On Thu, 24 Sep 2026 11:21:39 +0200
Andrea Parri <[email protected]> wrote:
> 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.
OK, this fix looks good to me. Let me pick this for probes/fixes and
rebase mine on top of it. Anyway I think we need both.
Thank you,
>
> - 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
>
--
Masami Hiramatsu (Google) <[email protected]>
