md: handle operation chaining in raid5_run_ops

From: Dan Williams <dan.j.williams@intel.com>

Neil said:
> At the end of ops_run_compute5 you have:
>         /* ack now if postxor is not set to be run */
>         if (tx && !test_bit(STRIPE_OP_POSTXOR, &s->ops_run))
>                 async_tx_ack(tx);
>
> It looks odd having that test there.  Would it fit in raid5_run_ops
> better?

The intended global interpretation is that raid5_run_ops can build a chain
of xor and memcpy operations.  When MD registers the compute-xor it tells
async_tx to keep the operation handle around so that another item in the
dependency chain can be submitted. If we are just computing a block to
satisfy a read then we can terminate the chain immediately.  raid5_run_ops
gives a better context for this test since it cares about the entire chain.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Neil Brown <neilb@suse.de>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index c712460..456c3c2 100644 (file)
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -574,8 +574,7 @@ static void ops_complete_compute5(void *stripe_head_ref)
        release_stripe(sh);
 }
 
-static struct dma_async_tx_descriptor *
-ops_run_compute5(struct stripe_head *sh, unsigned long ops_request)
+static struct dma_async_tx_descriptor *ops_run_compute5(struct stripe_head *sh)
 {
        /* kernel stack size limits the total number of disks */
        int disks = sh->disks;
@@ -605,10 +604,6 @@ ops_run_compute5(struct stripe_head *sh, unsigned long ops_request)
                        ASYNC_TX_XOR_ZERO_DST, NULL,
                        ops_complete_compute5, sh);
 
-       /* ack now if postxor is not set to be run */
-       if (tx && !test_bit(STRIPE_OP_POSTXOR, &ops_request))
-               async_tx_ack(tx);
-
        return tx;
 }
 
@@ -813,8 +808,12 @@ static void raid5_run_ops(struct stripe_head *sh, unsigned long ops_request)
                overlap_clear++;
        }
 
-       if (test_bit(STRIPE_OP_COMPUTE_BLK, &ops_request))
-               tx = ops_run_compute5(sh, ops_request);
+       if (test_bit(STRIPE_OP_COMPUTE_BLK, &ops_request)) {
+               tx = ops_run_compute5(sh);
+               /* terminate the chain if postxor is not set to be run */
+               if (tx && !test_bit(STRIPE_OP_POSTXOR, &ops_request))
+                       async_tx_ack(tx);
+       }
 
        if (test_bit(STRIPE_OP_PREXOR, &ops_request))
                tx = ops_run_prexor(sh, tx);