From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 5525721C16A; Mon, 3 Aug 2026 22:49:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785797358; cv=none; b=uT3BA4GOTf58lS9+Gv8l3tg87SAxIJd8bTQU1Y0v7db5Ri40TknHbUf/ubhm8y4wj+1offTYw0Iw1vVFCNbOS501QD2IU2jukbj7aHgwvA8BtCjQvPMn9gCxQKC84wlFW7TmcwJQlFxNJglECmQ8JW8+F+yN0EVTsk3FDvIDp14= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785797358; c=relaxed/simple; bh=ASv9q3+xfLRGKOhSf7EjnwSfh468LoNVhgU5FT1wnyI=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=FUVdvzwrNqwL5Y3UYDBpWiMGY/1vVDV4lT5926rsPNMgqWm41p1UfIRNAL1a9W9XTLp8XxUx7E6Gz0/GCBZr1R7rQhMNBlx/AgvkTqzSdufickbXUk3mS4cPTlw825ms55O7GJFT9jMRdAwSsHIL4d1R8QCvsmlgkK1oSCIUyeg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=I+Bjn1jH; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="I+Bjn1jH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8D6E21F000E9; Mon, 3 Aug 2026 22:49:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785797357; bh=hb4MB0ev5cZX0v2JbE6xC67R9BeB4dvfLrD64khG9DU=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=I+Bjn1jHPLd4B0Vohh0kcxN4xjgd1lGAQ2+1y82l73ejCgDR/eK0s9HT0SqDMkiTM faOnZlLoEB2CSRmPEZSBshoKqCStZQN9mp8DGTbbRGKtS2FB0GEhW+YSu1G+EB/NSX 60ix6PbbL0YTbCOdJNy0QwRCAAJ1zSQXh7kFnR1+vI6AUl2NDOCk1K5h4Hdvn9EaB9 rmIgFRG+Pv731QesOSExwuGC3F97TdGxJrVYeuCk94U6Y1KtZ4vFgcMqGSkG7dZ6JP Jb/o0MmgjHt42/w85CZJWfqXA0Wa7jrABxLTNpUvdKl4n5Q0WHKM3ZUt7MXHGnZqkv 3GM7t2eBXpj1A== Date: Tue, 4 Aug 2026 07:49:13 +0900 From: Masami Hiramatsu (Google) To: Michael Wu Cc: Steven Rostedt , Mathieu Desnoyers , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH v4] tracing: Fix race between update_event_fields and, event_define_fields Message-Id: <20260804074913.d5ffb39d70afc48aec556eb6@kernel.org> In-Reply-To: <6c515e5f-66f4-0dc7-00f2-69f319c8ad87@allwinnertech.com> References: <6c515e5f-66f4-0dc7-00f2-69f319c8ad87@allwinnertech.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 3 Aug 2026 17:40:55 +0800 Michael Wu wrote: > event_define_fields() (pri=1 MODULE_STATE_COMING notifier, locked by > event_mutex) populates class->fields via list_add(), while > update_event_fields() (called from the pri=0 notifier path via > trace_event_update_all) traverses class->fields protected only by > trace_event_sem. These are two different locks guarding the same > data structure, so during cross-module loading a reader on one CPU can > observe partially initialized list nodes being concurrently added by a > writer on another CPU. > > On arm64 with weak memory ordering, __list_add() writes to two > different cache lines: > > next->prev = new; // (1) ordinary store > new->next = next; // (2) ordinary store > new->prev = prev; // (3) ordinary store > WRITE_ONCE(prev->next, new); // (4) release store > > The store buffer can drain (2) and (4) independently since they target > different cache lines. A remote CPU may observe (4) before (2): it > sees prev->next pointing to the new node, but the new node's link.next > is still zero (kmem_cache_alloc zero-initialized via KMEM_CACHE with > SLAB_PANIC). Since offsetof(struct ftrace_event_field, link) == 0, > list_for_each_entry() derives field == NULL from link.next == 0 and > crashes at field->type (offset 0x18): > > Unable to handle kernel access ... at virtual address 0000000000000018 > pc : update_event_fields+0xf8/0x368 > Call trace: > update_event_fields+0xf8/0x368 > trace_event_update_all+0x7c/0x2b4 > trace_module_notify+0x4c/0x1dc > notifier_call_chain+0x84/0x168 > blocking_notifier_call_chain_robust+0x64/0xd4 > load_module+0x10c8/0x123c > __arm64_sys_finit_module+0x230/0x31c > > Fix by holding event_mutex in trace_event_update_all() before > down_write(&trace_event_sem). This serializes the reader against > event_define_fields() which takes event_mutex for the entire > write-side critical section. The lock ordering event_mutex -> > trace_event_sem is already established at three other sites > (trace_remove_event_call, event_trace_add_tracer, > event_trace_del_tracer), so this introduces no ordering conflict. > > An alternative of placing mutex_lock inside update_event_fields() was > considered but rejected -- it would invert the established lock > ordering (trace_event_sem -> event_mutex) and risk ABBA deadlock. > Looks good to me! Acked-by: Masami Hiramatsu (Google) Thank you, > Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event types as well") > Cc: stable@vger.kernel.org > Signed-off-by: Michael Wu > --- > kernel/trace/trace_events.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c > index 956692856fa8..9632788da5af 100644 > --- a/kernel/trace/trace_events.c > +++ b/kernel/trace/trace_events.c > @@ -3566,6 +3566,7 @@ void trace_event_update_all(struct trace_eval_map **map, int len) > int last_i; > int i; > > + mutex_lock(&event_mutex); > down_write(&trace_event_sem); > list_for_each_entry_safe(call, p, &ftrace_events, list) { > /* events are usually grouped together with systems */ > @@ -3604,6 +3605,7 @@ void trace_event_update_all(struct trace_eval_map **map, int len) > cond_resched(); > } > up_write(&trace_event_sem); > + mutex_unlock(&event_mutex); > } > > static bool event_in_systems(struct trace_event_call *call, > -- > 2.29.0 -- Masami Hiramatsu (Google)