[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 at mediatek.com>
                                   ^-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 at 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 <Tze-nan.Wu at mediatek.com> wrote:
> 
>> From: "Tze-nan Wu" <Tze-nan.Wu at mediatek.com>
>>
>> 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 at vger.kernel.org
>> Cc: npiggin at gmail.com
>> Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
>> Reviewed-by: Cheng-Jui Wang <cheng-jui.wang at mediatek.com>
>> Signed-off-by: Tze-nan Wu <Tze-nan.Wu at mediatek.com>
>> ---
>> 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);
>