Re: [RESEND PATCH v2 1/2] ftrace: disable preemption between ftrace_test_recursion_trylock/unlock()
From: Miroslav Benes <mbenes@suse.cz>
Date: 2021-10-13 07:55:24
Also in:
linux-riscv, linuxppc-dev, lkml
quoted hunk ↗ jump to hunk
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;
}[...]
quoted hunk ↗ jump to hunk
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
quoted hunk ↗ jump to hunk
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/ (local). Miroslav