Changes in kernel/generic/src/synch/waitq.c [b169619:e88eb48] in mainline

File:

• kernel/generic/src/synch/waitq.c (modified) (10 diffs)

kernel/generic/src/synch/waitq.c

@@ -1 +1 @@
 /*
  * Copyright (c) 2001-2004 Jakub Jermar
- * Copyright (c) 2022 Jiří Zárevúcky
  * All rights reserved.
  *
@@ -49 +48 @@
 #include <synch/waitq.h>
 #include <synch/spinlock.h>
-#include <preemption.h>
 #include <proc/thread.h>
 #include <proc/scheduler.h>
@@ -59 +57 @@
 #include <adt/list.h>
 #include <arch/cycle.h>
-#include <memw.h>
+#include <mem.h>
+
+static void waitq_sleep_timed_out(void *);
+static void waitq_complete_wakeup(waitq_t *);
 
 /** Initialize wait queue
@@ -75 +76 @@
 }
 
-/**
- * Initialize wait queue with an initial number of queued wakeups
- * (or a wakeup debt if negative).
- */
-void waitq_initialize_with_count(waitq_t *wq, int count)
-{
-        waitq_initialize(wq);
-        wq->wakeup_balance = count;
+/** Handle timeout during waitq_sleep_timeout() call
+ *
+ * This routine is called when waitq_sleep_timeout() times out.
+ * Interrupts are disabled.
+ *
+ * It is supposed to try to remove 'its' thread from the wait queue;
+ * it can eventually fail to achieve this goal when these two events
+ * overlap. In that case it behaves just as though there was no
+ * timeout at all.
+ *
+ * @param data Pointer to the thread that called waitq_sleep_timeout().
+ *
+ */
+void waitq_sleep_timed_out(void *data)
+{
+        thread_t *thread = (thread_t *) data;
+        bool do_wakeup = false;
+        DEADLOCK_PROBE_INIT(p_wqlock);
+
+        irq_spinlock_lock(&threads_lock, false);
+        if (!thread_exists(thread))
+                goto out;
+
+grab_locks:
+        irq_spinlock_lock(&thread->lock, false);
+
+        waitq_t *wq;
+        if ((wq = thread->sleep_queue)) {  /* Assignment */
+                if (!irq_spinlock_trylock(&wq->lock)) {
+                        irq_spinlock_unlock(&thread->lock, false);
+                        DEADLOCK_PROBE(p_wqlock, DEADLOCK_THRESHOLD);
+                        /* Avoid deadlock */
+                        goto grab_locks;
+                }
+
+                list_remove(&thread->wq_link);
+                thread->saved_context = thread->sleep_timeout_context;
+                do_wakeup = true;
+                if (thread->sleep_composable)
+                        wq->ignore_wakeups++;
+                thread->sleep_queue = NULL;
+                irq_spinlock_unlock(&wq->lock, false);
+        }
+
+        thread->timeout_pending = false;
+        irq_spinlock_unlock(&thread->lock, false);
+
+        if (do_wakeup)
+                thread_ready(thread);
+
+out:
+        irq_spinlock_unlock(&threads_lock, false);
+}
+
+/** Interrupt sleeping thread.
+ *
+ * This routine attempts to interrupt a thread from its sleep in
+ * a waitqueue. If the thread is not found sleeping, no action
+ * is taken.
+ *
+ * The threads_lock must be already held and interrupts must be
+ * disabled upon calling this function.
+ *
+ * @param thread Thread to be interrupted.
+ *
+ */
+void waitq_interrupt_sleep(thread_t *thread)
+{
+        bool do_wakeup = false;
+        DEADLOCK_PROBE_INIT(p_wqlock);
+
+        /*
+         * The thread is quaranteed to exist because
+         * threads_lock is held.
+         */
+
+grab_locks:
+        irq_spinlock_lock(&thread->lock, false);
+
+        waitq_t *wq;
+        if ((wq = thread->sleep_queue)) {  /* Assignment */
+                if (!(thread->sleep_interruptible)) {
+                        /*
+                         * The sleep cannot be interrupted.
+                         */
+                        irq_spinlock_unlock(&thread->lock, false);
+                        return;
+                }
+
+                if (!irq_spinlock_trylock(&wq->lock)) {
+                        /* Avoid deadlock */
+                        irq_spinlock_unlock(&thread->lock, false);
+                        DEADLOCK_PROBE(p_wqlock, DEADLOCK_THRESHOLD);
+                        goto grab_locks;
+                }
+
+                if ((thread->timeout_pending) &&
+                    (timeout_unregister(&thread->sleep_timeout)))
+                        thread->timeout_pending = false;
+
+                list_remove(&thread->wq_link);
+                thread->saved_context = thread->sleep_interruption_context;
+                if (thread->sleep_composable)
+                        wq->ignore_wakeups++;
+                do_wakeup = true;
+                thread->sleep_queue = NULL;
+                irq_spinlock_unlock(&wq->lock, false);
+        }
+
+        irq_spinlock_unlock(&thread->lock, false);
+
+        if (do_wakeup)
+                thread_ready(thread);
 }
 
@@ -88 +194 @@
         (((flags) & SYNCH_FLAGS_NON_BLOCKING) && ((usec) == 0))
 
-errno_t waitq_sleep(waitq_t *wq)
-{
-        return _waitq_sleep_timeout(wq, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
-}
-
-errno_t waitq_sleep_timeout(waitq_t *wq, uint32_t usec)
-{
-        return _waitq_sleep_timeout(wq, usec, SYNCH_FLAGS_NON_BLOCKING);
-}
-
 /** Sleep until either wakeup, timeout or interruption occurs
  *
+ * This is a sleep implementation which allows itself to time out or to be
+ * interrupted from the sleep, restoring a failover context.
+ *
  * Sleepers are organised in a FIFO fashion in a structure called wait queue.
  *
- * Other functions as waitq_sleep() and all the *_timeout() functions are
- * implemented using this function.
+ * This function is really basic in that other functions as waitq_sleep()
+ * and all the *_timeout() functions use it.
  *
  * @param wq    Pointer to wait queue.
@@ -109 +208 @@
  * @param flags Specify mode of the sleep.
  *
+ * @param[out] blocked  On return, regardless of the return code,
+ *                      `*blocked` is set to `true` iff the thread went to
+ *                      sleep.
+ *
  * The sleep can be interrupted only if the
  * SYNCH_FLAGS_INTERRUPTIBLE bit is specified in flags.
@@ -122 +225 @@
  * call will immediately return, reporting either success or failure.
  *
- * @return ETIMEOUT, meaning that the sleep timed out, or a nonblocking call
- *                   returned unsuccessfully.
- * @return EINTR, meaning that somebody interrupted the sleeping thread.
+ * @return EAGAIN, meaning that the sleep failed because it was requested
+ *                 as SYNCH_FLAGS_NON_BLOCKING, but there was no pending wakeup.
+ * @return ETIMEOUT, meaning that the sleep timed out.
+ * @return EINTR, meaning that somebody interrupted the sleeping
+ *         thread. Check the value of `*blocked` to see if the thread slept,
+ *         or if a pending interrupt forced it to return immediately.
  * @return EOK, meaning that none of the above conditions occured, and the
- *              thread was woken up successfuly by `waitq_wake_*()`.
- *
- */
-errno_t _waitq_sleep_timeout(waitq_t *wq, uint32_t usec, unsigned int flags)
+ *              thread was woken up successfuly by `waitq_wakeup()`. Check
+ *              the value of `*blocked` to see if the thread slept or if
+ *              the wakeup was already pending.
+ *
+ */
+errno_t waitq_sleep_timeout(waitq_t *wq, uint32_t usec, unsigned int flags, bool *blocked)
 {
         assert((!PREEMPTION_DISABLED) || (PARAM_NON_BLOCKING(flags, usec)));
-        return waitq_sleep_timeout_unsafe(wq, usec, flags, waitq_sleep_prepare(wq));
+
+        ipl_t ipl = waitq_sleep_prepare(wq);
+        bool nblocked;
+        errno_t rc = waitq_sleep_timeout_unsafe(wq, usec, flags, &nblocked);
+        waitq_sleep_finish(wq, nblocked, ipl);
+
+        if (blocked != NULL) {
+                *blocked = nblocked;
+        }
+        return rc;
 }
 
@@ -145 +262 @@
  *
  */
-wait_guard_t waitq_sleep_prepare(waitq_t *wq)
-{
-        ipl_t ipl = interrupts_disable();
+ipl_t waitq_sleep_prepare(waitq_t *wq)
+{
+        ipl_t ipl;
+
+restart:
+        ipl = interrupts_disable();
+
+        if (THREAD) {  /* Needed during system initiailzation */
+                /*
+                 * Busy waiting for a delayed timeout.
+                 * This is an important fix for the race condition between
+                 * a delayed timeout and a next call to waitq_sleep_timeout().
+                 * Simply, the thread is not allowed to go to sleep if
+                 * there are timeouts in progress.
+                 *
+                 */
+                irq_spinlock_lock(&THREAD->lock, false);
+
+                if (THREAD->timeout_pending) {
+                        irq_spinlock_unlock(&THREAD->lock, false);
+                        interrupts_restore(ipl);
+                        goto restart;
+                }
+
+                irq_spinlock_unlock(&THREAD->lock, false);
+        }
+
         irq_spinlock_lock(&wq->lock, false);
-        return (wait_guard_t) {
-                .ipl = ipl,
-        };
-}
-
-errno_t waitq_sleep_unsafe(waitq_t *wq, wait_guard_t guard)
-{
-        return waitq_sleep_timeout_unsafe(wq, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE, guard);
+        return ipl;
+}
+
+/** Finish waiting in a wait queue.
+ *
+ * This function restores interrupts to the state that existed prior
+ * to the call to waitq_sleep_prepare(). If necessary, the wait queue
+ * lock is released.
+ *
+ * @param wq       Wait queue.
+ * @param blocked  Out parameter of waitq_sleep_timeout_unsafe().
+ * @param ipl      Interrupt level returned by waitq_sleep_prepare().
+ *
+ */
+void waitq_sleep_finish(waitq_t *wq, bool blocked, ipl_t ipl)
+{
+        if (blocked) {
+                /*
+                 * Wait for a waitq_wakeup() or waitq_unsleep() to complete
+                 * before returning from waitq_sleep() to the caller. Otherwise
+                 * the caller might expect that the wait queue is no longer used
+                 * and deallocate it (although the wakeup on a another cpu has
+                 * not yet completed and is using the wait queue).
+                 *
+                 * Note that we have to do this for EOK and EINTR, but not
+                 * necessarily for ETIMEOUT where the timeout handler stops
+                 * using the waitq before waking us up. To be on the safe side,
+                 * ensure the waitq is not in use anymore in this case as well.
+                 */
+                waitq_complete_wakeup(wq);
+        } else {
+                irq_spinlock_unlock(&wq->lock, false);
+        }
+
+        interrupts_restore(ipl);
 }
 
@@ -162 +330 @@
  *
  * This function implements logic of sleeping in a wait queue.
- * This call must be preceded by a call to waitq_sleep_prepare().
+ * This call must be preceded by a call to waitq_sleep_prepare()
+ * and followed by a call to waitq_sleep_finish().
  *
  * @param wq    See waitq_sleep_timeout().
@@ -173 +342 @@
  *
  */
-errno_t waitq_sleep_timeout_unsafe(waitq_t *wq, uint32_t usec, unsigned int flags, wait_guard_t guard)
-{
-        errno_t rc;
+errno_t waitq_sleep_timeout_unsafe(waitq_t *wq, uint32_t usec, unsigned int flags, bool *blocked)
+{
+        *blocked = false;
+
+        /* Checks whether to go to sleep at all */
+        if (wq->missed_wakeups) {
+                wq->missed_wakeups--;
+                return EOK;
+        } else {
+                if (PARAM_NON_BLOCKING(flags, usec)) {
+                        /* Return immediately instead of going to sleep */
+                        return EAGAIN;
+                }
+        }
 
         /*
-         * If true, and this thread's sleep returns without a wakeup
-         * (timed out or interrupted), waitq ignores the next wakeup.
-         * This is necessary for futex to be able to handle those conditions.
+         * Now we are firmly decided to go to sleep.
+         *
          */
-        bool sleep_composable = (flags & SYNCH_FLAGS_FUTEX);
-        bool interruptible = (flags & SYNCH_FLAGS_INTERRUPTIBLE);
-
-        if (wq->closed) {
-                rc = EOK;
-                goto exit;
-        }
-
-        /* Checks whether to go to sleep at all */
-        if (wq->wakeup_balance > 0) {
-                wq->wakeup_balance--;
-
-                rc = EOK;
-                goto exit;
-        }
-
-        if (PARAM_NON_BLOCKING(flags, usec)) {
-                /* Return immediately instead of going to sleep */
-                rc = ETIMEOUT;
-                goto exit;
-        }
-
-        /* Just for debugging output. */
-        atomic_store_explicit(&THREAD->sleep_queue, wq, memory_order_relaxed);
+        irq_spinlock_lock(&THREAD->lock, false);
+
+        THREAD->sleep_composable = (flags & SYNCH_FLAGS_FUTEX);
+
+        if (flags & SYNCH_FLAGS_INTERRUPTIBLE) {
+                /*
+                 * If the thread was already interrupted,
+                 * don't go to sleep at all.
+                 */
+                if (THREAD->interrupted) {
+                        irq_spinlock_unlock(&THREAD->lock, false);
+                        return EINTR;
+                }
+
+                /*
+                 * Set context that will be restored if the sleep
+                 * of this thread is ever interrupted.
+                 */
+                THREAD->sleep_interruptible = true;
+                if (!context_save(&THREAD->sleep_interruption_context)) {
+                        /* Short emulation of scheduler() return code. */
+                        THREAD->last_cycle = get_cycle();
+                        irq_spinlock_unlock(&THREAD->lock, false);
+                        return EINTR;
+                }
+        } else
+                THREAD->sleep_interruptible = false;
+
+        if (usec) {
+                /* We use the timeout variant. */
+                if (!context_save(&THREAD->sleep_timeout_context)) {
+                        /* Short emulation of scheduler() return code. */
+                        THREAD->last_cycle = get_cycle();
+                        irq_spinlock_unlock(&THREAD->lock, false);
+                        return ETIMEOUT;
+                }
+
+                THREAD->timeout_pending = true;
+                timeout_register(&THREAD->sleep_timeout, (uint64_t) usec,
+                    waitq_sleep_timed_out, THREAD);
+        }
+
+        list_append(&THREAD->wq_link, &wq->sleepers);
 
         /*
-         * This thread_t field is synchronized exclusively via
-         * waitq lock of the waitq currently listing it.
+         * Suspend execution.
+         *
          */
-        list_append(&THREAD->wq_link, &wq->sleepers);
-
-        /* Needs to be run when interrupts are still disabled. */
-        deadline_t deadline = usec > 0 ?
-            timeout_deadline_in_usec(usec) : DEADLINE_NEVER;
-
-        while (true) {
-                bool terminating = (thread_wait_start() == THREAD_TERMINATING);
-                if (terminating && interruptible) {
-                        rc = EINTR;
-                        goto exit;
-                }
-
-                irq_spinlock_unlock(&wq->lock, false);
-
-                bool timed_out = (thread_wait_finish(deadline) == THREAD_WAIT_TIMEOUT);
-
-                /*
-                 * We always need to re-lock the WQ, since concurrently running
-                 * waitq_wakeup() may still not have exitted.
-                 * If we didn't always do this, we'd risk waitq_wakeup() that woke us
-                 * up still running on another CPU even after this function returns,
-                 * and that would be an issue if the waitq is allocated locally to
-                 * wait for a one-off asynchronous event. We'd need more external
-                 * synchronization in that case, and that would be a pain.
-                 *
-                 * On the plus side, always regaining a lock simplifies cleanup.
-                 */
-                irq_spinlock_lock(&wq->lock, false);
-
-                if (!link_in_use(&THREAD->wq_link)) {
-                        /*
-                         * We were woken up by the desired event. Return success,
-                         * regardless of any concurrent timeout or interruption.
-                         */
-                        rc = EOK;
-                        goto exit;
-                }
-
-                if (timed_out) {
-                        rc = ETIMEOUT;
-                        goto exit;
-                }
-
-                /* Interrupted for some other reason. */
-        }
-
-exit:
-        if (THREAD)
-                list_remove(&THREAD->wq_link);
-
-        if (rc != EOK && sleep_composable)
-                wq->wakeup_balance--;
-
-        if (THREAD)
-                atomic_store_explicit(&THREAD->sleep_queue, NULL, memory_order_relaxed);
-
+        THREAD->state = Sleeping;
+        THREAD->sleep_queue = wq;
+
+        /*
+         * Must be before entry to scheduler, because there are multiple
+         * return vectors.
+         */
+        *blocked = true;
+
+        irq_spinlock_unlock(&THREAD->lock, false);
+
+        /* wq->lock is released in scheduler_separated_stack() */
+        scheduler();
+
+        return EOK;
+}
+
+/** Wake up first thread sleeping in a wait queue
+ *
+ * Wake up first thread sleeping in a wait queue. This is the SMP- and IRQ-safe
+ * wrapper meant for general use.
+ *
+ * Besides its 'normal' wakeup operation, it attempts to unregister possible
+ * timeout.
+ *
+ * @param wq   Pointer to wait queue.
+ * @param mode Wakeup mode.
+ *
+ */
+void waitq_wakeup(waitq_t *wq, wakeup_mode_t mode)
+{
+        irq_spinlock_lock(&wq->lock, true);
+        _waitq_wakeup_unsafe(wq, mode);
+        irq_spinlock_unlock(&wq->lock, true);
+}
+
+/** If there is a wakeup in progress actively waits for it to complete.
+ *
+ * The function returns once the concurrently running waitq_wakeup()
+ * exits. It returns immediately if there are no concurrent wakeups
+ * at the time.
+ *
+ * Interrupts must be disabled.
+ *
+ * Example usage:
+ * @code
+ * void callback(waitq *wq)
+ * {
+ *     // Do something and notify wait_for_completion() that we're done.
+ *     waitq_wakeup(wq);
+ * }
+ * void wait_for_completion(void)
+ * {
+ *     waitq wg;
+ *     waitq_initialize(&wq);
+ *     // Run callback() in the background, pass it wq.
+ *     do_asynchronously(callback, &wq);
+ *     // Wait for callback() to complete its work.
+ *     waitq_sleep(&wq);
+ *     // callback() completed its work, but it may still be accessing
+ *     // wq in waitq_wakeup(). Therefore it is not yet safe to return
+ *     // from waitq_sleep() or it would clobber up our stack (where wq
+ *     // is stored). waitq_sleep() ensures the wait queue is no longer
+ *     // in use by invoking waitq_complete_wakeup() internally.
+ *
+ *     // waitq_sleep() returned, it is safe to free wq.
+ * }
+ * @endcode
+ *
+ * @param wq  Pointer to a wait queue.
+ */
+static void waitq_complete_wakeup(waitq_t *wq)
+{
+        assert(interrupts_disabled());
+
+        irq_spinlock_lock(&wq->lock, false);
         irq_spinlock_unlock(&wq->lock, false);
-        interrupts_restore(guard.ipl);
-        return rc;
-}
-
-static void _wake_one(waitq_t *wq)
-{
-        /* Pop one thread from the queue and wake it up. */
-        thread_t *thread = list_get_instance(list_first(&wq->sleepers), thread_t, wq_link);
+}
+
+/** Internal SMP- and IRQ-unsafe version of waitq_wakeup()
+ *
+ * This is the internal SMP- and IRQ-unsafe version of waitq_wakeup(). It
+ * assumes wq->lock is already locked and interrupts are already disabled.
+ *
+ * @param wq   Pointer to wait queue.
+ * @param mode If mode is WAKEUP_FIRST, then the longest waiting
+ *             thread, if any, is woken up. If mode is WAKEUP_ALL, then
+ *             all waiting threads, if any, are woken up. If there are
+ *             no waiting threads to be woken up, the missed wakeup is
+ *             recorded in the wait queue.
+ *
+ */
+void _waitq_wakeup_unsafe(waitq_t *wq, wakeup_mode_t mode)
+{
+        size_t count = 0;
+
+        assert(interrupts_disabled());
+        assert(irq_spinlock_locked(&wq->lock));
+
+        if (wq->ignore_wakeups > 0) {
+                if (mode == WAKEUP_FIRST) {
+                        wq->ignore_wakeups--;
+                        return;
+                }
+                wq->ignore_wakeups = 0;
+        }
+
+loop:
+        if (list_empty(&wq->sleepers)) {
+                wq->missed_wakeups++;
+                if ((count) && (mode == WAKEUP_ALL))
+                        wq->missed_wakeups--;
+
+                return;
+        }
+
+        count++;
+        thread_t *thread = list_get_instance(list_first(&wq->sleepers),
+            thread_t, wq_link);
+
+        /*
+         * Lock the thread prior to removing it from the wq.
+         * This is not necessary because of mutual exclusion
+         * (the link belongs to the wait queue), but because
+         * of synchronization with waitq_sleep_timed_out()
+         * and thread_interrupt_sleep().
+         *
+         * In order for these two functions to work, the following
+         * invariant must hold:
+         *
+         * thread->sleep_queue != NULL <=> thread sleeps in a wait queue
+         *
+         * For an observer who locks the thread, the invariant
+         * holds only when the lock is held prior to removing
+         * it from the wait queue.
+         *
+         */
+        irq_spinlock_lock(&thread->lock, false);
         list_remove(&thread->wq_link);
-        thread_wakeup(thread);
-}
-
-/**
- * Meant for implementing condvar signal.
- * Always wakes one thread if there are any sleeping,
- * has no effect if no threads are waiting for wakeup.
- */
-void waitq_signal(waitq_t *wq)
-{
+
+        if ((thread->timeout_pending) &&
+            (timeout_unregister(&thread->sleep_timeout)))
+                thread->timeout_pending = false;
+
+        thread->sleep_queue = NULL;
+        irq_spinlock_unlock(&thread->lock, false);
+
+        thread_ready(thread);
+
+        if (mode == WAKEUP_ALL)
+                goto loop;
+}
+
+/** Get the missed wakeups count.
+ *
+ * @param wq    Pointer to wait queue.
+ * @return      The wait queue's missed_wakeups count.
+ */
+int waitq_count_get(waitq_t *wq)
+{
+        int cnt;
+
         irq_spinlock_lock(&wq->lock, true);
-
-        if (!list_empty(&wq->sleepers))
-                _wake_one(wq);
-
+        cnt = wq->missed_wakeups;
         irq_spinlock_unlock(&wq->lock, true);
-}
-
-/**
- * Wakes up one thread sleeping on this waitq.
- * If there are no threads waiting, saves the wakeup so that the next sleep
- * returns immediately. If a previous failure in sleep created a wakeup debt
- * (see SYNCH_FLAGS_FUTEX) this debt is annulled and no thread is woken up.
- */
-void waitq_wake_one(waitq_t *wq)
+
+        return cnt;
+}
+
+/** Set the missed wakeups count.
+ *
+ * @param wq    Pointer to wait queue.
+ * @param val   New value of the missed_wakeups count.
+ */
+void waitq_count_set(waitq_t *wq, int val)
 {
         irq_spinlock_lock(&wq->lock, true);
-
-        if (!wq->closed) {
-                if (wq->wakeup_balance < 0 || list_empty(&wq->sleepers))
-                        wq->wakeup_balance++;
-                else
-                        _wake_one(wq);
-        }
-
+        wq->missed_wakeups = val;
         irq_spinlock_unlock(&wq->lock, true);
 }
 
-static void _wake_all(waitq_t *wq)
-{
-        while (!list_empty(&wq->sleepers))
-                _wake_one(wq);
-}
-
-/**
- * Wakes up all threads currently waiting on this waitq
- * and makes all future sleeps return instantly.
- */
-void waitq_close(waitq_t *wq)
-{
-        irq_spinlock_lock(&wq->lock, true);
-        wq->wakeup_balance = 0;
-        wq->closed = true;
-        _wake_all(wq);
-        irq_spinlock_unlock(&wq->lock, true);
-}
-
-/**
- * Wakes up all threads currently waiting on this waitq
- */
-void waitq_wake_all(waitq_t *wq)
-{
-        irq_spinlock_lock(&wq->lock, true);
-        wq->wakeup_balance = 0;
-        _wake_all(wq);
-        irq_spinlock_unlock(&wq->lock, true);
-}
-
 /** @}
  */