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SubjectRe: [patch] SMP alternatives: skip with UP kernels.
Gerd Hoffmann wrote:

[ depending on CONFIG_SMP ]

>> Okay, thanks. Yes, I agree such would make sense.
>
> Patch below. It simply returns in case the tables are empty and nothing
> is do to, thus avoids printing the confusing message.

It does avoid the message but well, that's also _all_ it avoids. Why do
you want to do it this way? If I apply the attached, things compile and
work fine which seems to confirm that, yes, all this smp_alternatives
code is simply dead baggage on !CONFIG_SMP.

Yes, the #ifdef in arch/i386/kernel/module.c is a bit clumsy. Just proof
of concept, might be abstracted out a bit better. It does lose most of
alternatives.c which would seem to be good thing.

Unless I'm missing some points, I believe this code should not be
compiled in for !CONFIG_SMP.

One other point, although probably not a relevant one. Your changes in
module_finalize() potentially change behaviour. It used to be:

for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
void *seg;
if (strcmp(".altinstructions", secstrings + s->sh_name))
continue;
seg = (void *)s->sh_addr;
apply_alternatives(seg, seg + s->sh_size);
}

which means that any .altinstructions section would be patched. It's now:

for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
...
if (!strcmp(".altinstructions", secstrings + s->sh_name))
alt = s;
...
}
if (alt) {
/* patch .altinstructions */
void *aseg = (void *)alt->sh_addr;
apply_alternatives(aseg, aseg + alt->sh_size);
}

which means that only the last such section would. I suppose there's
only one such section anyway, but not seeing a break in the original
does make me wonder if this was originally done on purpose.

Rene.

Index: local/arch/i386/kernel/alternative.c
===================================================================
--- local.orig/arch/i386/kernel/alternative.c 2006-05-11 20:28:56.000000000 +0200
+++ local/arch/i386/kernel/alternative.c 2006-05-11 20:32:24.000000000 +0200
@@ -118,6 +118,8 @@ void apply_alternatives(struct alt_instr
}
}

+#ifdef CONFIG_SMP
+
static void alternatives_smp_save(struct alt_instr *start, struct alt_instr *end)
{
struct alt_instr *a;
@@ -283,10 +285,13 @@ void alternatives_smp_switch(int smp)
spin_unlock_irqrestore(&smp_alt, flags);
}

+#endif /* CONFIG_SMP */
+
void __init alternative_instructions(void)
{
apply_alternatives(__alt_instructions, __alt_instructions_end);

+#ifdef CONFIG_SMP
/* switch to patch-once-at-boottime-only mode and free the
* tables in case we know the number of CPUs will never ever
* change */
@@ -318,4 +323,5 @@ void __init alternative_instructions(voi
_text, _etext);
alternatives_smp_switch(0);
}
+#endif
}
Index: local/arch/i386/kernel/module.c
===================================================================
--- local.orig/arch/i386/kernel/module.c 2006-05-11 20:28:56.000000000 +0200
+++ local/arch/i386/kernel/module.c 2006-05-11 20:29:00.000000000 +0200
@@ -112,12 +112,14 @@ int module_finalize(const Elf_Ehdr *hdr,
char *secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;

for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
- if (!strcmp(".text", secstrings + s->sh_name))
- text = s;
if (!strcmp(".altinstructions", secstrings + s->sh_name))
alt = s;
+#ifdef CONFIG_SMP
if (!strcmp(".smp_locks", secstrings + s->sh_name))
locks= s;
+ if (!strcmp(".text", secstrings + s->sh_name))
+ text = s;
+#endif
}

if (alt) {
@@ -126,16 +128,20 @@ int module_finalize(const Elf_Ehdr *hdr,
apply_alternatives(aseg, aseg + alt->sh_size);
}
if (locks && text) {
+#ifdef CONFIG_SMP
void *lseg = (void *)locks->sh_addr;
void *tseg = (void *)text->sh_addr;
alternatives_smp_module_add(me, me->name,
lseg, lseg + locks->sh_size,
tseg, tseg + text->sh_size);
+#endif
}
return 0;
}

void module_arch_cleanup(struct module *mod)
{
+#ifdef CONFIG_SMP
alternatives_smp_module_del(mod);
+#endif
}
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