linux-kernel - Re: How is Code in do_sys_settimeofday() safe in case of SMP and Nest Kernel Path?

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Message-ID: <a2ebde260609300909l5f33c152xa331f7600be67f6b@mail.gmail.com>
Date:	Sun, 1 Oct 2006 00:09:24 +0800
From:	"Dong Feng" <middle.fengdong@...il.com>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	"Christoph Lameter" <clameter@....com>, "Andi Kleen" <ak@...e.de>,
	"Arjan van de Ven" <arjan@...radead.org>,
	"Paul Mackerras" <paulus@...ba.org>,
	"David Howells" <dhowells@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: How is Code in do_sys_settimeofday() safe in case of SMP and Nest Kernel Path?

On Sat, 30 Sep 2006, Nick Piggin wrote
>
> You should write a patch and send it to Mister Morton.
>
> --
> SUSE Labs, Novell Inc.
> Send instant messages to your online friends http://au.messenger.yahoo.com
>
>

This is a patch for your review.

--- kernel/time.c.orig	2006-09-30 23:21:29.000000000 +0800
+++ kernel/time.c	2006-09-30 23:38:18.000000000 +0800
@@ -107,7 +107,16 @@ asmlinkage long sys_gettimeofday(struct
 			return -EFAULT;
 	}
 	if (unlikely(tz != NULL)) {
-		if (copy_to_user(tz, &sys_tz, sizeof(sys_tz)))
+		struct timezone ktz;
+		unsigned long seq;
+
+		do {
+                	seq = read_seqbegin(&xtime_lock);
+			ktz.tz_minuteswest = sys_tz.tz_minuteswest;
+			ktz.tz_dsttime = sys_tz.tz_dsttime;
+        	} while (unlikely(read_seqretry(&xtime_lock, seq)));
+
+		if (copy_to_user(tz, &ktz, sizeof(ktz)))
 			return -EFAULT;
 	}
 	return 0;
@@ -164,12 +173,16 @@ int do_sys_settimeofday(struct timespec

 	if (tz) {
 		/* SMP safe, global irq locking makes it work. */
-		sys_tz = *tz;
-		if (firsttime) {
-			firsttime = 0;
-			if (!tv)
-				warp_clock();
+		write_seqlock_irq(&xtime_lock);
+		{
+			sys_tz = *tz;
+			if (firsttime) {
+				firsttime = 0;
+				if (!tv)
+					warp_clock();
+			}
 		}
+		write_sequnlock_irq(&xtime_lock);
 	}
 	if (tv)
 	{
-
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