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SubjectRe: [PATCH 6/8] livepatch: Remove Nop structures when unused
On Fri, 23 Mar 2018, Petr Mladek wrote:

> Replaced patches are removed from the stack when the transition is
> finished. It means that Nop structures will never be needed again
> and can be removed. Why should we care?
>
> + Nop structures make false feeling that the function is patched
> even though the ftrace handler has no effect.
>
> + Ftrace handlers are not completely for free. They cause slowdown that
> might be visible in some workloads. The ftrace-related slowdown might
> actually be the reason why the function is not longer patched in
> the new cumulative patch. One would expect that cumulative patch
> would allow to solve these problems as well.
>
> + Cumulative patches are supposed to replace any earlier version of
> the patch. The amount of NOPs depends on which version was replaced.
> This multiplies the amount of scenarios that might happen.
>
> One might say that NOPs are innocent. But there are even optimized
> NOP instructions for different processor, for example, see
> arch/x86/kernel/alternative.c. And klp_ftrace_handler() is much
> more complicated.
>
> + It sounds natural to clean up a mess that is not longer needed.
> It could only be worse if we do not do it.
>
> This patch allows to unpatch and free the dynamic structures independently
> when the transition finishes.
>
> The free part is a bit tricky because kobject free callbacks are called
> asynchronously. We could not wait for them easily. Fortunately, we do
> not have to. Any further access can be avoided by removing them from
> the dynamic lists.
>
> Finally, the patch become the first on the stack when enabled. The replace
> functionality will not longer be needed. Let's clear patch->replace to
> avoid the special handling when it is eventually disabled/enabled again.
>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
> include/linux/livepatch.h | 6 ++++++
> kernel/livepatch/core.c | 42 +++++++++++++++++++++++++++++++++++-------
> kernel/livepatch/core.h | 1 +
> kernel/livepatch/patch.c | 31 ++++++++++++++++++++++++++-----
> kernel/livepatch/patch.h | 1 +
> kernel/livepatch/transition.c | 26 +++++++++++++++++++++++++-
> 6 files changed, 94 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index d6e6d8176995..1635b30bb1ec 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -172,6 +172,9 @@ struct klp_patch {
> #define klp_for_each_object_static(patch, obj) \
> for (obj = patch->objs; obj->funcs || obj->name; obj++)
>
> +#define klp_for_each_object_safe(patch, obj, tmp_obj) \
> + list_for_each_entry_safe(obj, tmp_obj, &patch->obj_list, node)
> +
> #define klp_for_each_object(patch, obj) \
> list_for_each_entry(obj, &patch->obj_list, node)
>
> @@ -180,6 +183,9 @@ struct klp_patch {
> func->old_name || func->new_func || func->old_sympos; \
> func++)
>
> +#define klp_for_each_func_safe(obj, func, tmp_func) \
> + list_for_each_entry_safe(func, tmp_func, &obj->func_list, node)
> +
> #define klp_for_each_func(obj, func) \
> list_for_each_entry(func, &obj->func_list, node)

Is there a benefit of the newly added iterators?

Miroslav

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