From: Chris Mason <chris.mason@oracle.com>
Date: Tue, 5 Jul 2011 19:07:27 +0000 (-0500)
Subject: IPC reduce lock contention in semctl
X-Git-Tag: v2.6.39-400.9.0~942
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=a8fc9c3f989c474f44e6d4b4f126961207261a1e;p=users%2Fjedix%2Flinux-maple.git

IPC reduce lock contention in semctl

Signed-off-by: Chris Mason <chris.mason@oracle.com>
---

diff --git a/ipc/sem.c b/ipc/sem.c
index 3cf16b408f6d..1dada2a9de72 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -196,6 +196,12 @@ static inline void sem_lock_and_putref(struct sem_array *sma)
 	ipc_rcu_putref(sma);
 }
 
+static inline void sem_read_lock_and_putref(struct sem_array *sma)
+{
+	ipc_read_lock_by_ptr(&sma->sem_perm);
+	ipc_rcu_putref(sma);
+}
+
 static inline void sem_getref_and_unlock(struct sem_array *sma)
 {
 	ipc_rcu_getref(sma);
@@ -1017,8 +1023,9 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 	ushort* sem_io = fast_sem_io;
 	int nsems;
 	struct list_head tasks;
+	int write_locked = 0;
 
-	sma = sem_lock_check(ns, semid);
+	sma = sem_read_lock_check(ns, semid);
 	if (IS_ERR(sma))
 		return PTR_ERR(sma);
 
@@ -1042,7 +1049,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 		int i;
 
 		if(nsems > SEMMSL_FAST) {
-			sem_getref_and_unlock(sma);
+			sem_getref_and_read_unlock(sma);
 
 			sem_io = ipc_alloc(sizeof(ushort)*nsems);
 			if(sem_io == NULL) {
@@ -1050,17 +1057,16 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 				return -ENOMEM;
 			}
 
-			sem_lock_and_putref(sma);
+			sem_read_lock_and_putref(sma);
 			if (sma->sem_perm.deleted) {
-				sem_unlock(sma);
 				err = -EIDRM;
-				goto out_free;
+				goto out_unlock;
 			}
 		}
 
 		for (i = 0; i < sma->sem_nsems; i++)
 			sem_io[i] = sma->sem_base[i].semval;
-		sem_unlock(sma);
+		sem_read_unlock(sma);
 		err = 0;
 		if(copy_to_user(array, sem_io, nsems*sizeof(ushort)))
 			err = -EFAULT;
@@ -1072,7 +1078,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 		struct sem_undo *un;
 		LIST_HEAD(pending);
 
-		sem_getref_and_unlock(sma);
+		sem_getref_and_read_unlock(sma);
 
 		if(nsems > SEMMSL_FAST) {
 			sem_io = ipc_alloc(sizeof(ushort)*nsems);
@@ -1095,11 +1101,19 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 				goto out_free;
 			}
 		}
-		sem_lock_and_putref(sma);
+		sem_read_lock_and_putref(sma);
+
+		/* no new undos can come in while we have the read lock
+		 * but we have to take the write lock if there are any
+		 */
+		if (!list_empty(&sma->list_id)) {
+			sem_getref_and_read_unlock(sma);
+			sem_lock_and_putref(sma);
+			write_locked = 1;
+		}
 		if (sma->sem_perm.deleted) {
-			sem_unlock(sma);
 			err = -EIDRM;
-			goto out_free;
+			goto out_unlock;
 		}
 
 		for (i = 0; i < nsems; i++) {
@@ -1154,7 +1168,16 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 		if (val > SEMVMX || val < 0)
 			goto out_unlock;
 
-		//assert_spin_locked(&sma->sem_perm.lock);
+		if (!list_empty(&sma->list_id)) {
+			sem_getref_and_read_unlock(sma);
+			sem_lock_and_putref(sma);
+			write_locked = 1;
+		}
+		if (sma->sem_perm.deleted) {
+			err = -EIDRM;
+			goto out_unlock;
+		}
+
 		list_for_each_entry(un, &sma->list_id, list_id)
 			un->semadj[semnum] = 0;
 
@@ -1173,7 +1196,10 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 	}
 	}
 out_unlock:
-	sem_unlock(sma);
+	if (write_locked)
+		sem_unlock(sma);
+	else
+		sem_read_unlock(sma);
 	wake_up_sem_queue_do(&tasks);
 
 out_free:
diff --git a/ipc/util.h b/ipc/util.h
index 25e9808bac68..3fe3e6a01ac0 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -164,6 +164,12 @@ static inline void ipc_lock_by_ptr(struct kern_ipc_perm *perm)
 	write_lock(&perm->lock);
 }
 
+static inline void ipc_read_lock_by_ptr(struct kern_ipc_perm *perm)
+{
+	rcu_read_lock();
+	read_lock(&perm->lock);
+}
+
 static inline void ipc_unlock(struct kern_ipc_perm *perm)
 {
 	write_unlock(&perm->lock);