From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout03.his.huawei.com (canpmsgout03.his.huawei.com [113.46.200.218]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6B6423043C8; Mon, 22 Dec 2025 07:12:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.218 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766387532; cv=none; b=Pjedt4qwY594E3pP7zAWCkjeKR/evCf0SJEZMDIVh/EfqtZWoXHb5S7ch/5e92b0JX4JdNK4xveRZRIk6ulg4LDCbNHMawJgoxlKdqd56S9s+Yd+Wcf/FqP852unuc4qvgt9JnuY/akEW9pT4I1nylWq24Isx8DISxy37vH1qOw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766387532; c=relaxed/simple; bh=aUNIWiozwaKGPbZ7we6g9cMR6WPhGKJa26i9/Y5kZg0=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=pWZgsf28vbxTAJOnuVQCtRjYdI7+cdBnbj4Fbrvczc8oQfSb2pqYNfo+wRHLeGG12wAQCV5nMXsIqFFhpeFVFqat/CL4pH6y7RvNjbr/Dc1EQckqIzYtICEWyQ+KMfRhYwe2lWICpQOLHYRi4NPz7i8Fw3SwujXs6niBJsukd9E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=3cPYQPNt; arc=none smtp.client-ip=113.46.200.218 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="3cPYQPNt" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=ztb9+3n+GCcK2m5Ba62kPS3wLNEwIpgkFPKdZJKVyaU=; b=3cPYQPNtrXs76shTgfJzQEZ5A/8Iyb/JOHVAymP21fUFzHnurhqUI5HJAh734tIEbPvcfPg+o /EDU3CwB7WNJON+BpJ8T5S/u19oLgX6DxzzVw1iaYVePcIJOQe2gpLvmfp+X7K6APDw63Tx4QHd eWn/jvEwIdlGfG2ABvpphr4= Received: from mail.maildlp.com (unknown [172.19.162.197]) by canpmsgout03.his.huawei.com (SkyGuard) with ESMTPS id 4dZTlQ0hvxzpT0h; Mon, 22 Dec 2025 15:09:06 +0800 (CST) Received: from dggpemr500006.china.huawei.com (unknown [7.185.36.185]) by mail.maildlp.com (Postfix) with ESMTPS id E315740363; Mon, 22 Dec 2025 15:11:58 +0800 (CST) Received: from [100.103.109.15] (100.103.109.15) by dggpemr500006.china.huawei.com (7.185.36.185) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Mon, 22 Dec 2025 15:11:57 +0800 Message-ID: <597d8f33-106d-45bf-a6da-637ab05604e4@huawei.com> Date: Mon, 22 Dec 2025 15:11:57 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] rcu: Fix rcu_read_unlock() deadloop due to softirq To: Joel Fernandes CC: , , , , , , , , , , , , , References: <20251218074950.1936014-1-yaokai34@huawei.com> <20251219151400.GA820904@joelbox2> <16e20d68-8d19-45bd-b4a1-f2c067ca44a6@nvidia.com> Content-Language: en-US From: Yao Kai In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: kwepems500002.china.huawei.com (7.221.188.17) To dggpemr500006.china.huawei.com (7.185.36.185) On 12/19/2025 11:44 PM, Joel Fernandes wrote: > > > On 12/19/2025 10:38 AM, Joel Fernandes wrote: >> >> >> On 12/19/2025 10:14 AM, Joel Fernandes wrote: >>> On Thu, Dec 18, 2025 at 03:49:50PM +0800, Yao Kai wrote: >>>> Commit 5f5fa7ea89dc ("rcu: Don't use negative nesting depth in >>>> __rcu_read_unlock()") removes the recursion-protection code from >>>> __rcu_read_unlock(). Therefore, we could invoke the deadloop in >>>> raise_softirq_irqoff() with ftrace enabled as follows: >>>> >>>> WARNING: CPU: 0 PID: 0 at kernel/trace/trace.c:3021 __ftrace_trace_stack.constprop.0+0x172/0x180 >>>> Modules linked in: my_irq_work(O) >>>> CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G O 6.18.0-rc7-dirty #23 PREEMPT(full) >>>> Tainted: [O]=OOT_MODULE >>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014 >>>> RIP: 0010:__ftrace_trace_stack.constprop.0+0x172/0x180 >>>> RSP: 0018:ffffc900000034a8 EFLAGS: 00010002 >>>> RAX: 0000000000000000 RBX: 0000000000000004 RCX: 0000000000000000 >>>> RDX: 0000000000000003 RSI: ffffffff826d7b87 RDI: ffffffff826e9329 >>>> RBP: 0000000000090009 R08: 0000000000000005 R09: ffffffff82afbc4c >>>> R10: 0000000000000008 R11: 0000000000011d7a R12: 0000000000000000 >>>> R13: ffff888003874100 R14: 0000000000000003 R15: ffff8880038c1054 >>>> FS: 0000000000000000(0000) GS:ffff8880fa8ea000(0000) knlGS:0000000000000000 >>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> CR2: 000055b31fa7f540 CR3: 00000000078f4005 CR4: 0000000000770ef0 >>>> PKRU: 55555554 >>>> Call Trace: >>>> >>>> trace_buffer_unlock_commit_regs+0x6d/0x220 >>>> trace_event_buffer_commit+0x5c/0x260 >>>> trace_event_raw_event_softirq+0x47/0x80 >>>> raise_softirq_irqoff+0x6e/0xa0 >>>> rcu_read_unlock_special+0xb1/0x160 >>>> unwind_next_frame+0x203/0x9b0 >>>> __unwind_start+0x15d/0x1c0 >>>> arch_stack_walk+0x62/0xf0 >>>> stack_trace_save+0x48/0x70 >>>> __ftrace_trace_stack.constprop.0+0x144/0x180 >>>> trace_buffer_unlock_commit_regs+0x6d/0x220 >>>> trace_event_buffer_commit+0x5c/0x260 >>>> trace_event_raw_event_softirq+0x47/0x80 >>>> raise_softirq_irqoff+0x6e/0xa0 >>>> rcu_read_unlock_special+0xb1/0x160 >>>> unwind_next_frame+0x203/0x9b0 >>>> __unwind_start+0x15d/0x1c0 >>>> arch_stack_walk+0x62/0xf0 >>>> stack_trace_save+0x48/0x70 >>>> __ftrace_trace_stack.constprop.0+0x144/0x180 >>>> trace_buffer_unlock_commit_regs+0x6d/0x220 >>>> trace_event_buffer_commit+0x5c/0x260 >>>> trace_event_raw_event_softirq+0x47/0x80 >>>> raise_softirq_irqoff+0x6e/0xa0 >>>> rcu_read_unlock_special+0xb1/0x160 >>>> unwind_next_frame+0x203/0x9b0 >>>> __unwind_start+0x15d/0x1c0 >>>> arch_stack_walk+0x62/0xf0 >>>> stack_trace_save+0x48/0x70 >>>> __ftrace_trace_stack.constprop.0+0x144/0x180 >>>> trace_buffer_unlock_commit_regs+0x6d/0x220 >>>> trace_event_buffer_commit+0x5c/0x260 >>>> trace_event_raw_event_softirq+0x47/0x80 >>>> raise_softirq_irqoff+0x6e/0xa0 >>>> rcu_read_unlock_special+0xb1/0x160 >>>> __is_insn_slot_addr+0x54/0x70 >>>> kernel_text_address+0x48/0xc0 >>>> __kernel_text_address+0xd/0x40 >>>> unwind_get_return_address+0x1e/0x40 >>>> arch_stack_walk+0x9c/0xf0 >>>> stack_trace_save+0x48/0x70 >>>> __ftrace_trace_stack.constprop.0+0x144/0x180 >>>> trace_buffer_unlock_commit_regs+0x6d/0x220 >>>> trace_event_buffer_commit+0x5c/0x260 >>>> trace_event_raw_event_softirq+0x47/0x80 >>>> __raise_softirq_irqoff+0x61/0x80 >>>> __flush_smp_call_function_queue+0x115/0x420 >>>> __sysvec_call_function_single+0x17/0xb0 >>>> sysvec_call_function_single+0x8c/0xc0 >>>> >>>> >>>> Commit b41642c87716 ("rcu: Fix rcu_read_unlock() deadloop due to IRQ work") >>>> fixed the infinite loop in rcu_read_unlock_special() for IRQ work by >>>> setting a flag before calling irq_work_queue_on(). We fix this issue by >>>> setting the same flag before calling raise_softirq_irqoff() and rename the >>>> flag to defer_qs_pending for more common. >>>> >>>> Fixes: 5f5fa7ea89dc ("rcu: Don't use negative nesting depth in __rcu_read_unlock()") >>>> Reported-by: Tengda Wu >>>> Signed-off-by: Yao Kai >>> >>> Good change! It is the exact same pattern and both IRQ work and softirq >>> raising are for deferred QS purposes. >>> >>> Reviewed-by: Joel Fernandes >> Could you please update the following comment in tree.h to say "An IRQ work or >> softirq": >> >> /* >> * An IRQ work (deferred_qs_iw) is used by RCU to get the scheduler's attention. >> * to report quiescent states at the soonest possible time. >> * The request can be in one of the following states: >> * - DEFER_QS_IDLE: An IRQ work is yet to be scheduled. >> * - DEFER_QS_PENDING: An IRQ work was scheduled but either not yet run, or it >> * ran and we still haven't reported a quiescent state. >> */ >> #define DEFER_QS_IDLE 0 >> #define DEFER_QS_PENDING 1 >> >> And resend the patch with my Review tag? > > Oh, and update the variable name in the comment too (deferred_qs_iw) :) and all > the occurences of "IRQ work" to "IRQ work or softirq". > > - Joel > > Thanks for review. I will send a new patch later. - Kai