linux-kernel - Re: [PATCH] regmap: maple: Drop the RCU read lock while syncing registers

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Date:   Tue, 23 May 2023 17:24:01 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org, alsa-devel@...a-project.org
Subject: Re: [PATCH] regmap: maple: Drop the RCU read lock while syncing
 registers



On 5/23/23 17:18, Mark Brown wrote:
> Unfortunately the maple tree requires us to explicitly lock it so we need
> to take the RCU read lock while iterating. When syncing this means that we
> end up trying to write out register values while holding the RCU read lock
> which triggers lockdep issues since that is an atomic context but most
> buses can't be used in atomic context. Pause the iteration and drop the
> lock for each register we check to avoid this.
> 
> Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>

Closes: https://github.com/thesofproject/linux/issues/4371
Tested-by: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>

> Signed-off-by: Mark Brown <broonie@...nel.org>
> ---
>  drivers/base/regmap/regcache-maple.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/regmap/regcache-maple.c b/drivers/base/regmap/regcache-maple.c
> index 9b1b559107ef..c2e3a0f6c218 100644
> --- a/drivers/base/regmap/regcache-maple.c
> +++ b/drivers/base/regmap/regcache-maple.c
> @@ -203,15 +203,18 @@ static int regcache_maple_sync(struct regmap *map, unsigned int min,
>  
>  	mas_for_each(&mas, entry, max) {
>  		for (r = max(mas.index, lmin); r <= min(mas.last, lmax); r++) {
> +			mas_pause(&mas);
> +			rcu_read_unlock();
>  			ret = regcache_sync_val(map, r, entry[r - mas.index]);
>  			if (ret != 0)
>  				goto out;
> +			rcu_read_lock();
>  		}
>  	}
>  
> -out:
>  	rcu_read_unlock();
>  
> +out:
>  	map->cache_bypass = false;
>  
>  	return ret;
> 
> ---
> base-commit: 44c026a73be8038f03dbdeef028b642880cf1511
> change-id: 20230523-regcache-maple-sync-lock-57ea356dc60b
> 
> Best regards,