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McKenney" , , , , , John 'Warthog9' Hawley Subject: Re: [PATCH v5] ring-buffer: Ensure proper resetting of atomic variables in ring_buffer_reset_online_cpus Message-ID: <20230425211737.757208b3@gandalf.local.home> In-Reply-To: <20230426010446.10753-1-Tze-nan.Wu@mediatek.com> References: <20230426010446.10753-1-Tze-nan.Wu@mediatek.com> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-6.7 required=5.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_PASS, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org For some reason, this email did not make it to linux-trace-kernel@vger.kernel.org, and therefore did not make it into patchwork? John? -- Steve On Wed, 26 Apr 2023 09:04:44 +0800 Tze-nan.Wu wrote: > From: "Tze-nan Wu" > > In ring_buffer_reset_online_cpus, the buffer_size_kb write operation > may permanently fail if the cpu_online_mask changes between two > for_each_online_buffer_cpu loops. The number of increases and decreases > on both cpu_buffer->resize_disabled and cpu_buffer->record_disabled may be > inconsistent, causing some CPUs to have non-zero values for these atomic > variables after the function returns. > > This issue can be reproduced by "echo 0 > trace" while hotplugging cpu. > After reproducing success, we can find out buffer_size_kb will not be > functional anymore. > > To prevent leaving 'resize_disabled' and 'record_disabled' non-zero after > ring_buffer_reset_online_cpus returns, we ensure that each atomic variable > has been set up before atomic_sub() to it. > > Cc: stable@vger.kernel.org > Cc: npiggin@gmail.com > Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU") > Reviewed-by: Cheng-Jui Wang > Signed-off-by: Tze-nan Wu > --- > Changes from v4 to v5: https://lore.kernel.org/lkml/20230412112401.25081-1-Tze-nan.Wu@mediatek.com/ > - Move the define before the function > --- > kernel/trace/ring_buffer.c | 16 +++++++++++++--- > 1 file changed, 13 insertions(+), 3 deletions(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 76a2d91eecad..253ef85a9ec3 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -5345,6 +5345,9 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu) > } > EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); > > +/* Flag to ensure proper resetting of atomic variables */ > +#define RESET_BIT (1 << 30) > + > /** > * ring_buffer_reset_online_cpus - reset a ring buffer per CPU buffer > * @buffer: The ring buffer to reset a per cpu buffer of > @@ -5361,20 +5364,27 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer) > for_each_online_buffer_cpu(buffer, cpu) { > cpu_buffer = buffer->buffers[cpu]; > > - atomic_inc(&cpu_buffer->resize_disabled); > + atomic_add(RESET_BIT, &cpu_buffer->resize_disabled); > atomic_inc(&cpu_buffer->record_disabled); > } > > /* Make sure all commits have finished */ > synchronize_rcu(); > > - for_each_online_buffer_cpu(buffer, cpu) { > + for_each_buffer_cpu(buffer, cpu) { > cpu_buffer = buffer->buffers[cpu]; > > + /* > + * If a CPU came online during the synchronize_rcu(), then > + * ignore it. > + */ > + if (!(atomic_read(&cpu_buffer->resize_disabled) & RESET_BIT)) > + continue; > + > reset_disabled_cpu_buffer(cpu_buffer); > > atomic_dec(&cpu_buffer->record_disabled); > - atomic_dec(&cpu_buffer->resize_disabled); > + atomic_sub(RESET_BIT, &cpu_buffer->resize_disabled); > } > > mutex_unlock(&buffer->mutex);