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

[John 'Warthog9' Hawley]

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

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);