2 files changed, 397 insertions, 0 deletions

diff --git a/firmware/kernel/include/mrsw_lock.h b/firmware/kernel/include/mrsw_lock.h
new file mode 100644
index 0000000000..fbfe1d405d
--- /dev/null
+++ b/firmware/kernel/include/mrsw_lock.h
@@ -0,0 +1,54 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2014 by Michael Sevakis
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef MRSW_LOCK_H
+#define MRSW_LOCK_H
+
+/* Multi-reader, single-writer object that allows mutltiple readers or a
+ * single writer thread access to a critical section.
+ *
+ * Readers and writers, as with a mutex, may claim the lock recursively for
+ * the same type of access they were already granted.
+ *
+ * Writers are trivially granted read access by ignoring the request; the
+ * object is not changed.
+ *
+ * Reader promotion to writer is NOT supported and a reader attempting write
+ * access will result in a deadlock. That could change but for now, be warned.
+ *
+ * Full priority inheritance is implemented.
+ */
+struct mrsw_lock
+{
+    int volatile count; /* rd/wr counter; >0 = reader(s), <0 = writer */
+    struct thread_entry *queue;
+    struct blocker_splay splay; /* priority inheritance info
+                                   for waiters */
+    uint8_t rdrecursion[MAXTHREADS]; /* per-thread reader recursion counts */
+    IF_COP( struct corelock cl; )
+};
+
+void mrsw_init(struct mrsw_lock *mrsw);
+void mrsw_read_acquire(struct mrsw_lock *mrsw);
+void mrsw_read_release(struct mrsw_lock *mrsw);
+void mrsw_write_acquire(struct mrsw_lock *mrsw);
+void mrsw_write_release(struct mrsw_lock *mrsw);
+
+#endif /* MRSW_LOCK_H */
diff --git a/firmware/kernel/mrsw_lock.c b/firmware/kernel/mrsw_lock.c
new file mode 100644
index 0000000000..42f43caec3
--- /dev/null
+++ b/firmware/kernel/mrsw_lock.c
@@ -0,0 +1,343 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2014 by Michael Sevakis
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#include <string.h>
+#include "config.h"
+#include "system.h"
+#include "thread.h"
+#include "kernel.h"
+#include "kernel-internal.h"
+
+#ifdef HAVE_PRIORITY_SCHEDULING
+
+static FORCE_INLINE void
+mrsw_reader_claim(struct mrsw_lock *mrsw, struct thread_entry *current,
+                  int count, unsigned int slotnum)
+{
+    /* no need to lock this; if a reader can claim, noone is in the queue */
+    threadbit_set_bit(&mrsw->splay.mask, slotnum);
+    mrsw->splay.blocker.thread = count == 1 ? current : NULL;
+}
+
+static FORCE_INLINE void
+mrsw_reader_relinquish(struct mrsw_lock *mrsw, struct thread_entry *current,
+                       int count, unsigned int slotnum)
+{
+    /* If no writer is queued or has ownership then noone is queued;
+       if a writer owns it, then the reader would be blocked instead.
+       Therefore, if the queue has threads, then the next after the
+       owning readers is a writer and this is not the last reader. */
+    if (mrsw->queue)
+    {
+        corelock_lock(&mrsw->splay.cl);
+    }
+
+    threadbit_clear_bit(&mrsw->splay.mask, slotnum);
+
+    if (count == 0)
+    {
+        /* There is noone waiting; we'd be calling mrsw_wakeup_writer()
+           at this time instead */
+        mrsw->splay.blocker.thread = NULL;
+        return;
+    }
+
+    if (count == 1)
+    {
+        KERNEL_ASSERT(threadbit_popcount(&mrsw->splay.mask) == 1,
+                      "mrsw_reader_relinquish() - "
+                      "threadbits has wrong popcount: %d\n",
+                      threadbit_popcount(&mrsw->splay.mask));
+        /* switch owner to sole remaining reader */
+        slotnum = threadbit_ffs(&mrsw->splay.mask);
+        mrsw->splay.blocker.thread = thread_id_entry(slotnum);
+    }
+
+    if (mrsw->queue)
+    {
+        priority_disinherit(current, &mrsw->splay.blocker);
+        corelock_unlock(&mrsw->splay.cl);
+    }
+}
+
+static FORCE_INLINE unsigned int
+mrsw_reader_wakeup_writer(struct mrsw_lock *mrsw, unsigned int slotnum)
+{
+    threadbit_clear_bit(&mrsw->splay.mask, slotnum);
+    return wakeup_thread(&mrsw->queue, WAKEUP_TRANSFER);
+}
+
+static FORCE_INLINE unsigned int
+mrsw_writer_wakeup_writer(struct mrsw_lock *mrsw)
+{
+    return wakeup_thread(&mrsw->queue, WAKEUP_TRANSFER);
+}
+
+static FORCE_INLINE unsigned int
+mrsw_writer_wakeup_readers(struct mrsw_lock *mrsw)
+{
+    unsigned int result = wakeup_thread(&mrsw->queue, WAKEUP_TRANSFER_MULTI);
+    mrsw->count = thread_self_entry()->retval;
+    return result;
+}
+
+#else /* !HAVE_PRIORITY_SCHEDULING */
+
+#define mrsw_reader_claim(mrsw, current, count, slotnum) \
+    do {} while (0)
+
+#define mrsw_reader_relinquish(mrsw, current, count, slotnum) \
+    do {} while (0)
+
+static FORCE_INLINE unsigned int
+mrsw_reader_wakeup_writer(struct mrsw_lock *mrsw)
+{
+    mrsw->splay.blocker.thread = mrsw->queue;
+    return wakeup_thread(&mrsw->queue);
+}
+
+static FORCE_INLINE unsigned int
+mrsw_writer_wakeup_writer(struct mrsw_lock *mrsw)
+{
+    mrsw->splay.blocker.thread = mrsw->queue;
+    return wakeup_thread(&mrsw->queue);
+}
+
+static FORCE_INLINE unsigned int
+mrsw_writer_wakeup_readers(struct mrsw_lock *mrsw)
+{
+    mrsw->splay.blocker.thread = NULL;
+    for (int count = 0; mrsw->queue && mrsw->queue->retval != 0; count++)
+        wakeup_thread(&mrsw->queue);
+    return THREAD_OK;
+}
+
+#endif /* HAVE_PRIORITY_SCHEDULING */
+
+/** Public interface **/
+
+/* Initializes a multi-reader, single-writer object */
+void mrsw_init(struct mrsw_lock *mrsw)
+{
+    mrsw->count = 0;
+    mrsw->queue = NULL;
+    mrsw->splay.blocker.thread = NULL;
+#ifdef HAVE_PRIORITY_SCHEDULING
+    mrsw->splay.blocker.priority = PRIORITY_IDLE;
+    threadbit_clear(&mrsw->splay.mask);
+    corelock_init(&mrsw->splay.cl);
+    memset(mrsw->rdrecursion, 0, sizeof (mrsw->rdrecursion));
+#endif /* HAVE_PRIORITY_SCHEDULING */
+    corelock_init(&mrsw->cl);
+}
+
+/* Request reader thread lock. Any number of reader threads may enter which
+ * also locks-out all writer threads. Same thread may safely acquire read
+ * access recursively. The current writer is ignored and gets access. */
+void mrsw_read_acquire(struct mrsw_lock *mrsw)
+{
+    struct thread_entry *current = thread_self_entry();
+
+    if (current == mrsw->splay.blocker.thread IF_PRIO( && mrsw->count < 0 ))
+        return; /* Read request while holding write access; pass */
+
+#ifdef HAVE_PRIORITY_SCHEDULING
+    /* Track recursion counts for each thread:
+       IF_PRIO, mrsw->count just tracks unique owners  */
+    unsigned int slotnum = THREAD_ID_SLOT(current->id);
+    if (threadbit_test_bit(&mrsw->splay.mask, slotnum))
+    {
+        KERNEL_ASSERT(mrsw->rdrecursion[slotnum] < UINT8_MAX,
+                      "mrsw_read_acquire() - "
+                      "Thread %s did too many claims\n",
+                      current->name);
+        mrsw->rdrecursion[slotnum]++;
+        return;
+    }
+#endif /* HAVE_PRIORITY_SCHEDULING */
+
+    corelock_lock(&mrsw->cl);
+
+    int count = mrsw->count;
+
+    if (LIKELY(count >= 0 && !mrsw->queue))
+    {
+        /* Lock open to readers:
+           IFN_PRIO, mrsw->count tracks reader recursion */
+        mrsw->count = ++count;
+        mrsw_reader_claim(mrsw, current, count, slotnum);
+        corelock_unlock(&mrsw->cl);
+        return;
+    }
+
+    /* A writer owns it or is waiting; block... */
+    IF_COP( current->obj_cl = &mrsw->cl; )
+    IF_PRIO( current->blocker = &mrsw->splay.blocker; )
+    current->bqp = &mrsw->queue;
+    current->retval = 1; /* indicate multi-wake candidate */
+
+    disable_irq();
+    block_thread(current, TIMEOUT_BLOCK);
+
+    corelock_unlock(&mrsw->cl);
+
+    /* ...and turn control over to next thread */
+    switch_thread();
+}
+
+/* Release reader thread lockout of writer thread. The last reader to
+ * leave opens up access to writer threads. The current writer is ignored. */
+void mrsw_read_release(struct mrsw_lock *mrsw)
+{
+    struct thread_entry *current = thread_self_entry();
+
+    if (current == mrsw->splay.blocker.thread IF_PRIO( && mrsw->count < 0 ))
+        return; /* Read release while holding write access; ignore */
+
+#ifdef HAVE_PRIORITY_SCHEDULING
+    unsigned int slotnum = THREAD_ID_SLOT(current->id);
+    KERNEL_ASSERT(threadbit_test_bit(&mrsw->splay.mask, slotnum),
+                  "mrsw_read_release() -"
+                  " thread '%s' not reader\n",
+                  current->name);
+
+    uint8_t *rdcountp = &mrsw->rdrecursion[slotnum];
+    unsigned int rdcount = *rdcountp;
+    if (rdcount > 0)
+    {
+        /* Reader is releasing recursive claim */
+        *rdcountp = rdcount - 1;
+        return;
+    }
+#endif /* HAVE_PRIORITY_SCHEDULING */
+
+    corelock_lock(&mrsw->cl);
+    int count = mrsw->count;
+
+    KERNEL_ASSERT(count > 0, "mrsw_read_release() - no readers!\n");
+
+    unsigned int result = THREAD_NONE;
+    const int oldlevel = disable_irq_save();
+
+    if (--count == 0 && mrsw->queue)
+    {
+        /* No readers remain and a writer is waiting */
+        mrsw->count = -1;
+        result = mrsw_reader_wakeup_writer(mrsw IF_PRIO(, slotnum));
+    }
+    else
+    {
+        /* Giving up readership; we may be the last, or not */
+        mrsw->count = count;
+        mrsw_reader_relinquish(mrsw, current, count, slotnum);
+    }
+
+    restore_irq(oldlevel);
+    corelock_unlock(&mrsw->cl);
+
+#ifdef HAVE_PRIORITY_SCHEDULING
+    if (result & THREAD_SWITCH)
+        switch_thread();
+#endif /* HAVE_PRIORITY_SCHEDULING */
+    (void)result;
+}
+
+/* Acquire writer thread lock which provides exclusive access. If a thread
+ * that is holding read access calls this it will deadlock. The writer may
+ * safely call recursively. */
+void mrsw_write_acquire(struct mrsw_lock *mrsw)
+{
+    struct thread_entry *current = thread_self_entry();
+
+    if (current == mrsw->splay.blocker.thread)
+    {
+        /* Current thread already has write access */
+        mrsw->count--;
+        return;
+    }
+
+    corelock_lock(&mrsw->cl);
+
+    int count = mrsw->count;
+
+    if (LIKELY(count == 0))
+    {
+        /* Lock is open to a writer */
+        mrsw->count = -1;
+        mrsw->splay.blocker.thread = current;
+        corelock_unlock(&mrsw->cl);
+        return;
+    }
+
+    /* Readers present or a writer owns it - block... */
+    IF_COP( current->obj_cl = &mrsw->cl; )
+    IF_PRIO( current->blocker = &mrsw->splay.blocker; )
+    current->bqp = &mrsw->queue;
+    current->retval = 0; /* indicate single-wake candidate */
+
+    disable_irq();
+    block_thread(current, TIMEOUT_BLOCK);
+
+    corelock_unlock(&mrsw->cl);
+
+    /* ...and turn control over to next thread */
+    switch_thread();
+}
+
+/* Release writer thread lock and open the lock to readers and writers */
+void mrsw_write_release(struct mrsw_lock *mrsw)
+{
+    KERNEL_ASSERT(thread_self_entry() == mrsw->splay.blocker.thread,
+                  "mrsw_write_release->wrong thread (%s != %s)\n",
+                  thread_self_entry()->name,
+                  mrsw->splay.blocker.thread->name);
+
+    int count = mrsw->count;
+    if (count < -1)
+    {
+        /* This thread still owns write lock */
+        mrsw->count = count + 1;
+        return;
+    }
+
+    unsigned int result = THREAD_NONE;
+
+    corelock_lock(&mrsw->cl);
+    const int oldlevel = disable_irq_save();
+
+    if (mrsw->queue == NULL)           /* 'count' becomes zero */
+    {
+        mrsw->splay.blocker.thread = NULL;
+        mrsw->count = 0;
+    }
+    else if (mrsw->queue->retval == 0) /* 'count' stays -1 */
+        result = mrsw_writer_wakeup_writer(mrsw);
+    else                               /* 'count' becomes # of readers */
+        result = mrsw_writer_wakeup_readers(mrsw);
+
+    restore_irq(oldlevel);
+    corelock_unlock(&mrsw->cl);
+
+#ifdef HAVE_PRIORITY_SCHEDULING
+    if (result & THREAD_SWITCH)
+        switch_thread();
+#endif /* HAVE_PRIORITY_SCHEDULING */
+    (void)result;
+}