Writing 0 to /proc/sys/debug/kprobes-optimization while a kprobe is
jump-optimized never returns. The writer sleeps in D state forever with
kprobe_sysctl_mutex held, so any later read or write of that sysctl
hangs as well. For example, with vfs_read+9 as an optimizable address
in this build:
# cd /sys/kernel/tracing
# echo 'p:myprobe vfs_read+9' >> kprobe_events
# echo 1 > events/kprobes/myprobe/enable
# # wait until /sys/kernel/debug/kprobes/list shows [OPTIMIZED]
# echo 0 > /proc/sys/debug/kprobes-optimization
INFO: task sh:246 blocked for more than 10 seconds.
Call Trace:
<TASK>
__schedule+0x1176/0x4f70
schedule+0xdc/0x2c0
schedule_timeout+0x17b/0x260
wait_for_completion+0x173/0x3c0
wait_for_kprobe_optimizer_locked+0xbc/0x130
proc_kprobes_optimization_handler+0x156/0x1b0
proc_sys_call_handler+0x324/0x490
vfs_write+0x52d/0xfe0
ksys_write+0xff/0x200
do_syscall_64+0x106/0x630
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>
...
INFO: task cat:265 is blocked on a mutex likely owned by task sh:246.
wait_for_kprobe_optimizer_locked() reinitializes optimizer_completion,
asks the optimizer thread to flush and sleeps in wait_for_completion().
The thread drains the (un)optimizing lists, but calls complete() only
if completion_done() is true, i.e. if the completion is already done,
which never happens while someone waits. disarm_all_kprobes() and
kprobe_trace_self_tests_init() wait the same way.
Calling complete() unconditionally would not be enough: the waiter
drops kprobe_mutex while it sleeps, and nothing else serializes the
sysctl handler against the debugfs "enabled" file. A second flusher
that still finds the lists non-empty, e.g. because a disabled probe is
queued for unoptimizing, reinitializes the completion under the first:
sysctl write debugfs "enabled" write
unoptimize_all_kprobes()
wait_for_kprobe_optimizer_locked()
init_completion(c)
mutex_unlock(&kprobe_mutex)
wait_for_completion(c)
disarm_all_kprobes()
wait_for_kprobe_optimizer_locked()
init_completion(c)
// c->wait is reset, the first
// waiter is off the queue
mutex_unlock(&kprobe_mutex)
wait_for_completion(c)
kprobe_optimizer()
complete(c)
// wakes the debugfs writer only
where c is &optimizer_completion. Lining up the two writes during an
optimizer pass loses the sysctl writer this way.
Replace the completion with a counter of optimizer passes, bumped at the
end of each pass and signalled with wake_up_var_locked(), both under
kprobe_mutex. A flusher samples the count and waits with
wait_var_event_mutex(), which drops kprobe_mutex only while sleeping, so
a new count means a whole pass ran in the meantime. Nothing is
reinitialized, so several flushers can sleep in the wait at once.
Fixes: 73c12f209462 ("kprobes: Use dedicated kthread for kprobe optimizer")
Cc: [email protected]
Assisted-by: LLM
Signed-off-by: Andrea Parri <[email protected]>
---
This overlaps with two patches under discussion in the "rcu-tasks: build
Tasks RCU on Tasks Trace readers in trampolines" thread:
- Masami's RFC "kprobes: Make optprobe optimizer multi-generational and
asynchronous" [1] rewrites the same code. It drops the
completion_done() check, which fixes the hang reported here, but it
keeps init_completion() in wait_for_kprobe_optimizer_locked(), so the
concurrent-flusher case described above remains. The two patches
conflict textually.
- Josef's "kprobes: Expose the optprobe jump window to Tasks RCU" [2]
changes kprobe_optimizer() around synchronize_rcu_tasks(); its
kernel/kprobes.c hunks still apply on top of this patch.
[1]
https://lore.kernel.org/all/179017148080.466588.9116221556625712980.stgit@devnote2/
[2]
https://lore.kernel.org/all/20260922-b4-rcu-tasks-preempt-qs-v5-3-410f57770...@toxicpanda.com/
kernel/kprobes.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 6337da5cab9e7..4edd8ca5c6578 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -42,6 +42,7 @@
#include <linux/execmem.h>
#include <linux/cleanup.h>
#include <linux/wait.h>
+#include <linux/wait_bit.h>
#include <asm/sections.h>
#include <asm/cacheflush.h>
@@ -526,7 +527,8 @@ enum {
OPTIMIZER_ST_FLUSHING = 2,
};
-static DECLARE_COMPLETION(optimizer_completion);
+/* Bumped at the end of each kprobe_optimizer() pass, under 'kprobe_mutex' */
+static unsigned long optimizer_passes;
#define OPTIMIZE_DELAY 5
@@ -654,9 +656,9 @@ static void kprobe_optimizer(void)
do_free_cleaned_kprobes();
}
- /* Step 5: Kick optimizer again if needed. But if there is a flush
requested, */
- if (completion_done(&optimizer_completion))
- complete(&optimizer_completion);
+ /* Step 5: Wake up flushers, and kick optimizer again if needed. */
+ optimizer_passes++;
+ wake_up_var_locked(&optimizer_passes, &kprobe_mutex);
if (!list_empty(&optimizing_list) || !list_empty(&unoptimizing_list))
kick_kprobe_optimizer(); /*normal kick*/
@@ -708,7 +710,8 @@ static void wait_for_kprobe_optimizer_locked(void)
lockdep_assert_held(&kprobe_mutex);
while (!list_empty(&optimizing_list) ||
!list_empty(&unoptimizing_list)) {
- init_completion(&optimizer_completion);
+ unsigned long passes = optimizer_passes;
+
/*
* Set state to OPTIMIZER_ST_FLUSHING and wake up the thread if
it's
* idle. If it's already kicked, it will see the state change.
@@ -717,9 +720,12 @@ static void wait_for_kprobe_optimizer_locked(void)
OPTIMIZER_ST_FLUSHING) != OPTIMIZER_ST_FLUSHING)
wake_up(&kprobe_optimizer_wait);
- mutex_unlock(&kprobe_mutex);
- wait_for_completion(&optimizer_completion);
- mutex_lock(&kprobe_mutex);
+ /*
+ * kprobe_optimizer() holds 'kprobe_mutex' for a whole pass,
which
+ * this drops while sleeping, so a new count means a full pass
ran.
+ */
+ wait_var_event_mutex(&optimizer_passes,
+ optimizer_passes != passes, &kprobe_mutex);
}
}
--
2.53.0
