> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
> index a9f9c57..101e1fb 100644
> --- a/include/linux/trace_recursion.h
> +++ b/include/linux/trace_recursion.h
> @@ -208,13 +208,29 @@ static __always_inline void trace_clear_recursion(int bit)
> * Use this for ftrace callbacks. This will detect if the function
> * tracing recursed in the same context (normal vs interrupt),
> *
> + * The ftrace_test_recursion_trylock() will disable preemption,
> + * which is required for the variant of synchronize_rcu() that is
> + * used to allow patching functions where RCU is not watching.
> + * See klp_synchronize_transition() for more details.
> + *
I think that you misunderstood. Steven proposed to put the comment before
ftrace_test_recursion_trylock() call site in klp_ftrace_handler().
> * Returns: -1 if a recursion happened.
> * >= 0 if no recursion
> */
> static __always_inline int ftrace_test_recursion_trylock(unsigned long ip,
> unsigned long parent_ip)
> {
> - return trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
> + int bit;
> +
> + bit = trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
> + /*
> + * The zero bit indicate we are nested
> + * in another trylock(), which means the
> + * preemption already disabled.
> + */
> + if (bit > 0)
> + preempt_disable_notrace();
> +
> + return bit;
> }
[...]
> diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> index e8029ae..6e66ccd 100644
> --- a/kernel/livepatch/patch.c
> +++ b/kernel/livepatch/patch.c
> @@ -52,11 +52,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
Here
> bit = ftrace_test_recursion_trylock(ip, parent_ip);
> if (WARN_ON_ONCE(bit < 0))
> return;
> - /*
> - * A variant of synchronize_rcu() is used to allow patching functions
> - * where RCU is not watching, see klp_synchronize_transition().
> - */
> - preempt_disable_notrace();
>
> func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
> stack_node);
> @@ -120,7 +115,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> klp_arch_set_pc(fregs, (unsigned long)func->new_func);
>
> unlock:
> - preempt_enable_notrace();
> ftrace_test_recursion_unlock(bit);
> }
Side note... the comment will eventually conflict with peterz's
https://lore.kernel.org/all/20210929152429.125997206@xxxxxxxxxxxxx/.
Miroslav
