Re: [RFC][PATCH 1/3] test-ww_mutex: Use prng instead of rng to avoid hangs at bootup

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 08, 2023 at 06:26:41AM +0000, John Stultz wrote:
> Booting w/ qemu without kvm, I noticed we'd sometimes seem to get
> stuck in get_random_u32_below(). This seems potentially to be
> entropy exhaustion (with the test module linked statically, it
> runs pretty early in the bootup).
> 
> I'm not 100% sure on this, but this patch switches to use the
> prng instead since we don't need true randomness, just mixed up
> orders for testing ww_mutex lock acquisitions.
> 
> With this patch, I no longer see hangs in get_random_u32_below()
> 
> Feedback would be appreciated!

Jason, I thought part of the 'recent' random rework was avoiding the
exhaustion problem, could you please give an opinion on the below?

> Signed-off-by: John Stultz <jstultz@google.com>
> ---
>  kernel/locking/test-ww_mutex.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/test-ww_mutex.c b/kernel/locking/test-ww_mutex.c
> index 93cca6e69860..9bceba65858a 100644
> --- a/kernel/locking/test-ww_mutex.c
> +++ b/kernel/locking/test-ww_mutex.c
> @@ -9,7 +9,7 @@
>  #include <linux/delay.h>
>  #include <linux/kthread.h>
>  #include <linux/module.h>
> -#include <linux/random.h>
> +#include <linux/prandom.h>
>  #include <linux/slab.h>
>  #include <linux/ww_mutex.h>
>  
> @@ -386,6 +386,19 @@ struct stress {
>  	int nlocks;
>  };
>  
> +struct rnd_state rng;
> +DEFINE_SPINLOCK(rng_lock);
> +
> +static inline u32 prandom_u32_below(u32 ceil)
> +{
> +	u32 ret;
> +
> +	spin_lock(&rng_lock);
> +	ret = prandom_u32_state(&rng) % ceil;
> +	spin_unlock(&rng_lock);
> +	return ret;
> +}
> +
>  static int *get_random_order(int count)
>  {
>  	int *order;
> @@ -399,7 +412,7 @@ static int *get_random_order(int count)
>  		order[n] = n;
>  
>  	for (n = count - 1; n > 1; n--) {
> -		r = get_random_u32_below(n + 1);
> +		r = prandom_u32_below(n + 1);
>  		if (r != n) {
>  			tmp = order[n];
>  			order[n] = order[r];
> @@ -625,6 +638,8 @@ static int __init test_ww_mutex_init(void)
>  
>  	printk(KERN_INFO "Beginning ww mutex selftests\n");
>  
> +	prandom_seed_state(&rng, get_random_u64());
> +
>  	wq = alloc_workqueue("test-ww_mutex", WQ_UNBOUND, 0);
>  	if (!wq)
>  		return -ENOMEM;
> -- 
> 2.41.0.640.ga95def55d0-goog
>