Re: [PATCH v2 1/4] dma-mapping: benchmark: fix up kthread-related error handling

[Robin Murphy]

On 2024-05-04 12:47 pm, Fedor Pchelkin wrote:
> kthread creation failure is invalidly handled inside do_map_benchmark().
> The put_task_struct() calls on the error path are supposed to balance the
> get_task_struct() calls which only happen after all the kthreads are
> successfully created. Rollback using kthread_stop() for already created
> kthreads in case of such failure.
>
> In normal situation call kthread_stop_put() to gracefully stop kthreads
> and put their task refcounts. This should be done for all started
> kthreads.

Although strictly there were two overlapping bugs here, I'd agree that 
it really doesn't seem worth the bother of trying to distinguish them. 
I'm far from a kthread expert, but as best I can tell this looks like it 
achieves a sensible final state. Modulo one nit below,

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

> Found by Linux Verification Center (linuxtesting.org).
>
> Fixes: 65789daa8087 ("dma-mapping: add benchmark support for streaming DMA APIs")
> Suggested-by: Robin Murphy <robin.murphy@xxxxxxx>
> Signed-off-by: Fedor Pchelkin <pchelkin@xxxxxxxxx>
> ---
>   kernel/dma/map_benchmark.c | 16 ++++++++++------
>   1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c
> index 02205ab53b7e..2478957cf9f8 100644
> --- a/kernel/dma/map_benchmark.c
> +++ b/kernel/dma/map_benchmark.c
> @@ -118,6 +118,8 @@ static int do_map_benchmark(struct map_benchmark_data *map)
>   		if (IS_ERR(tsk[i])) {
>   			pr_err("create dma_map thread failed\n");
>   			ret = PTR_ERR(tsk[i]);
> +			while (--i >= 0)
> +				kthread_stop(tsk[i]);

I think this means we'd end up actually starting the threads purely to 
get them to see the KTHREAD_SHOULD_STOP flag and exit again? Not that 
I'm too fussed about optimising an unexpected error path, however I 
can't help but wonder if we might only need a put_task_struct() if we 
can safely assume that the threads have never been started at this point.

Thanks,
Robin.

>   			goto out;
>   		}
>   
> @@ -139,13 +141,17 @@ static int do_map_benchmark(struct map_benchmark_data *map)
>   
>   	msleep_interruptible(map->bparam.seconds * 1000);
>   
> -	/* wait for the completion of benchmark threads */
> +	/* wait for the completion of all started benchmark threads */
>   	for (i = 0; i < threads; i++) {
> -		ret = kthread_stop(tsk[i]);
> -		if (ret)
> -			goto out;
> +		int kthread_ret = kthread_stop_put(tsk[i]);
> +
> +		if (kthread_ret)
> +			ret = kthread_ret;
>   	}
>   
> +	if (ret)
> +		goto out;
> +
>   	loops = atomic64_read(&map->loops);
>   	if (likely(loops > 0)) {
>   		u64 map_variance, unmap_variance;
> @@ -170,8 +176,6 @@ static int do_map_benchmark(struct map_benchmark_data *map)
>   	}
>   
>   out:
> -	for (i = 0; i < threads; i++)
> -		put_task_struct(tsk[i]);
>   	put_device(map->dev);
>   	kfree(tsk);
>   	return ret;