[PATCH] fs/compat.c: rwsem instead of BKL around ioctl32_hash_table

Currently the BKL is used to synchronize access to ioctl32_hash_table in
fs/compat.c.  It seems that an rwsem would be more appropriate, since this
would allow multiple lookups to occur in parallel (and also serve the
general good of minimizing use of the BKL).

I added lock_kernel()/unlock_kernel() around the call to t->handler when a
compatibility handler is found in compat_sys_ioctl() to preserve the
expectation that the BKL will be held during driver ioctl operations.  It
should be safe to do lock_kernel() while holding ioctl32_sem because of the
magic BKL sleep semantics.
Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/compat.c

@@ -41,6 +41,7 @@
 #include <linux/nfsd/nfsd.h>
 #include <linux/nfsd/syscall.h>
 #include <linux/personality.h>
+#include <linux/rwsem.h>
 
 #include <net/sock.h>		/* siocdevprivate_ioctl */
 
@@ -247,7 +248,8 @@ asmlinkage long compat_fstatfs64(unsigned int fd, compat_size_t sz, struct compa
 /* ioctl32 stuff, used by sparc64, parisc, s390x, ppc64, x86_64, MIPS */
 
 #define IOCTL_HASHSIZE 256
-struct ioctl_trans *ioctl32_hash_table[IOCTL_HASHSIZE];
+static struct ioctl_trans *ioctl32_hash_table[IOCTL_HASHSIZE];
+static DECLARE_RWSEM(ioctl32_sem);
 
 extern struct ioctl_trans ioctl_start[];
 extern int ioctl_table_size;
@@ -302,12 +304,12 @@ int register_ioctl32_conversion(unsigned int cmd,
 	if (!new_t)
 		return -ENOMEM;
 
-	lock_kernel(); 
+	down_write(&ioctl32_sem);
 	for (t = ioctl32_hash_table[hash]; t; t = t->next) {
 		if (t->cmd == cmd) {
 			printk(KERN_ERR "Trying to register duplicated ioctl32 "
 					"handler %x\n", cmd);
-			unlock_kernel();
+			up_write(&ioctl32_sem);
 			kfree(new_t);
 			return -EINVAL; 
 		}
@@ -317,7 +319,7 @@ int register_ioctl32_conversion(unsigned int cmd,
 	new_t->handler = handler;
 	ioctl32_insert_translation(new_t);
 
-	unlock_kernel();
+	up_write(&ioctl32_sem);
 	return 0;
 }
 EXPORT_SYMBOL(register_ioctl32_conversion);
@@ -337,11 +339,11 @@ int unregister_ioctl32_conversion(unsigned int cmd)
 	unsigned long hash = ioctl32_hash(cmd);
 	struct ioctl_trans *t, *t1;
 
-	lock_kernel(); 
+	down_write(&ioctl32_sem);
 
 	t = ioctl32_hash_table[hash];
 	if (!t) { 
-		unlock_kernel();
+		up_write(&ioctl32_sem);
 		return -EINVAL;
 	} 
 
@@ -351,7 +353,7 @@ int unregister_ioctl32_conversion(unsigned int cmd)
 			       __builtin_return_address(0), cmd);
 		} else { 
 			ioctl32_hash_table[hash] = t->next;
-			unlock_kernel();
+			up_write(&ioctl32_sem);
 			kfree(t);
 			return 0;
 		}
@@ -366,7 +368,7 @@ int unregister_ioctl32_conversion(unsigned int cmd)
 				goto out;
 			} else { 
 				t->next = t1->next;
-				unlock_kernel();
+				up_write(&ioctl32_sem);
 				kfree(t1);
 				return 0;
 			}
@@ -376,7 +378,7 @@ int unregister_ioctl32_conversion(unsigned int cmd)
 	printk(KERN_ERR "Trying to free unknown 32bit ioctl handler %x\n",
 				cmd);
 out:
-	unlock_kernel();
+	up_write(&ioctl32_sem);
 	return -EINVAL;
 }
 EXPORT_SYMBOL(unregister_ioctl32_conversion); 
@@ -397,7 +399,7 @@ asmlinkage long compat_sys_ioctl(unsigned int fd, unsigned int cmd,
 		goto out;
 	}
 
-	lock_kernel();
+	down_read(&ioctl32_sem);
 
 	t = ioctl32_hash_table[ioctl32_hash (cmd)];
 
@@ -405,14 +407,16 @@ asmlinkage long compat_sys_ioctl(unsigned int fd, unsigned int cmd,
 		t = t->next;
 	if (t) {
 		if (t->handler) { 
+			lock_kernel();
 			error = t->handler(fd, cmd, arg, filp);
 			unlock_kernel();
+			up_read(&ioctl32_sem);
 		} else {
-			unlock_kernel();
+			up_read(&ioctl32_sem);
 			error = sys_ioctl(fd, cmd, arg);
 		}
 	} else {
-		unlock_kernel();
+		up_read(&ioctl32_sem);
 		if (cmd >= SIOCDEVPRIVATE && cmd <= (SIOCDEVPRIVATE + 15)) {
 			error = siocdevprivate_ioctl(fd, cmd, arg);
 		} else {