Re: [PATCH v3] ring-buffer: Prevent inconsistent operation on cpu_buffer->resize_disabled

[Steven Rostedt]

Please have each new patch be a new thread, and not a Cc to the previous
version of the patch. As it makes it hard to find in INBOXs.

On Mon, 10 Apr 2023 15:35:08 +0800
Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx> wrote:

> Write to buffer_size_kb can permanently fail, due to cpu_online_mask may
> changed between two for_each_online_buffer_cpu loops.
> The number of increasing and decreasing on cpu_buffer->resize_disable
> may be inconsistent, leading that the resize_disabled in some CPUs
> becoming none zero after ring_buffer_reset_online_cpus return.
> 
> 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.
> 
> Prevent the two "loops" in this function from iterating through different
> online cpus by copying cpu_online_mask at the entry of the function.
> 

The "Changes from" need to go below  the '---', otherwise they are added to
the git commit (we don't want it there).

> Changes from v1 to v3:
>   Declare the cpumask variable statically rather than dynamically.
> 
> Changes from v2 to v3:
>   Considering holding cpu_hotplug_lock too long because of the
>   synchronize_rcu(), maybe it's better to prevent the issue by copying
>   cpu_online_mask at the entry of the function as V1 does, instead of
>   using cpus_read_lock().
> 
> Link: https://lore.kernel.org/lkml/20230408052226.25268-1-Tze-nan.Wu@xxxxxxxxxxxx/
> Link: https://lore.kernel.org/oe-kbuild-all/202304082051.Dp50upfS-lkp@xxxxxxxxx/
> Link: https://lore.kernel.org/oe-kbuild-all/202304081615.eiaqpbV8-lkp@xxxxxxxxx/
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: npiggin@xxxxxxxxx
> Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Reviewed-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx>
> Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
> ---

This is where the "Changes from" go. And since this patch is not suppose to
be a Cc. But since it's still good to have a link to it. You could do:

Changes from v2 to v3: https://lore.kernel.org/linux-trace-kernel/20230409024616.31099-1-Tze-nan.Wu@xxxxxxxxxxxx/
  Considering holding cpu_hotplug_lock too long because of the
  synchronize_rcu(), maybe it's better to prevent the issue by copying
  cpu_online_mask at the entry of the function as V1 does, instead of
  using cpus_read_lock().


Where the previous version changes has the lore link to the previous patch,
in case someone wants to look at it.


>  kernel/trace/ring_buffer.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 76a2d91eecad..dc758930dacb 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -288,9 +288,6 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data);
>  #define for_each_buffer_cpu(buffer, cpu)		\
>  	for_each_cpu(cpu, buffer->cpumask)
>  
> -#define for_each_online_buffer_cpu(buffer, cpu)		\
> -	for_each_cpu_and(cpu, buffer->cpumask, cpu_online_mask)
> -
>  #define TS_SHIFT	27
>  #define TS_MASK		((1ULL << TS_SHIFT) - 1)
>  #define TS_DELTA_TEST	(~TS_MASK)
> @@ -5353,12 +5350,19 @@ EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
>  void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
>  {
>  	struct ring_buffer_per_cpu *cpu_buffer;
> +	cpumask_t reset_online_cpumask;

It's usually considered bad form to put a cpumask on the stack. As it can
be 128 bytes for a machine with 1024 CPUs (and yes they do exist). Also,
the mask size is set to NR_CPUS not the actual size, so you do not even
need to have it that big.


>  	int cpu;
>  
> +	/*
> +	 * Record cpu_online_mask here to make sure we iterate through the same
> +	 * online CPUs in the following two loops.
> +	 */
> +	cpumask_copy(&reset_online_cpumask, cpu_online_mask);
> +
>  	/* prevent another thread from changing buffer sizes */
>  	mutex_lock(&buffer->mutex);
>  
> -	for_each_online_buffer_cpu(buffer, cpu) {
> +	for_each_cpu_and(cpu, buffer->cpumask, &reset_online_cpumask) {
>  		cpu_buffer = buffer->buffers[cpu];
>  
>  		atomic_inc(&cpu_buffer->resize_disabled);

Anyway, we don't need to modify any of the above, and just do the following
instead of atomic_inc():

#define RESET_BIT	(1 << 30)

		atomic_add(&cpu_buffer->resize_disabled, RESET_BIT);


> @@ -5368,7 +5372,7 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
>  	/* Make sure all commits have finished */
>  	synchronize_rcu();
>  
> -	for_each_online_buffer_cpu(buffer, cpu) {
> +	for_each_cpu_and(cpu, buffer->cpumask, &reset_online_cpumask) {
>  		cpu_buffer = buffer->buffers[cpu];

Then here we can do:

		/*
		 * If a CPU came online during the synchronize_rcu(), then
		 * ignore it.
		 */
		if (!atomic_read(&cpu_buffer->resize_disabled) & RESET_BIT))
			continue;

		atomic_sub(&cpu_buffer->resize_disabled, RESET_BIT);


As the resize_disabled only needs to be set to something to make it
disabled.

-- Steve

>  
>  		reset_disabled_cpu_buffer(cpu_buffer);