RCU: eliminating explicit memory barriers from SysV IPC

PATCH New Paul E. McKenney 1 participant 1 message Oct 23, 2004

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Paul E. McKenney

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2004-10-23 20:54 UTC #0 lore raw

PATCH 3/3 RCU: eliminating explicit memory barriers from SysV IPC

This patch uses the rcu_assign_pointer() API to eliminate a number of explicit memory barriers from the SysV IPC code that uses RCU. It also restructures the ipc_ids structure so that the array size is stored in the same memory block as the array itself (see the new struct ipc_id_ary). This prevents the race that the earlier code was subject to, where a reader could see a mismatch between the size and the actual array. With the size stored with the array, the possibility of mismatch is eliminated -- with out the need for careful ordering and explicit memory barriers. This has been tested successfully on i386 and ppc64.

Signed-off-by <[email protected]>

---

 msg.c  |    2 -
 sem.c  |    2 -
 util.c |   80 +++++++++++++++++++++++++++++------------------------------------
 util.h |   13 +++++-----
 4 files changed, 45 insertions(+), 52 deletions(-)

ipc/msg.c +1 -1

diff -urpN -X ../dontdiff linux-2.5-rap/ipc/msg.c linux-2.5-iloi/ipc/msg.c
@@ -380,7 +380,7 @@ asmlinkage long sys_msgctl (int msqid, i
 		int success_return;
 		if (!buf)
 			return -EFAULT;
-		if(cmd == MSG_STAT && msqid >= msg_ids.size)
+		if(cmd == MSG_STAT && msqid >= msg_ids.entries->size)
 			return -EINVAL;
 
 		memset(&tbuf,0,sizeof(tbuf));

ipc/sem.c +1 -1

diff -urpN -X ../dontdiff linux-2.5-rap/ipc/sem.c linux-2.5-iloi/ipc/sem.c
@@ -523,7 +523,7 @@ static int semctl_nolock(int semid, int 
 		struct semid64_ds tbuf;
 		int id;
 
-		if(semid >= sem_ids.size)
+		if(semid >= sem_ids.entries->size)
 			return -EINVAL;
 
 		memset(&tbuf,0,sizeof(tbuf));

ipc/util.c +36 ~~-44~~

170 lines not on this page · raw message

ipc/util.h +7 -6

diff -urpN -X ../dontdiff linux-2.5-rap/ipc/util.h linux-2.5-iloi/ipc/util.h
@@ -15,18 +15,19 @@ void sem_init (void);
 void msg_init (void);
 void shm_init (void);
 
-struct ipc_ids {
+struct ipc_id_ary {
 	int size;
+	struct kern_ipc_perm *p[0];
+};
+
+struct ipc_ids {
 	int in_use;
 	int max_id;
 	unsigned short seq;
 	unsigned short seq_max;
 	struct semaphore sem;	
-	struct ipc_id* entries;
-};
-
-struct ipc_id {
-	struct kern_ipc_perm* p;
+	struct ipc_id_ary nullentry;
+	struct ipc_id_ary* entries;
 };
 
 void __init ipc_init_ids(struct ipc_ids* ids, int size);