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    SubjectRe: [PATCH v3 13/15] livepatch: change to a per-task consistency model
    On Fri 2016-12-23 10:24:35, Miroslav Benes wrote:
    > > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
    > > > > index 5efa262..e79ebb5 100644
    > > > > --- a/kernel/livepatch/patch.c
    > > > > +++ b/kernel/livepatch/patch.c
    > > > > @@ -29,6 +29,7 @@
    > > > > #include <linux/bug.h>
    > > > > #include <linux/printk.h>
    > > > > #include "patch.h"
    > > > > +#include "transition.h"
    > > > >
    > > > > static LIST_HEAD(klp_ops);
    > > > >
    > > > > @@ -54,15 +55,53 @@ static void notrace klp_ftrace_handler(unsigned long ip,
    > > > > {
    > > > > struct klp_ops *ops;
    > > > > struct klp_func *func;
    > > > > + int patch_state;
    > > > >
    > > > > ops = container_of(fops, struct klp_ops, fops);
    > > > >
    > > > > rcu_read_lock();
    > > > > +
    > > > > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
    > > > > stack_node);
    > > > > - if (WARN_ON_ONCE(!func))
    > > > > +
    > > > > + if (!func)
    > > > > goto unlock;
    > > >
    > > > Why do you removed the WARN_ON_ONCE(), please?
    > > >
    > > > We still add the function on the stack before registering
    > > > the ftrace handler. Also we unregister the ftrace handler
    > > > before removing the the last entry from the stack.
    > > >
    > > > AFAIK, unregister_ftrace_function() calls rcu_synchronize()'
    > > > to make sure that no-one is inside the handler once finished.
    > > > Mirek knows more about it.
    > >
    > > Hm, this is news to me. Mirek, please share :-)
    >
    > Well, I think the whole thing is well described in emails I exchanged with
    > Steven few months ago. See [1].
    >
    > [1] http://lkml.kernel.org/r/alpine.LNX.2.00.1608081041060.10833@pobox.suse.cz
    >
    > > > If this is not true, we have a problem. For example,
    > > > we call kfree(ops) after unregister_ftrace_function();
    > >
    > > Agreed.
    >
    > TL;DR - we should be ok as long as we do not do crazy things in the
    > handler, deliberate sleeping for example.
    >
    > WARN_ON_ONCE() may be crazy too. I think we discussed it long ago and we
    > came to an agreement to remove it.

    There are definitely situations where this might hurt. For example,
    when we redirect a function called under logbuf_lock.

    On the other hand, there is a work in progress[1][2] that will mitigate
    this risk a lot. Also this warning would be printed only when
    something goes wrong. IMHO, it is worth the risk. It will succeed
    in 99,999% cases and it might save us some headache when debugging
    random crashes of the system.

    Anyway, if there is a reason to remove the warning, it should be
    described. And if it is not strictly related to this patch, it should
    be handled separately.

    [1] https://lkml.kernel.org/r/20161221143605.2272-1-sergey.senozhatsky@gmail.com
    [2] https://lkml.kernel.org/r/1461333180-2897-1-git-send-email-sergey.senozhatsky@gmail.com

    Best Regards,
    Petr

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