- Add an api for doing smp safe locks in userland.

 - umtx_lock() is defined as an inline in umtx.h.  It tries to do an
   uncontested acquire of a lock which falls back to the _umtx_lock()
   system-call if that fails.
 - umtx_unlock() is also an inline which falls back to _umtx_unlock() if the
   uncontested unlock fails.
 - Locks are keyed off of the thr_id_t of the currently running thread which
   is currently just the pointer to the 'struct thread' in kernel.
 - _umtx_lock() uses the proc pointer to synchronize access to blocked thread
   queues which are stored in the first blocked thread.

1 files changed, 303 insertions, 0 deletions

diff --git a/sys/kern/kern_umtx.c b/sys/kern/kern_umtx.c
new file mode 100644
index 0000000..a8dfe76
--- /dev/null
+++ b/sys/kern/kern_umtx.c
@@ -0,0 +1,303 @@
+/*
+ * Copyright (c) 2002, Jeffrey Roberson <jeff@freebsd.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice unmodified, this list of conditions, and the following
+ *    disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ *
+ */
+
+#include <sys/param.h>
+#include <sys/kernel.h>
+#include <sys/lock.h>
+#include <sys/mutex.h>
+#include <sys/proc.h>
+#include <sys/signalvar.h>
+#include <sys/sysent.h>
+#include <sys/systm.h>
+#include <sys/sysproto.h>
+#include <sys/thr.h>
+#include <sys/umtx.h>
+
+int
+_umtx_lock(struct thread *td, struct _umtx_lock_args *uap)
+    /* struct umtx *umtx */
+{
+	struct umtx *umtx;
+	struct thread *blocked;
+	intptr_t owner;
+	int error;
+
+	error = 0;
+
+	/*
+	 * Care must be exercised when dealing with this structure.  It
+	 * can fault on any access.
+	 */
+	umtx = uap->umtx;	
+
+	PROC_LOCK(td->td_proc);
+
+	for (;;) {
+		/*
+		 * Try the uncontested case.  This should be done in userland.
+		 */
+		owner = casuptr((intptr_t *)&umtx->u_owner,
+		    UMTX_UNOWNED, (intptr_t)td);
+
+		/* The acquire succeeded. */
+		if (owner == (intptr_t)td) {
+			error = 0;
+			goto out;
+		}
+
+		/* The address was invalid. */
+		if (owner == -1) {
+			error = EFAULT;
+			goto out;
+		}
+
+		if (owner & UMTX_CONTESTED)
+			break;
+
+		/*
+		 * Set the contested bit so that a release in user space
+		 * knows to use the system call for unlock.  If this fails
+		 * either some one else has acquired the lock or it has been
+		 * released.
+		 */
+		owner = casuptr((intptr_t *)&umtx->u_owner, owner, owner | UMTX_CONTESTED);
+
+		/* The contested bit was set. */
+		if (owner & UMTX_CONTESTED)
+			break;
+
+		/* The address was invalid. */
+		if (owner == -1) {
+			error = EFAULT;
+			goto out;
+		}
+		/* We didn't set the contested bit, try again. */
+	}
+
+	/*
+	 * We are now protected from further races via the proc lock.
+	 * If userland messes with their mutex without using cmpset
+	 * they will deadlock themselves but they will still be
+	 * killable via signals.
+	 */
+
+	if ((owner = fuword(&umtx->u_blocked)) == -1) {
+		error = EFAULT;
+		goto out;
+	}
+
+	if (owner == UMTX_UNOWNED) {
+		if (suword(&umtx->u_blocked, (long)td) == -1) {
+			error = EFAULT;
+			goto out;
+		}
+		/*
+		 * Other blocked threads will reside here.
+		 */
+		STAILQ_INIT(&td->td_umtxq);
+	} else {
+		FOREACH_THREAD_IN_PROC(td->td_proc, blocked)
+			if (blocked == (struct thread *)(owner))
+				break;
+
+		if (blocked == NULL) {
+			error = EINVAL;
+			goto out;
+		}
+		/*
+		 * Insert us onto the end of the TAILQ.
+		 */
+		STAILQ_INSERT_TAIL(&blocked->td_umtxq, td, td_umtx);
+	}
+
+	for (;;) {
+		/*
+		 * Sleep until we can acquire the lock.  We must still deliver
+		 * signals so that they are not deferred until we acquire the
+		 * lock which may be never.  The threads actual priority is
+		 * used to maintain proper ordering.
+		 */
+
+		error = msleep(&td->td_umtx, &td->td_proc->p_mtx,
+		    td->td_priority | PCATCH, "umtx", 0);
+
+		/*
+		 * When we are woken up we need to see if we now own the lock
+		 * even if a signal was delivered.
+		 */
+		if ((owner = fuword(&umtx->u_owner)) == -1) {
+			error = EFAULT;
+			break;
+		}
+		owner &= ~UMTX_CONTESTED;
+		if ((struct thread *)owner == td) {
+			error = 0;
+			break;
+		}
+
+		/*
+		 * We may have signals to deliver.
+		 */
+		if (error)
+			break;
+	}
+
+out:
+	PROC_UNLOCK(td->td_proc);
+
+	return (error);
+}
+
+int
+_umtx_unlock(struct thread *td, struct _umtx_unlock_args *uap)
+    /* struct umtx *umtx */
+{
+	struct thread *td0;
+	struct umtx *umtx;
+	intptr_t owner;
+	intptr_t blocked;
+	int error;
+
+	error = 0;
+	umtx = uap->umtx;
+
+	PROC_LOCK(td->td_proc);
+
+	/*
+	 * Make sure we own this mtx.
+	 *
+	 * XXX Need a {fu,su}ptr this is not correct on arch where
+	 * sizeof(intptr_t) != sizeof(long).
+	 */
+	if ((owner = fuword(&umtx->u_owner)) == -1) {
+		error = EFAULT;
+		goto out;
+	}
+	if ((struct thread *)(owner & ~UMTX_CONTESTED) != td) {
+		error = EPERM;
+		goto out;
+	}
+	/*
+	 * If we own it but it isn't contested then we can just release and
+	 * return.
+	 */
+	if ((owner & UMTX_CONTESTED) == 0) {
+		owner = casuptr((intptr_t *)&umtx->u_owner,
+		    (intptr_t)td, UMTX_UNOWNED);
+
+		if (owner == -1)
+			error = EFAULT;
+		/*
+		 * If this failed someone modified the memory without going
+		 * through this api.
+		 */
+		else if (owner != UMTX_UNOWNED)
+			error = EINVAL;
+		else
+			error = 0;
+
+		goto out;
+	}
+
+	/*
+	 * Since we own the mutex and the proc lock we are free to inspect
+	 * the blocked queue.  It must have one valid entry since the
+	 * CONTESTED bit was set.
+	 */
+	blocked = fuword(&umtx->u_blocked);
+	if (blocked == -1){ 
+		error = EFAULT;
+		goto out;
+	}
+	if (blocked == 0) {
+		error = EINVAL;
+		goto out;
+	}
+
+	FOREACH_THREAD_IN_PROC(td->td_proc, td0)
+		if (td0 == (struct thread *)blocked)
+			break;
+
+	if (td0 == NULL) {
+		error = EINVAL;
+		goto out;
+	}
+
+	if (!STAILQ_EMPTY(&td0->td_umtxq)) {
+		struct thread *next;
+
+		blocked |= UMTX_CONTESTED;
+		next = STAILQ_FIRST(&td0->td_umtxq);
+		if (suword(&umtx->u_blocked, (long)next) == -1) {
+			error = EFAULT;
+			goto out;
+		}
+		STAILQ_REMOVE_HEAD(&td0->td_umtxq, td_umtx);
+
+		/*
+		 * Switch the queue over to the next blocked thread.
+		 */
+		if (!STAILQ_EMPTY(&td0->td_umtxq)) {
+			next->td_umtxq = td0->td_umtxq;
+			STAILQ_INIT(&td0->td_umtxq);
+		} else
+			STAILQ_INIT(&next->td_umtxq);
+	} else {
+		if (suword(&umtx->u_blocked, UMTX_UNOWNED) == -1) {
+			error = EFAULT;
+			goto out;
+		}
+	}
+	/*
+	 * Now directly assign this mutex to the first thread that was
+	 * blocked on it.
+	 */
+	owner = casuptr((intptr_t *)&umtx->u_owner, owner, blocked);
+
+	/*
+	 * This will only happen if someone modifies the lock without going
+	 * through this api.
+	 */
+	if (owner != blocked) {
+		error = EINVAL;
+		goto out;
+	}
+	if (owner == -1) {
+		error = EFAULT;
+		goto out;
+	}
+	/* Success. */
+	error = 0;
+	wakeup(&td0->td_umtx);
+
+out:
+	PROC_UNLOCK(td->td_proc);
+
+	return (error);
+}