Hi Dimitris, On Wed, 21 Aug 2013 15:02:35 +0200 David Jander <da...@protonic.nl> wrote:
> The functionality of rbtree_ctx->cached_rbnode is broken. Remove it to > avoid hitting the wrong rbnode when locating a register. > rbnode register ranges can overlap, which is not a problem as long as > every lookup for a register returns the same rbnode. Therefor we need > to start searching from the top of the rb-tree _always_. Some clarification to this patch. I want to be careful in "attacking" such a core function that is used in quite a lot of places around the kernel, and it strikes me that this has been gone unnoticed so far. Here's the explanation: 1. If a driver initializes a regmap with a RB-tree cache, and starts writing to registers in some arbitrary order, you might get overlapping rbnodes: Suppose I have this rb-tree (a real case I happen to have): # cat /sys/kernel/debug/regmap/1-000a/rbtree 2-19 (24) 4-1b (24) 20-37 (24) 22-39 (24) 3c-53 (24) 100-117 (24) 104-11c (25) 11e-135 (24) 8 nodes, 193 registers, average 24 registers, used 626 bytes 2. Suppose you just added node 20-37 by writing to previously uncached register 20. Now this node would be in rbtree_ctx->cached_rbnode, and would get search first by the next access. Let's say that next access is register 30, then rbtree_ctx->cached_rbnode would be a hit on the node 20-37. Otherwise the hit would be in node 22-39 which is higher up the tree and should be searched first. Two different possible hits for register 30 with different contents! That cannot be right, right? Therefor I dare to declare the rbtree_ctx->cached_rbnode functionality broken, and removed it. I stumbled upon this problem while debugging a non-functioning audio codec (SGTL5000). This patch fixes the problem. Using a flat-cache in sgtl5000.c also fixes it, so at least if I didn't fix this bug the right way, there sure is a bug that needs to be fixed in regcache-rbtree.c. Best regards, -- David Jander Protonic Holland. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
