[PATCH] md: comment add_stripe_bio

From: Dan Williams

Document the overloading of struct bio fields.

Signed-off-by: Dan Williams
---

[ drop this if you think it is too much commenting/unnecessary, but I figured I would leave some
breadcrumbs for the next guy. ]

drivers/md/raid5.c | 26 ++++++++++++++++++++++----
1 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 061375e..065b8dc 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -1231,10 +1231,13 @@ static void compute_block_2(struct stripe_head *sh, int dd_idx1, int dd_idx2)



-/*
- * Each stripe/dev can have one or more bion attached.
- * toread/towrite point to the first in a chain.
- * The bi_next chain must be in order.
+/* add_stripe_bio - attach a bio to the toread/towrite list for an
+ * rdev in the given stripe. This routine assumes that the toread/towrite
+ * lists are in submission order
+ * @sh: stripe targeted by bi->bi_sector
+ * @bi: bio to add (this routines assumes ownership of the bi->bi_next field)
+ * @dd_idx: data disk determined from the logical sector
+ * @forwrite: read/write flag
*/
static int add_stripe_bio(struct stripe_head *sh, struct bio *bi, int dd_idx, int forwrite)
{
@@ -1249,12 +1252,18 @@ static int add_stripe_bio(struct stripe_head *sh, struct bio *bi, int dd_idx, in

spin_lock(&sh->lock);
spin_lock_irq(&conf->device_lock);
+
+ /* pick the list to manipulate */
if (forwrite) {
bip = &sh->dev[dd_idx].towrite;
if (*bip == NULL && sh->dev[dd_idx].written == NULL)
firstwrite = 1;
} else
bip = &sh->dev[dd_idx].toread;
+
+ /* scroll through the list to see if this bio overlaps with a
+ * pending request
+ */
while (*bip && (*bip)->bi_sector < bi->bi_sector) {
if ((*bip)->bi_sector + ((*bip)->bi_size >> 9) > bi->bi_sector)
goto overlap;
@@ -1263,10 +1272,19 @@ static int add_stripe_bio(struct stripe_head *sh, struct bio *bi, int dd_idx, in
if (*bip && (*bip)->bi_sector < bi->bi_sector + ((bi->bi_size)>>9))
goto overlap;

+ /* add this bio into the chain and make sure we are not dropping
+ * a link that was previously established
+ */
BUG_ON(*bip && bi->bi_next && (*bip) != bi->bi_next);
if (*bip)
bi->bi_next = *bip;
+
+ /* attach the bio to the end of the list, if this is the first bio
+ * added then we are directly manipulating toread/towrite
+ */
*bip = bi;
+
+ /* keep count of the number of stripes that this bio touches */
bi->bi_phys_segments ++;
spin_unlock_irq(&conf->device_lock);
spin_unlock(&sh->lock);
-
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