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[34.140.183.197]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e906a06d1sm49602445e9.1.2026.05.12.06.26.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 12 May 2026 06:26:42 -0700 (PDT) Date: Tue, 12 May 2026 14:26:38 +0100 From: Vincent Donnefort To: David Carlier Cc: Steven Rostedt , Mathieu Desnoyers , Masami Hiramatsu , Catalin Marinas , Ryan Roberts , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] tracing: Fix nr_subbufs initialization in simple_ring_buffer_init_mm() Message-ID: References: <20260318171907.19640-1-devnexen@gmail.com> <20260319094126.109691-1-devnexen@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260319094126.109691-1-devnexen@gmail.com> On Thu, Mar 19, 2026 at 09:41:26AM +0000, David Carlier wrote: > nr_subbufs in the ring buffer metadata is always initialized to zero > because it is assigned from cpu_buffer->nr_pages before the page > initialization loop has run. While nr_subbufs is not currently read > by the kernel, it should reflect the actual buffer geometry in the > meta page for correctness. > > Move the assignment after the page loop so that cpu_buffer->nr_pages > holds the final count. Bug discover by Opus AI agent. nit: I believe we can put that new Assisted-by: tag now. > > Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer") > Signed-off-by: David Carlier > --- > kernel/trace/simple_ring_buffer.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c > index 02af2297ae5a..f731f14d0ff7 100644 > --- a/kernel/trace/simple_ring_buffer.c > +++ b/kernel/trace/simple_ring_buffer.c > @@ -395,7 +395,6 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer, > > memset(cpu_buffer->meta, 0, sizeof(*cpu_buffer->meta)); > cpu_buffer->meta->meta_page_size = PAGE_SIZE; > - cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages; > > /* The reader page is not part of the ring initially */ > page = load_page(desc->page_va[0]); > @@ -437,6 +436,7 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer, > return ret; > } > > + cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages; > /* Close the ring */ > bpage->link.next = &cpu_buffer->tail_page->link; > cpu_buffer->tail_page->link.prev = &bpage->link; > -- > 2.53.0 > Reviewed-by: Vincent Donnefort