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Bug in atomic_sub_if_positive

To: linux-mips@linux-mips.org
Subject: Bug in atomic_sub_if_positive
From: "Morten Larsen" <mlarsen@broadcom.com>
Date: Wed, 25 Jun 2008 23:07:24 -0700
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
Thread-index: AcjXUubV9JI/8zo6QlaMem/+HdxMfQ==
Thread-topic: Bug in atomic_sub_if_positive
As far as I can tell the branch optimization fixes in 2.6.21 introduced
a bug in atomic_sub_if_positive that causes it to return even when the
sc instruction fails. The result is that e.g. down_trylock becomes
unreliable as the semaphore counter is not always decremented.

Below is a patch that fixed the problem on my system.

Morten S. Larsen


--- a/include/asm-mips/atomic.h 2008-06-25 22:38:43.159739000 -0700
+++ b/include/asm-mips/atomic.h 2008-06-25 22:39:07.552065000 -0700
@@ -292,10 +292,10 @@ static __inline__ int atomic_sub_if_posi
                "       beqz    %0, 2f
\n"
                "        subu   %0, %1, %3
\n"
                "       .set    reorder
\n"
-               "1:
\n"
                "       .subsection 2
\n"
                "2:     b       1b
\n"
                "       .previous
\n"
+               "1:
\n"
                "       .set    mips0
\n"
                : "=&r" (result), "=&r" (temp), "=m" (v->counter)
                : "Ir" (i), "m" (v->counter)
@@ -682,10 +682,10 @@ static __inline__ long atomic64_sub_if_p
                "       beqz    %0, 2f
\n"
                "        dsubu  %0, %1, %3
\n"
                "       .set    reorder
\n"
-               "1:
\n"
                "       .subsection 2
\n"
                "2:     b       1b
\n"
                "       .previous
\n"
+               "1:
\n"
                "       .set    mips0
\n"
                : "=&r" (result), "=&r" (temp), "=m" (v->counter)
                : "Ir" (i), "m" (v->counter)


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