From: William Lee Irwin III <wli@holomorphy.com>
Rewrite alloc_pidmap() to clarify control flow by eliminating all usage of
goto, honor pid_max and first available pid after last_pid semantics, make
only a single pass over the used portion of the pid bitmap, and update
copyrights to reflect ongoing maintenance by Ingo and myself.
Signed-off-by: William Irwin <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/kernel/pid.c | 111 +++++++++++++++++++++------------------------------
1 files changed, 46 insertions(+), 65 deletions(-)
diff -puN kernel/pid.c~rewrite-alloc_pidmap kernel/pid.c
--- 25/kernel/pid.c~rewrite-alloc_pidmap 2004-09-12 23:08:33.485233512 -0700
+++ 25-akpm/kernel/pid.c 2004-09-12 23:08:33.490232752 -0700
@@ -1,8 +1,8 @@
/*
* Generic pidhash and scalable, time-bounded PID allocator
*
- * (C) 2002 William Irwin, IBM
- * (C) 2002 Ingo Molnar, Red Hat
+ * (C) 2002-2004 William Irwin, Oracle
+ * (C) 2002-2004 Ingo Molnar, Red Hat
*
* pid-structures are backing objects for tasks sharing a given ID to chain
* against. There is very little to them aside from hashing them and
@@ -38,6 +38,9 @@ int last_pid;
#define PIDMAP_ENTRIES (PID_MAX_LIMIT/PAGE_SIZE/8)
#define BITS_PER_PAGE (PAGE_SIZE*8)
#define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1)
+#define mk_pid(map, off) (((map) - pidmap_array)*BITS_PER_PAGE + (off))
+#define find_next_offset(map, off) \
+ find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
/*
* PID-map pages start out as NULL, they get allocated upon
@@ -53,8 +56,6 @@ typedef struct pidmap {
static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
{ [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };
-static pidmap_t *map_limit = pidmap_array + PIDMAP_ENTRIES;
-
static spinlock_t pidmap_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;
fastcall void free_pidmap(int pid)
@@ -66,15 +67,18 @@ fastcall void free_pidmap(int pid)
atomic_inc(&map->nr_free);
}
-/*
- * Here we search for the next map that has free bits left.
- * Normally the next map has free PIDs.
- */
-static inline pidmap_t *next_free_map(pidmap_t *map, int *max_steps)
+int alloc_pidmap(void)
{
- while (--*max_steps) {
- if (++map == map_limit)
- map = pidmap_array;
+ int i, offset, max_scan, pid, last = last_pid;
+ pidmap_t *map;
+
+ pid = last + 1;
+ if (pid >= pid_max)
+ pid = RESERVED_PIDS;
+ offset = pid & BITS_PER_PAGE_MASK;
+ map = &pidmap_array[pid/BITS_PER_PAGE];
+ max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
+ for (i = 0; i <= max_scan; ++i) {
if (unlikely(!map->page)) {
unsigned long page = get_zeroed_page(GFP_KERNEL);
/*
@@ -87,62 +91,39 @@ static inline pidmap_t *next_free_map(pi
else
map->page = (void *)page;
spin_unlock(&pidmap_lock);
-
- if (!map->page)
+ if (unlikely(!map->page))
break;
}
- if (atomic_read(&map->nr_free))
- return map;
- }
- return NULL;
-}
-
-int alloc_pidmap(void)
-{
- int pid, offset, max_steps = PIDMAP_ENTRIES + 1;
- pidmap_t *map;
-
- pid = last_pid + 1;
- if (pid >= pid_max)
- pid = RESERVED_PIDS;
-
- offset = pid & BITS_PER_PAGE_MASK;
- map = pidmap_array + pid / BITS_PER_PAGE;
-
- if (likely(map->page && !test_and_set_bit(offset, map->page))) {
- /*
- * There is a small window for last_pid updates to race,
- * but in that case the next allocation will go into the
- * slowpath and that fixes things up.
- */
-return_pid:
- atomic_dec(&map->nr_free);
- last_pid = pid;
- return pid;
- }
-
- if (!offset || !atomic_read(&map->nr_free)) {
-next_map:
- map = next_free_map(map, &max_steps);
- if (!map)
- goto failure;
- offset = 0;
+ if (likely(atomic_read(&map->nr_free))) {
+ do {
+ if (!test_and_set_bit(offset, map->page)) {
+ atomic_dec(&map->nr_free);
+ last_pid = pid;
+ return pid;
+ }
+ offset = find_next_offset(map, offset);
+ pid = mk_pid(map, offset);
+ /*
+ * find_next_offset() found a bit, the pid from it
+ * is in-bounds, and if we fell back to the last
+ * bitmap block and the final block was the same
+ * as the starting point, pid is before last_pid.
+ */
+ } while (offset < BITS_PER_PAGE && pid < pid_max &&
+ (i != max_scan || pid < last ||
+ !((last+1) & BITS_PER_PAGE_MASK)));
+ }
+ if (map < &pidmap_array[(pid_max-1)/BITS_PER_PAGE]) {
+ ++map;
+ offset = 0;
+ } else {
+ map = &pidmap_array[0];
+ offset = RESERVED_PIDS;
+ if (unlikely(last == offset))
+ break;
+ }
+ pid = mk_pid(map, offset);
}
- /*
- * Find the next zero bit:
- */
-scan_more:
- offset = find_next_zero_bit(map->page, BITS_PER_PAGE, offset);
- if (offset >= BITS_PER_PAGE)
- goto next_map;
- if (test_and_set_bit(offset, map->page))
- goto scan_more;
-
- /* we got the PID: */
- pid = (map - pidmap_array) * BITS_PER_PAGE + offset;
- goto return_pid;
-
-failure:
return -1;
}
_