• Petr Mladek's avatar
    livepatch: Remove Nop structures when unused · d697bad5
    Petr Mladek authored
    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 give the impression that the function is patched
        even though the ftrace handler has no effect.
    
      + Ftrace handlers do not come 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 no longer patched in
        the new cumulative patch. One would expect that cumulative patch
        would help 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 processors, 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 no 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.
    Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
    Acked-by: default avatarMiroslav Benes <mbenes@suse.cz>
    Acked-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
    Signed-off-by: default avatarJiri Kosina <jkosina@suse.cz>
    d697bad5
patch.h 1.12 KB