libsys: use linked list for free grants

With this change, obtaining an existing free grant is no longer an
operation of O(n) complexity.  As a result, the now-deprecated
getgrant/setgrant part of the grants API also no longer has a
performance advantage.

Change-Id: Ic19308a76924c6242f9784244a6b3600e561e0fe

diff --git a/minix/include/minix/safecopies.h b/minix/include/minix/safecopies.h
index 1276cc6bf3a09cccb876dcbb58007c778f8d9003..53580ff3d69d69da2b5f5a8488377e06c7108fcc 100644 (file)
--- a/minix/include/minix/safecopies.h
+++ b/minix/include/minix/safecopies.h
@@ -31,6 +31,10 @@ typedef struct {
                        size_t          cp_len;         /* size in bytes */
                        char            cp_reserved[8]; /* future use */
                } cp_magic;
+               struct {
+                       /* (free slot) */
+                       int             cp_next;        /* next free or -1 */
+               } cp_free;
        } cp_u;
        int cp_seq;                                     /* sequence number */
        char cp_reserved[4];                            /* future use */

diff --git a/minix/lib/libsys/safecopies.c b/minix/lib/libsys/safecopies.c
index f5b33c22327c17f5206d2daf6585646690a9c9ea..cec339f6aa871691c1a4617fdb199ae29fdf3054 100644 (file)
--- a/minix/lib/libsys/safecopies.c
+++ b/minix/lib/libsys/safecopies.c
@@ -42,6 +42,7 @@
 static cp_grant_t static_grants[NR_STATIC_GRANTS];
 static cp_grant_t *grants = NULL;
 static int ngrants = 0;
+static int freelist = -1;
 
 static void
 cpf_grow(void)
@@ -78,10 +79,16 @@ cpf_grow(void)
         * Make sure new slots are marked unused (CPF_USED is clear).
         * Also start with a zero sequence number, for consistency; since the
         * grant table is never shrunk, this introduces no issues by itself.
+        * Finally, form a new free list, in ascending order so that the lowest
+        * IDs get allocated first.  Both the zeroed sequence number and the
+        * ascending order are necessary so that the first grant to be
+        * allocated has a zero ID (see the live update comment below).
         */
        for(g = ngrants; g < new_size; g++) {
                new_grants[g].cp_flags = 0;
                new_grants[g].cp_seq = 0;
+               new_grants[g].cp_u.cp_free.cp_next =
+                   (g < new_size - 1) ? (g + 1) : freelist;
        }
 
        /* Inform kernel about new size (and possibly new location). */
@@ -92,6 +99,7 @@ cpf_grow(void)
 
        /* Update internal data. */
        if(grants && ngrants > 0 && grants != static_grants) free(grants);
+       freelist = ngrants;
        grants = new_grants;
        ngrants = new_size;
 }
@@ -105,17 +113,11 @@ cpf_new_grantslot(void)
  */
        int g;
 
-       /* Find free slot. */
-       for(g = 0; g < ngrants && (grants[g].cp_flags & CPF_USED); g++)
-               ;
-
-       assert(g <= ngrants);
-
-       /* No free slot found? */
-       if(g == ngrants) {
+       /* Obtain a free slot. */
+       if ((g = freelist) == -1) {
+               /* Table full - try to make the table larger. */
                cpf_grow();
-               assert(g <= ngrants); /* ngrants can't shrink. */
-               if(g == ngrants) {
+               if ((g = freelist) == -1) {
                        /* ngrants hasn't increased. */
                        errno = ENOSPC;
                        return -1;
@@ -129,6 +131,9 @@ cpf_new_grantslot(void)
        assert(g < ngrants);
        assert(!(grants[g].cp_flags & CPF_USED));
 
+       /* Take the slot off the free list, and return its slot number. */
+       freelist = grants[g].cp_u.cp_free.cp_next;
+
        return g;
 }
 
@@ -226,6 +231,14 @@ cpf_revoke(cp_grant_id_t grant)
        else
                grants[g].cp_seq = 0;
 
+       /*
+        * Put the grant back on the free list.  The list is single-headed, so
+        * the last freed grant will be the first to be reused.  Especially
+        * given the presence of sequence numbers, this is not a problem.
+        */
+       grants[g].cp_u.cp_free.cp_next = freelist;
+       freelist = g;
+
        return 0;
 }