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!
268 lines
6.7 KiB
C
268 lines
6.7 KiB
C
/* rwsem.c: R/W semaphores: contention handling functions
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from arch/i386/kernel/semaphore.c
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*/
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#include <linux/rwsem.h>
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#include <linux/sched.h>
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#include <linux/init.h>
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#include <linux/module.h>
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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unsigned int flags;
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#define RWSEM_WAITING_FOR_READ 0x00000001
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#define RWSEM_WAITING_FOR_WRITE 0x00000002
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};
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#if RWSEM_DEBUG
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#undef rwsemtrace
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void rwsemtrace(struct rw_semaphore *sem, const char *str)
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{
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printk("sem=%p\n", sem);
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printk("(sem)=%08lx\n", sem->count);
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if (sem->debug)
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printk("[%d] %s({%08lx})\n", current->pid, str, sem->count);
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}
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#endif
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here from up_xxxx(), then:
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* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
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* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
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* - there must be someone on the queue
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* - the spinlock must be held by the caller
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only woken if downgrading is false
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*/
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static inline struct rw_semaphore *
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__rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
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{
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struct rwsem_waiter *waiter;
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struct task_struct *tsk;
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struct list_head *next;
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signed long oldcount, woken, loop;
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rwsemtrace(sem, "Entering __rwsem_do_wake");
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if (downgrading)
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goto dont_wake_writers;
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/* if we came through an up_xxxx() call, we only only wake someone up
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* if we can transition the active part of the count from 0 -> 1
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*/
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try_again:
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oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
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- RWSEM_ACTIVE_BIAS;
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if (oldcount & RWSEM_ACTIVE_MASK)
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goto undo;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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/* try to grant a single write lock if there's a writer at the front
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* of the queue - note we leave the 'active part' of the count
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* incremented by 1 and the waiting part incremented by 0x00010000
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*/
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if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
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goto readers_only;
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/* We must be careful not to touch 'waiter' after we set ->task = NULL.
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* It is an allocated on the waiter's stack and may become invalid at
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* any time after that point (due to a wakeup from another source).
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*/
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list_del(&waiter->list);
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tsk = waiter->task;
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mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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goto out;
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/* don't want to wake any writers */
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dont_wake_writers:
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
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goto out;
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/* grant an infinite number of read locks to the readers at the front
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* of the queue
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* - note we increment the 'active part' of the count by the number of
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* readers before waking any processes up
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*/
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readers_only:
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woken = 0;
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do {
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woken++;
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if (waiter->list.next == &sem->wait_list)
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break;
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waiter = list_entry(waiter->list.next,
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struct rwsem_waiter, list);
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} while (waiter->flags & RWSEM_WAITING_FOR_READ);
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loop = woken;
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woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
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if (!downgrading)
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/* we'd already done one increment earlier */
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woken -= RWSEM_ACTIVE_BIAS;
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rwsem_atomic_add(woken, sem);
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next = sem->wait_list.next;
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for (; loop > 0; loop--) {
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waiter = list_entry(next, struct rwsem_waiter, list);
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next = waiter->list.next;
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tsk = waiter->task;
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mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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}
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sem->wait_list.next = next;
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next->prev = &sem->wait_list;
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out:
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rwsemtrace(sem, "Leaving __rwsem_do_wake");
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return sem;
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/* undo the change to count, but check for a transition 1->0 */
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undo:
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if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) != 0)
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goto out;
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goto try_again;
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}
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/*
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* wait for a lock to be granted
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*/
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static inline struct rw_semaphore *
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rwsem_down_failed_common(struct rw_semaphore *sem,
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struct rwsem_waiter *waiter, signed long adjustment)
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{
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struct task_struct *tsk = current;
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signed long count;
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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/* set up my own style of waitqueue */
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spin_lock_irq(&sem->wait_lock);
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waiter->task = tsk;
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get_task_struct(tsk);
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list_add_tail(&waiter->list, &sem->wait_list);
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/* we're now waiting on the lock, but no longer actively read-locking */
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count = rwsem_atomic_update(adjustment, sem);
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/* if there are no active locks, wake the front queued process(es) up */
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if (!(count & RWSEM_ACTIVE_MASK))
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sem = __rwsem_do_wake(sem, 0);
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spin_unlock_irq(&sem->wait_lock);
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/* wait to be given the lock */
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for (;;) {
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if (!waiter->task)
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break;
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schedule();
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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}
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tsk->state = TASK_RUNNING;
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return sem;
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}
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/*
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* wait for the read lock to be granted
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*/
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struct rw_semaphore fastcall __sched *
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rwsem_down_read_failed(struct rw_semaphore *sem)
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{
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struct rwsem_waiter waiter;
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rwsemtrace(sem, "Entering rwsem_down_read_failed");
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waiter.flags = RWSEM_WAITING_FOR_READ;
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rwsem_down_failed_common(sem, &waiter,
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RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
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rwsemtrace(sem, "Leaving rwsem_down_read_failed");
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return sem;
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}
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/*
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* wait for the write lock to be granted
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*/
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struct rw_semaphore fastcall __sched *
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rwsem_down_write_failed(struct rw_semaphore *sem)
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{
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struct rwsem_waiter waiter;
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rwsemtrace(sem, "Entering rwsem_down_write_failed");
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waiter.flags = RWSEM_WAITING_FOR_WRITE;
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rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
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rwsemtrace(sem, "Leaving rwsem_down_write_failed");
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return sem;
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}
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/*
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* handle waking up a waiter on the semaphore
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* - up_read/up_write has decremented the active part of count if we come here
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*/
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struct rw_semaphore fastcall *rwsem_wake(struct rw_semaphore *sem)
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{
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unsigned long flags;
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rwsemtrace(sem, "Entering rwsem_wake");
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spin_lock_irqsave(&sem->wait_lock, flags);
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/* do nothing if list empty */
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, 0);
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spin_unlock_irqrestore(&sem->wait_lock, flags);
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rwsemtrace(sem, "Leaving rwsem_wake");
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return sem;
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}
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/*
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* downgrade a write lock into a read lock
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* - caller incremented waiting part of count and discovered it still negative
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* - just wake up any readers at the front of the queue
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*/
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struct rw_semaphore fastcall *rwsem_downgrade_wake(struct rw_semaphore *sem)
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{
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unsigned long flags;
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rwsemtrace(sem, "Entering rwsem_downgrade_wake");
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spin_lock_irqsave(&sem->wait_lock, flags);
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/* do nothing if list empty */
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, 1);
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spin_unlock_irqrestore(&sem->wait_lock, flags);
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rwsemtrace(sem, "Leaving rwsem_downgrade_wake");
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return sem;
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}
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EXPORT_SYMBOL(rwsem_down_read_failed);
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EXPORT_SYMBOL(rwsem_down_write_failed);
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EXPORT_SYMBOL(rwsem_wake);
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EXPORT_SYMBOL(rwsem_downgrade_wake);
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#if RWSEM_DEBUG
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EXPORT_SYMBOL(rwsemtrace);
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#endif
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