Messages in this thread | | | Date | Wed, 13 Oct 2021 09:55:18 +0200 (CEST) | From | Miroslav Benes <> | Subject | Re: [RESEND PATCH v2 1/2] ftrace: disable preemption between ftrace_test_recursion_trylock/unlock() |
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> 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@infradead.org/.
Miroslav
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