lkml.org 
[lkml]   [2011]   [Oct]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] x86: Make variable_test_bit reference all of *addr
Date
This casts the addr that's fed to asm into a struct-array pointer, so
gcc knows that more than just the first long is needed. Since there's
no fixed size for all callers, an arbitrary size is chosen just to
ensure that it's probably good enough.

I noticed this warning on i686, with gcc-4.6.1-9.fc15:

CC arch/x86/kernel/kprobes.o
In file included from include/linux/bitops.h:22:0,
from include/linux/kernel.h:17,
from [...]/arch/x86/include/asm/percpu.h:44,
from [...]/arch/x86/include/asm/current.h:5,
from [...]/arch/x86/include/asm/processor.h:15,
from [...]/arch/x86/include/asm/atomic.h:6,
from include/linux/atomic.h:4,
from include/linux/mutex.h:18,
from include/linux/notifier.h:13,
from include/linux/kprobes.h:34,
from arch/x86/kernel/kprobes.c:43:
[...]/arch/x86/include/asm/bitops.h: In function ‘can_boost.part.1’:
[...]/arch/x86/include/asm/bitops.h:319:2: warning: use of memory input without lvalue in asm operand 1 is deprecated [enabled by default]

In investigating the impact of this warning, I discovered that only the
first long of the 32-byte twobyte_is_boostable[] was making into the
object file.

Jakub advised that variable_test_bit is incorrectly telling gcc that its
asm only uses a single long from the addr pointer, and he suggested the
struct-array cast to broaden the memory reference.

Signed-off-by: Josh Stone <jistone@redhat.com>
Cc: Jakub Jelinek <jakub@redhat.com>

---

An alternate fix would be to make kprobes' twobyte_is_boostable[]
volatile, which forces gcc to keep it around. I feel that's treating
the symptom though, rather than the cause in variable_test_bit().

IMO this is also a good candidate for -stable, for fixing the obviously bad
data behavior, but I'll let others judge...

---
arch/x86/include/asm/bitops.h | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 1775d6e..0565371 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -319,7 +319,8 @@ static inline int variable_test_bit(int nr, volatile const unsigned long *addr)
asm volatile("bt %2,%1\n\t"
"sbb %0,%0"
: "=r" (oldbit)
- : "m" (*(unsigned long *)addr), "Ir" (nr));
+ : "m" (*(struct { unsigned long _[0x10000]; } *)addr),
+ "Ir" (nr));

return oldbit;
}
--
1.7.6.4
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2011-10-07 02:03    [W:0.768 / U:0.708 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site