_sops) kfree(sops); @@ -1446,6 +1503,7 @@ void exit_sem(struct task_struct *tsk) for (;;) { struct sem_array *sma; struct sem_undo *un; + struct list_head tasks; int semid; int i; @@ -1509,10 +1567,11 @@ void exit_sem(struct task_struct *tsk) semaphore->sempid = task_tgid_vnr(current); } } - sma->sem_otime = get_seconds(); /* maybe some queued-up processes were waiting for this */ - update_queue(sma, -1); + INIT_LIST_HEAD(&tasks); + do_smart_update(sma, NULL, 0, 1, &tasks); sem_unlock(sma); + wake_up_sem_queue_do(&tasks); call_rcu(&un->rcu, free_un); } -- 1.8.2.3