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The inline assembly in __XCHG_CASE() uses a +Q constraint to hazard
against other accesses to the memory location being exchanged. However,
the pointer passed to the constraint is a u8 pointer, and thus the
hazard only applies to the first byte of the location.
GCC can take advantage of this, assuming that other portions of the
location are unchanged, as demonstrated with the following test case:
union u {
unsigned long l;
unsigned int i[2];
};
unsigned long update_char_hazard(union u *u)
{
unsigned int a, b;
a = u->i[1];
asm ("str %1, %0" : "+Q" (*(char *)&u->l) : "r" (0UL));
b = u->i[1];
return a ^ b;
}
unsigned long update_long_hazard(union u *u)
{
unsigned int a, b;
a = u->i[1];
asm ("str %1, %0" : "+Q" (*(long *)&u->l) : "r" (0UL));
b = u->i[1];
return a ^ b;
}
The linaro 15.08 GCC 5.1.1 toolchain compiles the above as follows when
using -O2 or above:
0000000000000000 <update_char_hazard>:
0: d2800001 mov x1, #0x0 // #0
4: f9000001 str x1, [x0]
8: d2800000 mov x0, #0x0 // #0
c: d65f03c0 ret
0000000000000010 <update_long_hazard>:
10: b9400401 ldr w1, [x0,#4]
14: d2800002 mov x2, #0x0 // #0
18: f9000002 str x2, [x0]
1c: b9400400 ldr w0, [x0,#4]
20: 4a000020 eor w0, w1, w0
24: d65f03c0 ret
This patch fixes the issue by passing an unsigned long pointer into the
+Q constraint, as we do for our cmpxchg code. This may hazard against
more than is necessary, but this is better than missing a necessary
hazard.
Fixes: 305d454aaa29 ("arm64: atomics: implement native {relaxed, acquire, release} atomics")
Cc: <stable@vger.kernel.org> # 4.4.x-
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.