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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /lib/rwsem.c
downloadlinux-linaro-precise-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.gz
Linux-2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
Diffstat (limited to 'lib/rwsem.c')
-rw-r--r--lib/rwsem.c268
1 files changed, 268 insertions, 0 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c
new file mode 100644
index 00000000000..7644089ec8f
--- /dev/null
+++ b/lib/rwsem.c
@@ -0,0 +1,268 @@
+/* rwsem.c: R/W semaphores: contention handling functions
+ *
+ * Written by David Howells (dhowells@redhat.com).
+ * Derived from arch/i386/kernel/semaphore.c
+ */
+#include <linux/rwsem.h>
+#include <linux/sched.h>
+#include <linux/init.h>
+#include <linux/module.h>
+
+struct rwsem_waiter {
+ struct list_head list;
+ struct task_struct *task;
+ unsigned int flags;
+#define RWSEM_WAITING_FOR_READ 0x00000001
+#define RWSEM_WAITING_FOR_WRITE 0x00000002
+};
+
+#if RWSEM_DEBUG
+#undef rwsemtrace
+void rwsemtrace(struct rw_semaphore *sem, const char *str)
+{
+ printk("sem=%p\n", sem);
+ printk("(sem)=%08lx\n", sem->count);
+ if (sem->debug)
+ printk("[%d] %s({%08lx})\n", current->pid, str, sem->count);
+}
+#endif
+
+/*
+ * handle the lock release when processes blocked on it that can now run
+ * - if we come here from up_xxxx(), then:
+ * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
+ * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
+ * - there must be someone on the queue
+ * - the spinlock must be held by the caller
+ * - woken process blocks are discarded from the list after having task zeroed
+ * - writers are only woken if downgrading is false
+ */
+static inline struct rw_semaphore *
+__rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
+{
+ struct rwsem_waiter *waiter;
+ struct task_struct *tsk;
+ struct list_head *next;
+ signed long oldcount, woken, loop;
+
+ rwsemtrace(sem, "Entering __rwsem_do_wake");
+
+ if (downgrading)
+ goto dont_wake_writers;
+
+ /* if we came through an up_xxxx() call, we only only wake someone up
+ * if we can transition the active part of the count from 0 -> 1
+ */
+ try_again:
+ oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
+ - RWSEM_ACTIVE_BIAS;
+ if (oldcount & RWSEM_ACTIVE_MASK)
+ goto undo;
+
+ waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+
+ /* try to grant a single write lock if there's a writer at the front
+ * of the queue - note we leave the 'active part' of the count
+ * incremented by 1 and the waiting part incremented by 0x00010000
+ */
+ if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
+ goto readers_only;
+
+ /* We must be careful not to touch 'waiter' after we set ->task = NULL.
+ * It is an allocated on the waiter's stack and may become invalid at
+ * any time after that point (due to a wakeup from another source).
+ */
+ list_del(&waiter->list);
+ tsk = waiter->task;
+ mb();
+ waiter->task = NULL;
+ wake_up_process(tsk);
+ put_task_struct(tsk);
+ goto out;
+
+ /* don't want to wake any writers */
+ dont_wake_writers:
+ waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+ if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
+ goto out;
+
+ /* grant an infinite number of read locks to the readers at the front
+ * of the queue
+ * - note we increment the 'active part' of the count by the number of
+ * readers before waking any processes up
+ */
+ readers_only:
+ woken = 0;
+ do {
+ woken++;
+
+ if (waiter->list.next == &sem->wait_list)
+ break;
+
+ waiter = list_entry(waiter->list.next,
+ struct rwsem_waiter, list);
+
+ } while (waiter->flags & RWSEM_WAITING_FOR_READ);
+
+ loop = woken;
+ woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
+ if (!downgrading)
+ /* we'd already done one increment earlier */
+ woken -= RWSEM_ACTIVE_BIAS;
+
+ rwsem_atomic_add(woken, sem);
+
+ next = sem->wait_list.next;
+ for (; loop > 0; loop--) {
+ waiter = list_entry(next, struct rwsem_waiter, list);
+ next = waiter->list.next;
+ tsk = waiter->task;
+ mb();
+ waiter->task = NULL;
+ wake_up_process(tsk);
+ put_task_struct(tsk);
+ }
+
+ sem->wait_list.next = next;
+ next->prev = &sem->wait_list;
+
+ out:
+ rwsemtrace(sem, "Leaving __rwsem_do_wake");
+ return sem;
+
+ /* undo the change to count, but check for a transition 1->0 */
+ undo:
+ if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) != 0)
+ goto out;
+ goto try_again;
+}
+
+/*
+ * wait for a lock to be granted
+ */
+static inline struct rw_semaphore *
+rwsem_down_failed_common(struct rw_semaphore *sem,
+ struct rwsem_waiter *waiter, signed long adjustment)
+{
+ struct task_struct *tsk = current;
+ signed long count;
+
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+
+ /* set up my own style of waitqueue */
+ spin_lock_irq(&sem->wait_lock);
+ waiter->task = tsk;
+ get_task_struct(tsk);
+
+ list_add_tail(&waiter->list, &sem->wait_list);
+
+ /* we're now waiting on the lock, but no longer actively read-locking */
+ count = rwsem_atomic_update(adjustment, sem);
+
+ /* if there are no active locks, wake the front queued process(es) up */
+ if (!(count & RWSEM_ACTIVE_MASK))
+ sem = __rwsem_do_wake(sem, 0);
+
+ spin_unlock_irq(&sem->wait_lock);
+
+ /* wait to be given the lock */
+ for (;;) {
+ if (!waiter->task)
+ break;
+ schedule();
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ }
+
+ tsk->state = TASK_RUNNING;
+
+ return sem;
+}
+
+/*
+ * wait for the read lock to be granted
+ */
+struct rw_semaphore fastcall __sched *
+rwsem_down_read_failed(struct rw_semaphore *sem)
+{
+ struct rwsem_waiter waiter;
+
+ rwsemtrace(sem, "Entering rwsem_down_read_failed");
+
+ waiter.flags = RWSEM_WAITING_FOR_READ;
+ rwsem_down_failed_common(sem, &waiter,
+ RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
+
+ rwsemtrace(sem, "Leaving rwsem_down_read_failed");
+ return sem;
+}
+
+/*
+ * wait for the write lock to be granted
+ */
+struct rw_semaphore fastcall __sched *
+rwsem_down_write_failed(struct rw_semaphore *sem)
+{
+ struct rwsem_waiter waiter;
+
+ rwsemtrace(sem, "Entering rwsem_down_write_failed");
+
+ waiter.flags = RWSEM_WAITING_FOR_WRITE;
+ rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
+
+ rwsemtrace(sem, "Leaving rwsem_down_write_failed");
+ return sem;
+}
+
+/*
+ * handle waking up a waiter on the semaphore
+ * - up_read/up_write has decremented the active part of count if we come here
+ */
+struct rw_semaphore fastcall *rwsem_wake(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+
+ rwsemtrace(sem, "Entering rwsem_wake");
+
+ spin_lock_irqsave(&sem->wait_lock, flags);
+
+ /* do nothing if list empty */
+ if (!list_empty(&sem->wait_list))
+ sem = __rwsem_do_wake(sem, 0);
+
+ spin_unlock_irqrestore(&sem->wait_lock, flags);
+
+ rwsemtrace(sem, "Leaving rwsem_wake");
+
+ return sem;
+}
+
+/*
+ * downgrade a write lock into a read lock
+ * - caller incremented waiting part of count and discovered it still negative
+ * - just wake up any readers at the front of the queue
+ */
+struct rw_semaphore fastcall *rwsem_downgrade_wake(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+
+ rwsemtrace(sem, "Entering rwsem_downgrade_wake");
+
+ spin_lock_irqsave(&sem->wait_lock, flags);
+
+ /* do nothing if list empty */
+ if (!list_empty(&sem->wait_list))
+ sem = __rwsem_do_wake(sem, 1);
+
+ spin_unlock_irqrestore(&sem->wait_lock, flags);
+
+ rwsemtrace(sem, "Leaving rwsem_downgrade_wake");
+ return sem;
+}
+
+EXPORT_SYMBOL(rwsem_down_read_failed);
+EXPORT_SYMBOL(rwsem_down_write_failed);
+EXPORT_SYMBOL(rwsem_wake);
+EXPORT_SYMBOL(rwsem_downgrade_wake);
+#if RWSEM_DEBUG
+EXPORT_SYMBOL(rwsemtrace);
+#endif