mmap_cache: Use two chains for hash collision.

struct sss_mc_rec had two hash members (hash1 and hash2) but only one next
member. This was a big problem in case of higher probability of hash collision.

structure sss_mc_rec will have two next members (next1, next2) with this patch.
next1 is related to hash1 and next2 is related to hash1.

Iterating over chains is changed, because we need to choose right next pointer.
Right next pointer will be chosen after comparing record hashes.
This behaviour is wrapped in function sss_mc_next_slot_with_hash.

Adding new record to chain is also changed. The situation is very similar to
iterating. We need to choose right next pointer (next1 or next2).
Right next pointer will be chosen after comparing record hashes.
Adding reference to next slot is wrapped in function
sss_mc_chain_slot_to_record_with_hash

Size of structure sss_mc_rec was increased from 32 bytes to 40 bytes.

Resolves:
https://fedorahosted.org/sssd/ticket/2049

1 files changed, 48 insertions, 18 deletions

diff --git a/src/responder/nss/nsssrv_mmap_cache.c b/src/responder/nss/nsssrv_mmap_cache.c
index 69fe2078..8655a1a0 100644
--- a/src/responder/nss/nsssrv_mmap_cache.c
+++ b/src/responder/nss/nsssrv_mmap_cache.c
@@ -93,6 +93,34 @@ struct sss_mc_ctx {
     else used = false; \
 } while (0)
 
+static inline
+uint32_t sss_mc_next_slot_with_hash(struct sss_mc_rec *rec,
+                                    uint32_t hash)
+{
+    if (rec->hash1 == hash) {
+        return rec->next1;
+    } else if (rec->hash2 == hash) {
+        return rec->next2;
+    } else {
+        /* it should never happen. */
+        return MC_INVALID_VAL;
+    }
+}
+
+static inline
+void sss_mc_chain_slot_to_record_with_hash(struct sss_mc_rec *rec,
+                                           uint32_t hash,
+                                           uint32_t slot)
+{
+    /* changing a single uint32_t is atomic, so there is no
+     * need to use barriers in this case */
+    if (rec->hash1 == hash) {
+        rec->next1 = slot;
+    } else if (rec->hash2 == hash) {
+        rec->next2 = slot;
+    }
+}
+
 /* This function will store corrupted memcache to disk for later
  * analysis. */
 static void  sss_mc_save_corrupted(struct sss_mc_ctx *mc_ctx)
@@ -196,13 +224,12 @@ static void sss_mc_add_rec_to_chain(struct sss_mc_ctx *mcc,
             /* rec already stored in hash chain */
             return;
         }
-        slot = cur->next;
+        slot = sss_mc_next_slot_with_hash(cur, hash);
     } while (slot != MC_INVALID_VAL);
     /* end of chain, append our record here */
 
-    /* changing a single uint32_t is atomic, so there is no
-     * need to use barriers in this case */
-    cur->next = MC_PTR_TO_SLOT(mcc->data_table, rec);
+    slot = MC_PTR_TO_SLOT(mcc->data_table, rec);
+    sss_mc_chain_slot_to_record_with_hash(cur, hash, slot);
 }
 
 static void sss_mc_rm_rec_from_chain(struct sss_mc_ctx *mcc,
@@ -230,19 +257,19 @@ static void sss_mc_rm_rec_from_chain(struct sss_mc_ctx *mcc,
     }
     cur = MC_SLOT_TO_PTR(mcc->data_table, slot, struct sss_mc_rec);
     if (cur == rec) {
-        mcc->hash_table[hash] = rec->next;
+        mcc->hash_table[hash] = sss_mc_next_slot_with_hash(rec, hash);
     } else {
-        slot = cur->next;
+        slot = sss_mc_next_slot_with_hash(cur, hash);
         while (slot != MC_INVALID_VAL) {
             prev = cur;
             cur = MC_SLOT_TO_PTR(mcc->data_table, slot, struct sss_mc_rec);
             if (cur == rec) {
-                /* changing a single uint32_t is atomic, so there is no
-                 * need to use barriers in this case */
-                prev->next = cur->next;
+                slot = sss_mc_next_slot_with_hash(cur, hash);
+
+                sss_mc_chain_slot_to_record_with_hash(prev, hash, slot);
                 slot = MC_INVALID_VAL;
             } else {
-                slot = cur->next;
+                slot = sss_mc_next_slot_with_hash(cur, hash);
             }
         }
     }
@@ -284,7 +311,8 @@ static void sss_mc_invalidate_rec(struct sss_mc_ctx *mcc,
                                         - sizeof(struct sss_mc_rec)));
     rec->len = MC_INVALID_VAL32;
     rec->expire = MC_INVALID_VAL64;
-    rec->next = MC_INVALID_VAL32;
+    rec->next1 = MC_INVALID_VAL32;
+    rec->next2 = MC_INVALID_VAL32;
     rec->hash1 = MC_INVALID_VAL32;
     rec->hash2 = MC_INVALID_VAL32;
     MC_LOWER_BARRIER(rec);
@@ -313,7 +341,7 @@ static bool sss_mc_is_valid_rec(struct sss_mc_ctx *mcc, struct sss_mc_rec *rec)
         return false;
     }
 
-    /* rec->next can be invalid if there are no next records */
+    /* next record can be invalid if there are no next records */
 
     if (rec->hash1 == MC_INVALID_VAL32) {
         return false;
@@ -322,7 +350,7 @@ static bool sss_mc_is_valid_rec(struct sss_mc_ctx *mcc, struct sss_mc_rec *rec)
         slot = mcc->hash_table[rec->hash1];
         while (slot != MC_INVALID_VAL32 && self != rec) {
             self = MC_SLOT_TO_PTR(mcc->data_table, slot, struct sss_mc_rec);
-            slot = self->next;
+            slot = sss_mc_next_slot_with_hash(self, rec->hash1);
         }
         if (self != rec) {
             return false;
@@ -333,7 +361,7 @@ static bool sss_mc_is_valid_rec(struct sss_mc_ctx *mcc, struct sss_mc_rec *rec)
         slot = mcc->hash_table[rec->hash2];
         while (slot != MC_INVALID_VAL32 && self != rec) {
             self = MC_SLOT_TO_PTR(mcc->data_table, slot, struct sss_mc_rec);
-            slot = self->next;
+            slot = sss_mc_next_slot_with_hash(self, rec->hash2);
         }
         if (self != rec) {
             return false;
@@ -527,7 +555,7 @@ static struct sss_mc_rec *sss_mc_find_record(struct sss_mc_ctx *mcc,
             break;
         }
 
-        slot = rec->next;
+        slot = sss_mc_next_slot_with_hash(rec, hash);
     }
 
     if (slot == MC_INVALID_VAL) {
@@ -583,7 +611,9 @@ static errno_t sss_mc_get_record(struct sss_mc_ctx **_mcc,
     /* mark as not valid yet */
     MC_RAISE_INVALID_BARRIER(rec);
     rec->len = rec_len;
-    rec->next = MC_INVALID_VAL;
+    rec->next1 = MC_INVALID_VAL;
+    rec->next2 = MC_INVALID_VAL;
+    rec->padding = MC_INVALID_VAL;
     MC_LOWER_BARRIER(rec);
 
     /* and now mark slots as used */
@@ -769,7 +799,7 @@ errno_t sss_mmap_cache_pw_invalidate_uid(struct sss_mc_ctx *mcc, uid_t uid)
             break;
         }
 
-        slot = rec->next;
+        slot = sss_mc_next_slot_with_hash(rec, hash);
     }
 
     if (slot == MC_INVALID_VAL) {
@@ -908,7 +938,7 @@ errno_t sss_mmap_cache_gr_invalidate_gid(struct sss_mc_ctx *mcc, gid_t gid)
             break;
         }
 
-        slot = rec->next;
+        slot = sss_mc_next_slot_with_hash(rec, hash);
     }
 
     if (slot == MC_INVALID_VAL) {