From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm2-f13.google.com (mail-wm2-f13.google.com [74.125.225.141]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0BD3344CACA for ; Thu, 24 Sep 2026 09:22:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=74.125.225.141 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790241726; cv=none; b=p4g3HpYYCRbfczD28N3/D2+vqVhY730QLvfz85cHI7+j9pk5WR5yJUzgt57UrdYLlHx7de38G9Jh3K+zQ+qvfAFdLxP+jGazr1sslKO2xIU9kcxD51mfZU0t5k+LIlEAMUzakpTy4kT6Ij/w98xqA+OLugtBrrE1PrPDP9c16dM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790241726; c=relaxed/simple; bh=fshvWfpZGmu6OxU4jA/4vo4ZS+70//Gu/JiFujJoev4=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=jKeDZeIAFAhp9gExpUC7QbjXY2CrHeJnl9pKvMZS3s8KRCbOrhaArX2rPfcfzUJzjl7nlji222dK4RpkC9ChEgA5DGPrK7hScgDvsokU8uWh2+QHeSXjvkIiVHVCNn+aGJwaEbG69TpKbsae2m7Zxu0KMElFFHduQlMm8Wili7M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=rH1lSRyI; arc=none smtp.client-ip=74.125.225.141 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="rH1lSRyI" Received: by mail-wm2-f13.google.com with SMTP id 5b1f17b1804b1-49e71cdb22bso13276485e9.2 for ; Thu, 24 Sep 2026 02:22:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1790241720; x=1790846520; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to:content-type; bh=3i4XwsbTztFzn8Zx6bLGOqiaI42zayiDsFTcP+jZwAo=; b=rH1lSRyIOraJK2AgSqW52qL48bUgwfHYeoxrky6PV/bQs3awQ3Hjgbe+rRjbICktSL 6DaDSwHTADxDLYG0icr8I9x98mpSbuCvsrGjBDvsfdK7VUNFEa3Y3wA3W1pmDokbZL/q mI89KfuMroCCR2uKzs3g79Yvutt6D9mwlE0IuJ9vHHD/DoRVQ/G30BIJ9RuCqbBmNt7L sIoP5UF/EFN7ePToMGJbjHPCVrxAYEclhZLiMZkXirl1oCygKCqQaZLVVQ9Ehem7AQIs qQ6IIbOZNiEVOeH1ozYeBPDwCKJvG8dN4MWuE2kihuSqy7dA15Ym5VdP4+dJaKNOWew3 6eYA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1790241720; x=1790846520; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=3i4XwsbTztFzn8Zx6bLGOqiaI42zayiDsFTcP+jZwAo=; b=DaXnTyEz5R2pU3xq7zWYLdMfuo5W2WkSLCiOwx8avrNC1xwGEE6An1tFL8PivodJz7 zpWukBB1MV+sAwwCw1WqWas4s5WO09lxmh9//2NwSW9WLs9qUVCguJxCkgJF/W6oUrif KWnENnUSarhY//wlD1YgZj80cWwlpu9lfrGcUYcEQ8ywhSTaryzZEIQEIBIf5+s3sytz UujbUSyzxbUrPuCuRJpP/R7bKTEQQXcP9LSmJZ4qUcM2itcK/iqA0cRvzyB+IPVmPUAg YgBkiCC/lDywbmcz8EPrlkyoeVM+PbGgKkkazFHIyNofGwsDPmWXzuILS1FEvUOHGUrp Vx6w== X-Forwarded-Encrypted: i=1; AKwUvBzAtiI2c+nF4+2xoElECTBbwW97LpSfg5pO1g+RYA8NxKZHjL0lTM84F+ZvwBE4g757y7Ub1ced37uwyi0fyxkxsms=@vger.kernel.org X-Gm-Message-State: AFuF++kOO36W4NsJC4cTdA4vngEzwZRsz9NpjaeqwrxGvjkzftbF4g0F ewUjq4kkl0n68SDHANkb1onWemN+aZV7kfaqUTPeX7KlxbVP/LvTohSd X-Gm-Gg: AYBFou1d1GDQYvP8VOTLAKTpQn2fqCiVoY9Q6WutbQU0lFFXbGqotS8Jg4JMxDzhvvV T/cSU6kH1v3dVGP2Vo2+oQ04ZFLV2+tLcGozaQB7z7eoqRdcSzJU8C8WvoMtaTc9Q2wdDt+wuvf UxB87lixaKh35E0+BchCJNEWFlYqz5FPVw2VyA4WuD6+mKrnS9M+SXo9MyMxT7h964ZrIzteGEI lQWb0q25Xjtd/5bKc2035XPMKnKCOxdYZdhJ9py/L8qNrAsR4rOorynL/lB+g7BMN1PJwUHNtKZ n76bgTIwClkJbSH2ItBKjf8K6+p9op0GB2bQpouglj6Mp8tTkdZ/hBTc/ArSA+z52tfwRboAwaY q2ppjGT9Q49KwG2GRcNa2n9IlU+qFzaLm0wsVeZJ9oaETpX2UyN/7F9kq2rTq2Pwq4S+4olxTUj skZ2G60qXzG5YzlhBULhir20vzccf8gjrnTjDdhdyOCTwFttCCb1IsTdkzPiqtGcqFzD3K+9e57 pDCn4A8g6miO8YyTgsSC/aUjsFzL0iGgNDJBZUN0+8iYQ6lpel3k4DStqOq5g20gxPqkAvhF7zF Y/Mt X-Received: by 2002:a05:600c:8b4c:b0:49c:fa20:cbfc with SMTP id 5b1f17b1804b1-49fe66eb0f0mr29202135e9.19.1790241720298; Thu, 24 Sep 2026 02:22:00 -0700 (PDT) Received: from andreayoga.localdomain (93-42-14-189.ip84.fastwebnet.it. [93.42.14.189]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49fe0c3731fsm116682435e9.2.2026.09.24.02.21.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Sep 2026 02:21:58 -0700 (PDT) From: Andrea Parri To: Naveen N Rao , "David S. Miller" , Masami Hiramatsu Cc: Andrea Parri , Steven Rostedt , Josef Bacik , linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH] kprobes: Fix permanent hang when flushing the kprobe optimizer Date: Thu, 24 Sep 2026 11:21:39 +0200 Message-ID: <20260924092142.199198-1-parri.andrea@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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: __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 ... 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: stable@vger.kernel.org Assisted-by: LLM Signed-off-by: Andrea Parri --- 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-v5-3-410f57770bad@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 #include #include +#include #include #include @@ -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