Re: [PATCH v5] ring-buffer: Ensure proper resetting of atomic variables in ring_buffer_reset_online_cpus

[Tze-nan Wu (吳澤南)]

On Tue, 2023-04-25 at 19:58 -0700, John 'Warthog9' Hawley wrote:
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
> 
> 
> Ran afoul of the taboo filters:
> 
> Illegal-Object: Syntax error in From: address found on
> vger.kernel.org:
>          From:   Tze-nan.Wu<Tze-nan.Wu@xxxxxxxxxxxx>
>                                    ^-missing end of mailbox
> 
> I've seen a few of these but I've been unable to replicate the
> problem.
> I've a suspicion but so far been unable to prove the theory on what's
> wrong.
> 
> - John
> 
  So I sent the patch again, but this time I changed the name field in
.gitconfig from "Tze-nan.Wu"(internal-use) to "Tze-nan Wu".
This can avoid the line "From:  Tze-nan.Wu<Tze-nan.Wu@xxxxxxxxxxxx>"
being added to the mail when we execute "git send-email", and it works,
the last patch I sent is not filter out anymore.

---
Here is the magore difference between the patches:
old patch(fail):
  From: "Tze-nan Wu" <Tze-nan.Wu@xxxxxxxxxxxx>   <== (modify by editor
from Tze-nan.Wu to Tze-nan Wu)
---
new patch(success):
  From: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
---

 - Tzenan
> On 4/25/23 18:17, Steven Rostedt wrote:
> > 
> > For some reason, this email did not make it to
> > linux-trace-kernel@xxxxxxxxxxxxxxx, and therefore did not make it
> > into
> > patchwork?
> > 
> > John?
> > 
> > -- Steve
> > 
> > 
> > On Wed, 26 Apr 2023 09:04:44 +0800
> > Tze-nan.Wu <Tze-nan.Wu@xxxxxxxxxxxx> wrote:
> > 
> > > From: "Tze-nan Wu" <Tze-nan.Wu@xxxxxxxxxxxx>
> > > 
> > > 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@xxxxxxxxxxxxxxx
> > > Cc: npiggin@xxxxxxxxx
> > > Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by
> > > avoiding synchronize_rcu for each CPU")
> > > Reviewed-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx>
> > > Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
> > > ---
> > > Changes from v4 to v5: 
> > > https://lore.kernel.org/lkml/20230412112401.25081-1-Tze-nan.Wu@xxxxxxxxxxxx/
> > >    - 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);