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libsys: use linked list for free grants

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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
This commit is contained in:
David van Moolenbroek 2015-12-29 15:37:11 +00:00 committed by Lionel Sambuc
parent ca779acd72
commit efc775b4c8
2 changed files with 27 additions and 10 deletions
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4
minix/include/minix/safecopies.h
View File

@ -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 */

33
minix/lib/libsys/safecopies.c
View File

@ -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;
}