On Mon, May 18, 2009 at 06:00:12PM -0700, Dan Williams wrote:

+/**
+ * async_raid6_2data_recov - asynchronously calculate two missing data blocks
+ * @disks: number of disks in the RAID-6 array
+ * @bytes: block size
+ * @faila: first failed drive index
+ * @failb: second failed drive index
+ * @blocks: array of source pointers where the last two entries are p and q
+ * @submit: submission/completion modifiers
+ */
+struct dma_async_tx_descriptor *
+async_raid6_2data_recov(int disks, size_t bytes, int faila, int failb,
+			struct page **blocks, struct async_submit_ctl *submit)
+{

[...]

+	/* Dx = A*(P+Pxy) + B*(Q+Qxy) */
+	srcs[0] = dp;
+	srcs[1] = dq;
+	coef[0] = raid6_gfexi[failb-faila];

Here it's essential that faila < failb. This should either be clearly
documented and checked for, or (better) the function should swap
faila and failb if they are in the wrong order.

+	p = blocks[disks-2];
+	q = blocks[disks-1];
+
+	/* Compute syndrome with zero for the missing data page
+	   Use the dead data page as temporary storage for delta q */
+	dq = blocks[faila];
+	blocks[faila] = (void *)raid6_empty_zero_page;
+	blocks[disks-1] = dq;
+
+	submit->flags &= ~ASYNC_TX_ACK;
+	submit->cb_fn = NULL;
+	submit->cb_param = NULL;
+	tx = async_gen_syndrome(blocks, 0, disks, bytes, submit);
+	submit->depend_tx = tx;
+
+	/* Restore pointer table */
+	blocks[faila]   = dq;
+	blocks[disks-1] = q;
+
+	/* Now, pick the proper data tables */
+	coef = raid6_gfinv[raid6_gfexp[faila]];

Pick data tables? The formula for recovering the data block is

        (q + dq) / g^faila

(with g being the generator). What this line actually does is computing
g^{-faila}.

Regards
Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe