linux-kernel - Re: [GIT PULL] asm-generic fixes

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Date:	Tue, 23 Jun 2009 21:56:45 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Mike Frysinger <vapier@...too.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Magnus Damm <magnus.damm@...il.com>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [GIT PULL] asm-generic fixes

On Tuesday 23 June 2009, Linus Torvalds wrote:
> You might need to make 'result', 'carry', and 'w' be 'unsigned int' too.

Yes, you're right.

> Now, it's possible (even likely) that even with a 64-bit word, we'll never 
> actually do large enough areas that 'result' would ever have very many 
> bits set in the 32+ bit region, and since we do end up folding to 16 bits 
> twice (once after the loop and once at the end), it probably gets things 
> right in most cases. But I doubt "probably" is strong enough. Somebody 
> should check.

I think it would overrun only if we have more than 65536 u32 words of
0xffffffff in a single IP packet, on a 64 bit machine. A more obvious
reason to change it is that it relies on from32to16() actually behaving
like a from47to16() function on 64-bit. Changing it to use unsigned int
throughout makes it both more obvious and more consistent between
32 and 64 bit unsigned long types.

> Or just see arch/alpha/lib/checksum.c, which does the whole 64-bit case.
> Maybe lib/checksum.c should be lib/checksum_{32,64}.c.

Mike Frysinger earlier suggested just making the do_csum function optional
in this file because this is the one that most architectures would override.

The alpha code is the only 64-bit platform implementing do_csum in C, so
if Richard wants to use the generic code in its current form, he could simply
override the do_csum implementation.

I've now added these two patches:

commit 217a8c7b6af924379a2083439b4bb606f332e7b1
Author: Arnd Bergmann <arnd@...db.de>
Date:   Tue Jun 23 21:37:26 2009 +0200

    lib/checksum.c: make do_csum optional
    
    Mike Frysinger suggested that do_csum should be optional
    so that an architecture can use the generic checksum code
    but still provide an optimized fast-path for the most
    critical function.
    
    This can mean an implementation using inline assembly,
    or in case of Alpha one using 64-bit arithmetic in C.
    
    Cc: Mike Frysinger <vapier@...too.org>
    Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/lib/checksum.c b/lib/checksum.c
index 886b48d..b08c2d0 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -37,6 +37,7 @@
 
 #include <asm/byteorder.h>
 
+#ifndef do_csum
 static inline unsigned short from32to16(unsigned int x)
 {
 	/* add up 16-bit and 16-bit for 16+c bit */
@@ -102,6 +103,7 @@ static unsigned int do_csum(const unsigned char *buff, int len)
 out:
 	return result;
 }
+#endif
 
 /*
  *	This is a version of ip_compute_csum() optimized for IP headers,

commit 5cb59758c3e2170b24e9c0d659eb6c03872155c0
Author: Arnd Bergmann <arnd@...db.de>
Date:   Tue Jun 23 21:22:58 2009 +0200

    lib/checksum.c: use 32-bit arithmetic consistently
    
    The use of 'unsigned long' variables in the 32-bit part of do_csum()
    is confusing at best, and potentially broken for long input on 64-bit
    machines.
    
    This changes the code to use 'unsigned int' instead, which makes
    the code behave in the same (correct) way on both 32 and 64 bit
    machines.
    
    Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
    Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/lib/checksum.c b/lib/checksum.c
index b2e2fd4..886b48d 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -37,7 +37,7 @@
 
 #include <asm/byteorder.h>
 
-static inline unsigned short from32to16(unsigned long x)
+static inline unsigned short from32to16(unsigned int x)
 {
 	/* add up 16-bit and 16-bit for 16+c bit */
 	x = (x & 0xffff) + (x >> 16);
@@ -49,7 +49,7 @@ static inline unsigned short from32to16(unsigned long x)
 static unsigned int do_csum(const unsigned char *buff, int len)
 {
 	int odd, count;
-	unsigned long result = 0;
+	unsigned int result = 0;
 
 	if (len <= 0)
 		goto out;
@@ -73,9 +73,9 @@ static unsigned int do_csum(const unsigned char *buff, int len)
 		}
 		count >>= 1;		/* nr of 32-bit words.. */
 		if (count) {
-			unsigned long carry = 0;
+			unsigned int carry = 0;
 			do {
-				unsigned long w = *(unsigned int *) buff;
+				unsigned int w = *(unsigned int *) buff;
 				count--;
 				buff += 4;
 				result += carry;
--
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