Changeset 3ede440 in mainline

Timestamp:

2019-02-01T22:30:53Z (6 years ago)

Author:

Jiří Zárevúcky <zarevucky.jiri@…>

Parents:

1a37496

git-author:

Jiří Zárevúcky <zarevucky.jiri@…> (2019-02-01 22:12:04)

git-committer:

Jiří Zárevúcky <zarevucky.jiri@…> (2019-02-01 22:30:53)

Message:

Change the meaning of counter in fibril mutex

Previously analogous to semaphore tokens, initially 1 and
counting downward into negative numbers.

Now the number of fibrils currently accessing or waiting for access
to the critical sections. Initially 0 (unlocked mutex), and counting
up. Never negative when used correctly. Also consistent with
readers/writers counts in rwlock.

This way is IMO easier to understand.

Location:

uspace/lib/c

Files:

• generic/thread/fibril_synch.c (modified) (4 diffs)
• include/fibril_synch.h (modified) (2 diffs)

uspace/lib/c/generic/thread/fibril_synch.c

@@ -165 +165 @@
 {
         fm->oi.owned_by = NULL;
-        fm->counter = 1;
+        fm->counter = 0;
         list_initialize(&fm->waiters);
 }
@@ -175 +175 @@
         futex_lock(&fibril_synch_futex);
 
-        if (fm->counter-- > 0) {
+        fm->counter++;
+
+        if (fm->counter == 1) {
+                /* I am the only one here. */
                 fm->oi.owned_by = f;
                 futex_unlock(&fibril_synch_futex);
@@ -196 +199 @@
 
         futex_lock(&fibril_synch_futex);
-        if (fm->counter > 0) {
-                fm->counter--;
+        if (fm->counter == 0) {
+                fm->counter++;
                 fm->oi.owned_by = (fibril_t *) fibril_get_id();
                 locked = true;
@@ -209 +212 @@
 {
         assert(fm->oi.owned_by == (fibril_t *) fibril_get_id());
-
-        if (fm->counter++ < 0) {
+        assert(fm->counter > 0);
+
+        fm->counter--;
+
+        if (fm->counter > 0) {
                 awaiter_t *wdp = list_pop(&fm->waiters, awaiter_t, link);
                 assert(wdp);

uspace/lib/c/include/fibril_synch.h

@@ -44 +44 @@
 typedef struct {
         fibril_owner_info_t oi;  /**< Keep this the first thing. */
-        int counter;
+        int counter;  /**< # of fibrils currently waiting for or running in the critical section. */
         list_t waiters;
 } fibril_mutex_t;
@@ -53 +53 @@
                         .owned_by = NULL \
                 }, \
-                .counter = 1, \
+                .counter = 0, \
                 .waiters = LIST_INITIALIZER((name).waiters), \
         }