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

I have booted this and run some basic 32-bit userspace on ppc64, and
also compile tested this for x86_64 and sparc64.

Thanks,
Roland

Signed-off-by: Roland Dreier <roland@topspin.com>

Index: linux-bk/fs/compat.c
===================================================================
--- linux-bk.orig/fs/compat.c 2004-09-01 00:57:03.000000000 -0700
+++ linux-bk/fs/compat.c 2004-09-01 07:21:20.000000000 -0700
@@ -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 @@
/* 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 @@
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 @@
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 @@
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 @@
__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 @@
goto out;
} else {
t->next = t1->next;
- unlock_kernel();
+ up_write(&ioctl32_sem);
kfree(t1);
return 0;
}
@@ -376,7 +378,7 @@
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 @@
goto out;
}

- lock_kernel();
+ down_read(&ioctl32_sem);

t = ioctl32_hash_table[ioctl32_hash (cmd)];

@@ -405,14 +407,16 @@
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 {
-
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