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Re: [PATCH] MIPS: unsigned result is always greater than 0

To: "David VomLehn (dvomlehn)" <dvomlehn@cisco.com>
Subject: Re: [PATCH] MIPS: unsigned result is always greater than 0
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 12 Jan 2009 01:26:54 +0000
Cc: Roel Kluin <roel.kluin@gmail.com>, linux-mips@linux-mips.org
In-reply-to: <FF038EB85946AA46B18DFEE6E6F8A2896606BB@xmb-rtp-218.amer.cisco.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <495E2E47.6080605@gmail.com> <FF038EB85946AA46B18DFEE6E6F8A2896606BB@xmb-rtp-218.amer.cisco.com>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.18 (2008-05-17)
On Tue, Jan 06, 2009 at 04:46:19PM -0500, David VomLehn (dvomlehn) wrote:

> I agree that the code as it exists is wrong, but, as I see it, the
> problem is that the type of result should be changed from unsigned long
> to int. This fixes the comparison so it works correctly. In addition,
> such a change means that result would be the same type as the counter
> element of atomic_t, avoiding possible surprises should longs be larger
> than ints.

Originally that code was intended only for R2000-class processors.  But
LL/SC don't work correctly on XKPHYS addresses on some R5000-class
processors, so we avoid these instructions on the affected processors
running 64-bit kernels and use the fallback method of disabling interrupts
anyway, so long indeed might be longer than int.  What in the end saves
us is that atomic_sub_if_positive's only caller atomic_dec_if_positive
is only being used by the RM9000 watchdog driver and the RM9000 doesn't
suffer from the mentioned CPU bug.

Still no reason not to fix it.  I changed the intermediate variables used in
all functions that operate on atomic_t to int rsp. long for the atomic64_*
functions for consistency - which as a nice side effect also shaves of
almost 2kB from the kernel due to the elemination of gcc-generated
non-sense sign extension code.

  Ralf

MIPS: atomic_*(): Change type of intermediate variables.

This shaves of 1912 bytes of an IP27 defconfig kernel and avoids
unexpected overflow behaviour in atomic_sub_if_positive.  Apply the same
changes to the atomic64_* functions for consistency.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/include/asm/atomic.h b/arch/mips/include/asm/atomic.h
index c996c3b..1b332e1 100644
--- a/arch/mips/include/asm/atomic.h
+++ b/arch/mips/include/asm/atomic.h
@@ -50,7 +50,7 @@
 static __inline__ void atomic_add(int i, atomic_t * v)
 {
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -62,7 +62,7 @@ static __inline__ void atomic_add(int i, atomic_t * v)
                : "=&r" (temp), "=m" (v->counter)
                : "Ir" (i), "m" (v->counter));
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -95,7 +95,7 @@ static __inline__ void atomic_add(int i, atomic_t * v)
 static __inline__ void atomic_sub(int i, atomic_t * v)
 {
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -107,7 +107,7 @@ static __inline__ void atomic_sub(int i, atomic_t * v)
                : "=&r" (temp), "=m" (v->counter)
                : "Ir" (i), "m" (v->counter));
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -135,12 +135,12 @@ static __inline__ void atomic_sub(int i, atomic_t * v)
  */
 static __inline__ int atomic_add_return(int i, atomic_t * v)
 {
-       unsigned long result;
+       int result;
 
        smp_llsc_mb();
 
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -154,7 +154,7 @@ static __inline__ int atomic_add_return(int i, atomic_t * v)
                : "Ir" (i), "m" (v->counter)
                : "memory");
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -187,12 +187,12 @@ static __inline__ int atomic_add_return(int i, atomic_t * 
v)
 
 static __inline__ int atomic_sub_return(int i, atomic_t * v)
 {
-       unsigned long result;
+       int result;
 
        smp_llsc_mb();
 
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -206,7 +206,7 @@ static __inline__ int atomic_sub_return(int i, atomic_t * v)
                : "Ir" (i), "m" (v->counter)
                : "memory");
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -247,12 +247,12 @@ static __inline__ int atomic_sub_return(int i, atomic_t * 
v)
  */
 static __inline__ int atomic_sub_if_positive(int i, atomic_t * v)
 {
-       unsigned long result;
+       int result;
 
        smp_llsc_mb();
 
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -270,7 +270,7 @@ static __inline__ int atomic_sub_if_positive(int i, 
atomic_t * v)
                : "Ir" (i), "m" (v->counter)
                : "memory");
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               int temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -429,7 +429,7 @@ static __inline__ int atomic_add_unless(atomic_t *v, int a, 
int u)
 static __inline__ void atomic64_add(long i, atomic64_t * v)
 {
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -441,7 +441,7 @@ static __inline__ void atomic64_add(long i, atomic64_t * v)
                : "=&r" (temp), "=m" (v->counter)
                : "Ir" (i), "m" (v->counter));
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -474,7 +474,7 @@ static __inline__ void atomic64_add(long i, atomic64_t * v)
 static __inline__ void atomic64_sub(long i, atomic64_t * v)
 {
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -486,7 +486,7 @@ static __inline__ void atomic64_sub(long i, atomic64_t * v)
                : "=&r" (temp), "=m" (v->counter)
                : "Ir" (i), "m" (v->counter));
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -514,12 +514,12 @@ static __inline__ void atomic64_sub(long i, atomic64_t * 
v)
  */
 static __inline__ long atomic64_add_return(long i, atomic64_t * v)
 {
-       unsigned long result;
+       long result;
 
        smp_llsc_mb();
 
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -533,7 +533,7 @@ static __inline__ long atomic64_add_return(long i, 
atomic64_t * v)
                : "Ir" (i), "m" (v->counter)
                : "memory");
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -566,12 +566,12 @@ static __inline__ long atomic64_add_return(long i, 
atomic64_t * v)
 
 static __inline__ long atomic64_sub_return(long i, atomic64_t * v)
 {
-       unsigned long result;
+       long result;
 
        smp_llsc_mb();
 
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -585,7 +585,7 @@ static __inline__ long atomic64_sub_return(long i, 
atomic64_t * v)
                : "Ir" (i), "m" (v->counter)
                : "memory");
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -626,12 +626,12 @@ static __inline__ long atomic64_sub_return(long i, 
atomic64_t * v)
  */
 static __inline__ long atomic64_sub_if_positive(long i, atomic64_t * v)
 {
-       unsigned long result;
+       long result;
 
        smp_llsc_mb();
 
        if (cpu_has_llsc && R10000_LLSC_WAR) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"
@@ -649,7 +649,7 @@ static __inline__ long atomic64_sub_if_positive(long i, 
atomic64_t * v)
                : "Ir" (i), "m" (v->counter)
                : "memory");
        } else if (cpu_has_llsc) {
-               unsigned long temp;
+               long temp;
 
                __asm__ __volatile__(
                "       .set    mips3                                   \n"

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