[uml-devel] broken mdelay

[uml-devel] broken mdelay

[Julian Bradfield]

A few years ago, I reported a problem with mdelay (that it basically
didn't work) - see

http://marc.theaimsgroup.com/?l=user-mode-linux-devel&m=103774842828993&w=2

and preceding messages.

I see the problem is still there in 2.6.18 (I find it because I insert an
mdelay(1000) into one of the ubd routines to simulate the effect of
busy-waiting on i/o in one of my practical exercises).

Here's the patch I currently use to fix it. I can't remember now how I
produced this patch four years ago, but it should make sense in
conjunction with the above referenced message.

--- arch/um/sys-i386/delay.c.dist	2006-10-26 14:50:38.069023000 +0100
+++ arch/um/sys-i386/delay.c	2006-10-26 14:50:42.527846000 +0100
@@ -3,7 +3,7 @@
 #include <linux/delay.h>
 #include <asm/param.h>
 
-void __delay(unsigned long time)
+static void __loop_delay(unsigned long loops)
 {
 	/* Stolen from the i386 __loop_delay */
 	int d0;
@@ -14,27 +14,28 @@
 		".align 16\n"
 		"2:\tdecl %0\n\tjns 2b"
 		:"=&a" (d0)
-		:"0" (time));
+		:"0" (loops));
 }
 
-void __udelay(unsigned long usecs)
+void __delay(unsigned long loops)
 {
-	int i, n;
-
-	n = (loops_per_jiffy * HZ * usecs) / MILLION;
-        for(i=0;i<n;i++)
-                cpu_relax();
+	__loop_delay(loops);
 }
 
-EXPORT_SYMBOL(__udelay);
-
-void __const_udelay(unsigned long usecs)
+inline void __const_udelay(unsigned long xloops)
 {
-	int i, n;
+	int d0;
+	__asm__("mull %0"
+		:"=d" (xloops), "=&a" (d0)
+		:"1" (xloops),"0" (loops_per_jiffy));
+        __delay(xloops * HZ);
+}
 
-	n = (loops_per_jiffy * HZ * usecs) / MILLION;
-        for(i=0;i<n;i++)
-                cpu_relax();
+void __udelay(unsigned long usecs)
+{
+	__const_udelay(usecs * 0x000010c6);  /* 2**32 / 1000000 */
 }
 
+EXPORT_SYMBOL(__udelay);
+
 EXPORT_SYMBOL(__const_udelay);

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Re: [uml-devel] broken mdelay

[Blaisorblade]

[-- Attachment #1: Type: text/plain, Size: 2614 bytes --]

On Thursday 26 October 2006 16:12, Julian Bradfield wrote:
> A few years ago, I reported a problem with mdelay (that it basically
> didn't work) - see
>
> http://marc.theaimsgroup.com/?l=user-mode-linux-devel&m=103774842828993&w=2
>
> and preceding messages.
>
> I see the problem is still there in 2.6.18 (I find it because I insert an
> mdelay(1000) into one of the ubd routines to simulate the effect of
> busy-waiting on i/o in one of my practical exercises).
>
> Here's the patch I currently use to fix it. I can't remember now how I
> produced this patch four years ago, but it should make sense in
> conjunction with the above referenced message.

I've not yet looked at that messages, however:

Direct invocations of __const_udelay (see include/asm-i386/delay.h) are still 
broken with the below patch IMHO, since you do not multiply the value passed 
there by that magic constant - look also at arch/i386/lib/delay.c. Please try 
if the attached patch works for you. Also, a number of other users complained 
about the same bug (hangs on shutdown result for device-mapper users in some 
particular conditions, for instance).

> --- arch/um/sys-i386/delay.c.dist	2006-10-26 14:50:38.069023000 +0100
> +++ arch/um/sys-i386/delay.c	2006-10-26 14:50:42.527846000 +0100
> @@ -3,7 +3,7 @@
>  #include <linux/delay.h>
>  #include <asm/param.h>
>
> -void __delay(unsigned long time)
> +static void __loop_delay(unsigned long loops)
>  {
>  	/* Stolen from the i386 __loop_delay */
>  	int d0;
> @@ -14,27 +14,28 @@
>  		".align 16\n"
>  		"2:\tdecl %0\n\tjns 2b"
>
>  		:"=&a" (d0)
>
> -		:"0" (time));
> +		:"0" (loops));
>  }
>
> -void __udelay(unsigned long usecs)
> +void __delay(unsigned long loops)
>  {
> -	int i, n;
> -
> -	n = (loops_per_jiffy * HZ * usecs) / MILLION;
> -        for(i=0;i<n;i++)
> -                cpu_relax();
> +	__loop_delay(loops);
>  }
>
> -EXPORT_SYMBOL(__udelay);
> -
> -void __const_udelay(unsigned long usecs)
> +inline void __const_udelay(unsigned long xloops)
>  {
> -	int i, n;
> +	int d0;
> +	__asm__("mull %0"
> +		:"=d" (xloops), "=&a" (d0)
> +		:"1" (xloops),"0" (loops_per_jiffy));
> +        __delay(xloops * HZ);
> +}
>
> -	n = (loops_per_jiffy * HZ * usecs) / MILLION;
> -        for(i=0;i<n;i++)
> -                cpu_relax();
> +void __udelay(unsigned long usecs)
> +{
> +	__const_udelay(usecs * 0x000010c6);  /* 2**32 / 1000000 */
>  }
>
> +EXPORT_SYMBOL(__udelay);
> +
>  EXPORT_SYMBOL(__const_udelay);

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade

[-- Attachment #2: fix-udelay.diff --]
[-- Type: text/x-diff, Size: 1784 bytes --]

Index: linux-2.6.git/arch/um/sys-i386/delay.c
===================================================================
--- linux-2.6.git.orig/arch/um/sys-i386/delay.c
+++ linux-2.6.git/arch/um/sys-i386/delay.c
@@ -27,14 +27,3 @@ void __udelay(unsigned long usecs)
 }
 
 EXPORT_SYMBOL(__udelay);
-
-void __const_udelay(unsigned long usecs)
-{
-	int i, n;
-
-	n = (loops_per_jiffy * HZ * usecs) / MILLION;
-        for(i=0;i<n;i++)
-                cpu_relax();
-}
-
-EXPORT_SYMBOL(__const_udelay);
Index: linux-2.6.git/arch/um/sys-x86_64/delay.c
===================================================================
--- linux-2.6.git.orig/arch/um/sys-x86_64/delay.c
+++ linux-2.6.git/arch/um/sys-x86_64/delay.c
@@ -28,14 +28,3 @@ void __udelay(unsigned long usecs)
 }
 
 EXPORT_SYMBOL(__udelay);
-
-void __const_udelay(unsigned long usecs)
-{
-	unsigned long i, n;
-
-	n = (loops_per_jiffy * HZ * usecs) / MILLION;
-        for(i=0;i<n;i++)
-                cpu_relax();
-}
-
-EXPORT_SYMBOL(__const_udelay);
Index: linux-2.6.git/include/asm-um/delay.h
===================================================================
--- linux-2.6.git.orig/include/asm-um/delay.h
+++ linux-2.6.git/include/asm-um/delay.h
@@ -1,9 +1,20 @@
 #ifndef __UM_DELAY_H
 #define __UM_DELAY_H
 
-#include "asm/arch/delay.h"
-#include "asm/archparam.h"
-
 #define MILLION 1000000
 
+/* Undefined on purpose */
+extern void __bad_udelay(void);
+extern void __bad_ndelay(void);
+
+extern void __udelay(unsigned long usecs);
+extern void __const_udelay(unsigned long usecs);
+extern void __delay(unsigned long loops);
+
+#define udelay(n) ((__builtin_constant_p(n) && (n) > 20000) ? \
+	__bad_udelay() : __udelay(n))
+
+/* It appears that ndelay is not used at all for UML. */
+#define ndelay(n) __bad_ndelay()
+
 #endif

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Re: [uml-devel] broken mdelay

[extmaillist]

Hello,

seems like Your patch fixed 64bit guests for me! 32bit ones running on 64bit guest seem to be still overflowing though.

Thanks!

NiK

> I've not yet looked at that messages, however:
> 
> Direct invocations of __const_udelay (see include/asm-i386/delay.h) are still 
> broken with the below patch IMHO, since you do not multiply the value passed 
> there by that magic constant - look also at arch/i386/lib/delay.c. Please try 
> if the attached patch works for you. Also, a number of other users complained 
> about the same bug (hangs on shutdown result for device-mapper users in some 
> particular conditions, for instance).
> 

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