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

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Message-Id: <20230523-regcache-maple-sync-lock-v1-1-530e4d68dfab@kernel.org>
Date:   Tue, 23 May 2023 23:18:19 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, alsa-devel@...a-project.org,
        Mark Brown <broonie@...nel.org>
Subject: [PATCH] regmap: maple: Drop the RCU read lock while syncing
 registers

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>
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,
-- 
Mark Brown <broonie@...nel.org>