linux-kernel - Re: [PATCH] kexec jump: fix compiling warning on xchg(&kexec_lock, 0) in kernel

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Message-Id: <20080813134118.e1ab1292.akpm@linux-foundation.org>
Date:	Wed, 13 Aug 2008 13:41:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	ebiederm@...ssion.com, ying.huang@...el.com, pavel@....cz,
	nigel@...el.suspend2.net, rjw@...k.pl, vgoyal@...hat.com,
	mingo@...e.hu, linux-kernel@...r.kernel.org,
	kexec@...ts.infradead.org
Subject: Re: [PATCH] kexec jump: fix compiling warning on xchg(&kexec_lock,
 0) in kernel_kexec()

On Wed, 13 Aug 2008 13:31:24 -0700 (PDT)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, 13 Aug 2008, Andrew Morton wrote:
> > 
> > #2:
> 
> I thought you said there were things that want to sleep in the region?

be reasonable - that was over five minutes ago.

--- a/kernel/kexec.c~kexec-use-a-bitop-for-locking-rather-than-xchg
+++ a/kernel/kexec.c
@@ -12,7 +12,7 @@
 #include <linux/slab.h>
 #include <linux/fs.h>
 #include <linux/kexec.h>
-#include <linux/spinlock.h>
+#include <linux/mutex.h>
 #include <linux/list.h>
 #include <linux/highmem.h>
 #include <linux/syscalls.h>
@@ -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_MUTEX(kexec_mutex);
 
 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 (!mutex_trylock(&kexec_mutex))
 		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);
+	mutex_unlock(&kexec_mutex);
 	kimage_free(image);
 
 	return result;
@@ -1063,10 +1056,7 @@ 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
+	/* Take the kexec_mutex 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 (mutex_trylock(&kexec_mutex)) {
 		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);
+		mutex_unlock(&kexec_mutex);
 	}
 }
 
@@ -1434,7 +1422,7 @@ int kernel_kexec(void)
 {
 	int error = 0;
 
-	if (xchg(&kexec_lock, 1))
+	if (!mutex_trylock(&kexec_mutex))
 		return -EBUSY;
 	if (!kexec_image) {
 		error = -EINVAL;
@@ -1498,8 +1486,6 @@ int kernel_kexec(void)
 #endif
 
  Unlock:
-	if (!xchg(&kexec_lock, 0))
-		BUG();
-
+	mutex_unlock(&kexec_mutex);
 	return error;
 }
_

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