percpu_rw_semaphore: reimplement to not block the readers unnecessarily
Quoted 5 lines · Grand poo-bah Linus
Now, I doubt you'll find an architecture or C compiler where this will actually ever make a difference, but the fact remains that you shouldn't use signed integers for counters like this. You should use unsigned, and you should rely on the well-defined modulo-2**n semantics.
Actually, this is another C standard undefined case that recent versions of GCC exploit for optimization.
When using signed integers, GCC's optimizer assumes that, if b > 0, then a + b > a.
For example, the loop:
for (i = 1; i; i++) /* Code */
will never terminate! Feed the following to gcc -O2 and see for yourself:
extern void foo(int x);
void bar(void)
{
int i;
for (i = 0; i >= 0; i++)
foo(i);
}here's what I get:
.file "test.c"
.text
.p2align 4,,15
.globl bar
.type bar, @function
bar:
.LFB0:
.cfi_startproc
pushl %ebx
.cfi_def_cfa_offset 8
.cfi_offset 3, -8
movl $1, %ebx
subl $24, %esp
.cfi_def_cfa_offset 32
.p2align 4,,7
.p2align 3
.L2:
movl %ebx, (%esp)
addl $1, %ebx
call foo
jmp .L2 .cfi_endproc
.LFE0:
.size bar, .-bar
.ident "GCC: (Debian 4.7.2-4) 4.7.2"
.section .note.GNU-stack,"",@progbits
Notice the lack of test in the "jmp .L2" loop.
It can even handle more complicated cases like:
void bar(int j)
{
int i = 0;
do {
foo(i);
if (j >= 0)
i += j;
else
i -= j;
} while (i >= 0);
}... which gcc -O3 neatly splits into two infinite loops, as if I had written:
void bar(int j)
{
int i = 0;
if (j >= 0)
for (; ; i += j)
foo(i);
else
for (; ; i -= j)
foo(i);
}Quoted 10 lines · George Spelvin
Quoted 5 lines · Grand poo-bah Linus
Now, I doubt you'll find an architecture or C compiler where this will actually ever make a difference, but the fact remains that you shouldn't use signed integers for counters like this. You should use unsigned, and you should rely on the well-defined modulo-2**n semantics.
Actually, this is another C standard undefined case that recent versions of GCC exploit for optimization.
^^^^^^^^^^^^
This is another thing,
Quoted 4 lines
For example, the loop:
for (i = 1; i; i++) /* Code */will never terminate! Feed the following to gcc -O2 and see for yourself:
Yes, because ...
Quoted 1 line
Notice the lack of test in the "jmp .L2" loop.
Exactly.
But if we have like
int A, B;
int sum(void)
{
return A + B;
}then I doubt there is any architecture (at least supported by linux) which can generate the different code if you do s/int/unsigned/.
Anyway I agree, unsigned makes more sense, and I changed this patch accordingly..
Oleg.