On Wed, 13 Aug 2008 13:13:13 -0700 (PDT)
Linus Torvalds <torvalds at linux-foundation.org> wrote:
> > Used a bitop to preserve the runtime checking in there. spin_unlock()
> > doesn't return the previous lockedness.
>
> Umm. spin_unlock does a lot more when you have lock debugging on, and
> doesn't do useless crap when it isn't.
>
> > A bitop seems a better fit to me. We never spin on that lock (it
> > always uses test_and_set), so why use a "spin"lock?
>
> ..because an atomic bitop is not the same as a lock.
>
> The memory ordering guarantees are different. Yes, they are sufficient,
> but that's because we've had to make them so to account for CRAP CODE that
> uses bit operations as if they were locks.
>
> Don't continue that. It's WRONG.
#2:
(The xchg(kexec_crash_image) stuff is still in there)
--- a/kernel/kexec.c~kexec-use-a-bitop-for-locking-rather-than-xchg
+++ a/kernel/kexec.c
@@ -924,19 +924,14 @@ static int kimage_load_segment(struct ki
*/
struct kimage *kexec_image;
struct kimage *kexec_crash_image;
-/*
- * A home grown binary mutex.
- * Nothing can wait so this mutex is safe to use
- * in interrupt context :)
- */
-static int kexec_lock;
+
+static DEFINE_SPINLOCK(kexec_lock);
asmlinkage long sys_kexec_load(unsigned long entry, unsigned long nr_segments,
struct kexec_segment __user *segments,
unsigned long flags)
{
struct kimage **dest_image, *image;
- int locked;
int result;
/* We only trust the superuser with rebooting the system. */
@@ -972,8 +967,7 @@ asmlinkage long sys_kexec_load(unsigned
*
* KISS: always take the mutex.
*/
- locked = xchg(&kexec_lock, 1);
- if (locked)
+ if (!spin_trylock(&kexec_lock))
return -EBUSY;
dest_image = &kexec_image;
@@ -1015,8 +1009,7 @@ asmlinkage long sys_kexec_load(unsigned
image = xchg(dest_image, image);
out:
- locked = xchg(&kexec_lock, 0); /* Release the mutex */
- BUG_ON(!locked);
+ spin_unlock(&kexec_lock);
kimage_free(image);
return result;
@@ -1063,9 +1056,6 @@ asmlinkage long compat_sys_kexec_load(un
void crash_kexec(struct pt_regs *regs)
{
- int locked;
-
-
/* Take the kexec_lock here to prevent sys_kexec_load
* running on one cpu from replacing the crash kernel
* we are using after a panic on a different cpu.
@@ -1074,8 +1064,7 @@ void crash_kexec(struct pt_regs *regs)
* of memory the xchg(&kexec_crash_image) would be
* sufficient. But since I reuse the memory...
*/
- locked = xchg(&kexec_lock, 1);
- if (!locked) {
+ if (spin_trylock(&kexec_lock)) {
if (kexec_crash_image) {
struct pt_regs fixed_regs;
crash_setup_regs(&fixed_regs, regs);
@@ -1083,8 +1072,7 @@ void crash_kexec(struct pt_regs *regs)
machine_crash_shutdown(&fixed_regs);
machine_kexec(kexec_crash_image);
}
- locked = xchg(&kexec_lock, 0);
- BUG_ON(!locked);
+ spin_unlock(&kexec_lock);
}
}
@@ -1434,7 +1422,7 @@ int kernel_kexec(void)
{
int error = 0;
- if (xchg(&kexec_lock, 1))
+ if (!spin_trylock(&kexec_lock))
return -EBUSY;
if (!kexec_image) {
error = -EINVAL;
@@ -1498,8 +1486,6 @@ int kernel_kexec(void)
#endif
Unlock:
- if (!xchg(&kexec_lock, 0))
- BUG();
-
+ spin_unlock(&kexec_lock);
return error;
}
_
