2 files changed, 56 insertions, 1 deletions

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 17e188973428..74a567afb830 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -3116,7 +3116,7 @@ static inline int should_fail_request(struct bio *bio)
  * bi_sector for remaps as it sees fit.  So the values of these fields
  * should NOT be depended on after the call to generic_make_request.
  */
-void generic_make_request(struct bio *bio)
+static inline void __generic_make_request(struct bio *bio)
 {
         request_queue_t *q;
         sector_t maxsector;
@@ -3215,6 +3215,57 @@ end_io:
         } while (ret);
 }
 
+/*
+ * We only want one ->make_request_fn to be active at a time,
+ * else stack usage with stacked devices could be a problem.
+ * So use current->bio_{list,tail} to keep a list of requests
+ * submited by a make_request_fn function.
+ * current->bio_tail is also used as a flag to say if
+ * generic_make_request is currently active in this task or not.
+ * If it is NULL, then no make_request is active.  If it is non-NULL,
+ * then a make_request is active, and new requests should be added
+ * at the tail
+ */
+void generic_make_request(struct bio *bio)
+{
+        if (current->bio_tail) {
+                /* make_request is active */
+                *(current->bio_tail) = bio;
+                bio->bi_next = NULL;
+                current->bio_tail = &bio->bi_next;
+                return;
+        }
+        /* following loop may be a bit non-obvious, and so deserves some
+         * explanation.
+         * Before entering the loop, bio->bi_next is NULL (as all callers
+         * ensure that) so we have a list with a single bio.
+         * We pretend that we have just taken it off a longer list, so
+         * we assign bio_list to the next (which is NULL) and bio_tail
+         * to &bio_list, thus initialising the bio_list of new bios to be
+         * added.  __generic_make_request may indeed add some more bios
+         * through a recursive call to generic_make_request.  If it
+         * did, we find a non-NULL value in bio_list and re-enter the loop
+         * from the top.  In this case we really did just take the bio
+         * of the top of the list (no pretending) and so fixup bio_list and
+         * bio_tail or bi_next, and call into __generic_make_request again.
+         *
+         * The loop was structured like this to make only one call to
+         * __generic_make_request (which is important as it is large and
+         * inlined) and to keep the structure simple.
+         */
+        BUG_ON(bio->bi_next);
+        do {
+                current->bio_list = bio->bi_next;
+                if (bio->bi_next == NULL)
+                        current->bio_tail = &current->bio_list;
+                else
+                        bio->bi_next = NULL;
+                __generic_make_request(bio);
+                bio = current->bio_list;
+        } while (bio);
+        current->bio_tail = NULL; /* deactivate */
+}
+
 EXPORT_SYMBOL(generic_make_request);
 
 /**
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 17b72d88c4cb..e38c436ee12b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -88,6 +88,7 @@ struct sched_param {
 
 struct exec_domain;
 struct futex_pi_state;
+struct bio;
 
 /*
  * List of flags we want to share for kernel threads,
@@ -1014,6 +1015,9 @@ struct task_struct {
 /* journalling filesystem info */
         void *journal_info;
 
+/* stacked block device info */
+        struct bio *bio_list, **bio_tail;
+
 /* VM state */
         struct reclaim_state *reclaim_state;