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

[Tze-nan Wu (吳澤南)]

Hi Steve,

On Tue, 2023-04-11 at 12:44 -0400, Steven Rostedt wrote:
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
> 
> 
> 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.
> 

No problem, got it.

> 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).
> 

Will remember this, won't happened next time :)

> > 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.
> 

Sure, a link here is really helpful.
Will follow this format in the future.

> 
> >  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.
> 

Never thought about that until you told me,
I will keep it in mind before declare a cpumask next time.

> 
> >       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)

Maybe we should use for_each_buffer_cpu(buffer, cpu) here?
since a CPU may also came offline during synchronize_rcu().

> > {
> >               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
> 

Thanks for all your suggestions, learn a lot from here, really
appriciate :).
I will upload a v4 patch in new thread as soon as the new patch pass my
test.

-- Tzenan


> > 
> >               reset_disabled_cpu_buffer(cpu_buffer);
> 
>