On Wed, Jan 17, 2024 at 09:26:46PM -0500, Steven Rostedt wrote:

On Thu, 18 Jan 2024 02:18:42 +0000
Chen Zhongjin [off-list ref] wrote:

quoted

There is a deadlock scenario in kprobe_optimizer():

pid A				pid B			pid C
kprobe_optimizer()		do_exit()		perf_kprobe_init()
mutex_lock(&kprobe_mutex)	exit_tasks_rcu_start()	mutex_lock(&kprobe_mutex)
synchronize_rcu_tasks()		zap_pid_ns_processes()	// waiting kprobe_mutex
// waiting tasks_rcu_exit_srcu	kernel_wait4()
				// waiting pid C exit

To avoid this deadlock loop, use synchronize_rcu_tasks_rude() in kprobe_optimizer()
rather than synchronize_rcu_tasks(). synchronize_rcu_tasks_rude() can also promise
that all preempted tasks have scheduled, but it will not wait tasks_rcu_exit_srcu.

Did lockdep detect this? If not, we should fix that.

I'm also thinking if we should find another solution, as this seems more of
a work around than a fix.

My suggestion is at 526b12e4-4bb0-47b1-bece-66b47bfc0a92@paulmck-laptop.

Better suggestions are of course welcome.  ;-)

quoted

Fixes: a30b85df7d59 ("kprobes: Use synchronize_rcu_tasks() for optprobe with CONFIG_PREEMPT=y")
Signed-off-by: Chen Zhongjin <redacted>
---
v1 -> v2: Add Fixes tag
---
 arch/Kconfig     | 2 +-
 kernel/kprobes.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index f4b210ab0612..dc6a18854017 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -104,7 +104,7 @@ config STATIC_CALL_SELFTEST
 config OPTPROBES
 	def_bool y
 	depends on KPROBES && HAVE_OPTPROBES
-	select TASKS_RCU if PREEMPTION
+	select TASKS_RUDE_RCU

Is this still a bug if PREEMPTION is not enabled?

Both "select" clauses would be needed for this patch, if I understand
correctly.

							Thanx, Paul

-- Steve

quoted

 
 config KPROBES_ON_FTRACE
 	def_bool y

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index d5a0ee40bf66..09056ae50c58 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c

@@ -623,7 +623,7 @@ static void kprobe_optimizer(struct work_struct *work)
 	 * Note that on non-preemptive kernel, this is transparently converted
 	 * to synchronoze_sched() to wait for all interrupts to have completed.
 	 */
-	synchronize_rcu_tasks();
+	synchronize_rcu_tasks_rude();
 
 	/* Step 3: Optimize kprobes after quiesence period */
 	do_optimize_kprobes();