[RFC] Add support for semaphore-like structure with support for asynchronous I/O

[RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

In NFSv4 we often want to serialize asynchronous RPC calls with ordinary
RPC calls (OPEN and CLOSE for instance). On paper, semaphores would
appear to fit the bill, however there is no support for asynchronous I/O
with semaphores.
<rant>What's more, trying to add that type of support is an exercise in
futility: there are currently 23 slightly different arch-dependent and
over-optimized versions of semaphores (not counting the different
versions of read/write semaphores).</rant>

Anyhow, the following is a simple implementation of semaphores designed
to satisfy the needs of those I/O subsystems that want to support
asynchronous behaviour too. Please comment.

Cheers,
  Trond

--------------------------------------
NFS: Add support for iosems.

 These act rather like semaphores, but also have support for asynchronous
 I/O, using the wait_queue_t callback features.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
---
 include/linux/iosem.h |   63 ++++++++++++++++++++++++++++++
 lib/Makefile          |    2 
 lib/iosem.c           |  103 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 167 insertions(+), 1 deletion(-)

Index: linux-2.6.12-rc1/include/linux/iosem.h
===================================================================
--- /dev/null
+++ linux-2.6.12-rc1/include/linux/iosem.h
@@ -0,0 +1,63 @@
+/*
+ * include/linux/iosem.h
+ *
+ * Copyright (C) 2005 Trond Myklebust <Trond.Myklebust@netapp.com>
+ *
+ * Definitions for iosems. These can act as mutexes, but unlike
+ * semaphores, their code is 100% arch-independent, and can therefore
+ * easily be expanded in order to provide for things like
+ * asynchronous I/O.
+ */
+
+#ifndef __LINUX_SEM_LOCK_H
+#define __LINUX_SEM_LOCK_H
+
+#ifdef __KERNEL__
+#include <linux/wait.h>
+#include <linux/workqueue.h>
+
+struct iosem {
+	unsigned long state;
+	wait_queue_head_t wait;
+};
+
+#define IOSEM_LOCK_EXCLUSIVE (24)
+/* #define IOSEM_LOCK_SHARED (25) */
+
+struct iosem_wait {
+	struct iosem *lock;
+	wait_queue_t wait;
+};
+
+struct iosem_work {
+	struct work_struct work;
+	struct iosem_wait waiter;
+};
+
+extern void FASTCALL(iosem_lock(struct iosem *lk));
+extern void FASTCALL(iosem_unlock(struct iosem *lk));
+extern int iosem_lock_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
+
+static inline void init_iosem(struct iosem *lk)
+{
+	lk->state = 0;
+	init_waitqueue_head(&lk->wait);
+}
+
+static inline void init_iosem_waiter(struct iosem_wait *waiter)
+{
+	waiter->lock = NULL;
+	init_waitqueue_entry(&waiter->wait, current);
+	INIT_LIST_HEAD(&waiter->wait.task_list);
+}
+
+static inline void init_iosem_work(struct iosem_work *wk, void (*func)(void *), void *data)
+{
+	INIT_WORK(&wk->work, func, data);
+}
+
+extern int FASTCALL(iosem_lock_and_schedule_work(struct iosem *lk, struct iosem_work *wk));
+extern int iosem_lock_and_schedule_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
+
+#endif /* __KERNEL__ */
+#endif /* __LINUX_SEM_LOCK_H */
Index: linux-2.6.12-rc1/lib/iosem.c
===================================================================
--- /dev/null
+++ linux-2.6.12-rc1/lib/iosem.c
@@ -0,0 +1,103 @@
+/*
+ * linux/fs/nfs/iosem.c
+ *
+ * Copyright (C) 2005 Trond Myklebust <Trond.Myklebust@netapp.com>
+ *
+ * A set of primitives for semaphore-like locks that also support notification
+ * callbacks for waiters.
+ */
+#include <linux/config.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/wait.h>
+#include <linux/iosem.h>
+
+static int fastcall __iosem_lock(struct iosem *lk, struct iosem_wait *waiter)
+{
+	int ret;
+
+	spin_lock(&lk->wait.lock);
+	if (lk->state != 0) {
+		waiter->lock = lk;
+		add_wait_queue_exclusive_locked(&lk->wait, &waiter->wait);
+		ret = -EINPROGRESS;
+	} else {
+		lk->state |= 1 << IOSEM_LOCK_EXCLUSIVE;
+		ret = 0;
+	}
+	spin_unlock(&lk->wait.lock);
+	return ret;
+}
+
+void fastcall iosem_unlock(struct iosem *lk)
+{
+	spin_lock(&lk->wait.lock);
+	lk->state &= ~(1 << IOSEM_LOCK_EXCLUSIVE);
+	wake_up_locked(&lk->wait);
+	spin_unlock(&lk->wait.lock);
+}
+EXPORT_SYMBOL(iosem_unlock);
+
+int iosem_lock_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+	struct iosem_wait *waiter = container_of(wait, struct iosem_wait, wait);
+	unsigned long *lk_state = &waiter->lock->state;
+	int ret = 0;
+
+	if (*lk_state == 0) {
+		ret = default_wake_function(wait, mode, sync, key);
+		if (ret) {
+			*lk_state |= 1 << IOSEM_LOCK_EXCLUSIVE;
+			list_del_init(&wait->task_list);
+		}
+	}
+	return ret;
+}
+
+void fastcall iosem_lock(struct iosem *lk)
+{
+	struct iosem_wait waiter;
+
+	might_sleep();
+
+	init_iosem_waiter(&waiter);
+	waiter.wait.func = iosem_lock_wake_function;
+
+	set_current_state(TASK_UNINTERRUPTIBLE);
+	if (__iosem_lock(lk, &waiter))
+		schedule();
+	__set_current_state(TASK_RUNNING);
+
+	BUG_ON(!list_empty(&waiter.wait.task_list));
+}
+EXPORT_SYMBOL(iosem_lock);
+
+int iosem_lock_and_schedule_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+	struct iosem_wait *waiter = container_of(wait, struct iosem_wait, wait);
+	struct iosem_work *wk = container_of(waiter, struct iosem_work, waiter);
+	unsigned long *lk_state = &waiter->lock->state;
+	int ret = 0;
+
+	if (*lk_state == 0) {
+		ret = schedule_work(&wk->work);
+		if (ret) {
+			*lk_state |= 1 << IOSEM_LOCK_EXCLUSIVE;
+			list_del_init(&wait->task_list);
+		}
+	}
+	return ret;
+}
+
+int fastcall iosem_lock_and_schedule_work(struct iosem *lk, struct iosem_work *wk)
+{
+	int ret;
+
+	init_iosem_waiter(&wk->waiter);
+	wk->waiter.wait.func = iosem_lock_and_schedule_function;
+	ret = __iosem_lock(lk, &wk->waiter);
+	if (ret == 0)
+		ret = schedule_work(&wk->work);
+	return ret;
+}
+EXPORT_SYMBOL(iosem_lock_and_schedule_work);
Index: linux-2.6.12-rc1/lib/Makefile
===================================================================
--- linux-2.6.12-rc1.orig/lib/Makefile
+++ linux-2.6.12-rc1/lib/Makefile
@@ -8,7 +8,7 @@ lib-y := errno.o ctype.o string.o vsprin
 	 bitmap.o extable.o kobject_uevent.o prio_tree.o sha1.o \
 	 halfmd4.o
 
-obj-y += sort.o parser.o
+obj-y += sort.o parser.o iosem.o
 
 ifeq ($(CONFIG_DEBUG_KOBJECT),y)
 CFLAGS_kobject.o += -DDEBUG

-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Andrew Morton]

Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
>
> In NFSv4 we often want to serialize asynchronous RPC calls with ordinary
> RPC calls (OPEN and CLOSE for instance). On paper, semaphores would
> appear to fit the bill, however there is no support for asynchronous I/O
> with semaphores.
> <rant>What's more, trying to add that type of support is an exercise in
> futility: there are currently 23 slightly different arch-dependent and
> over-optimized versions of semaphores (not counting the different
> versions of read/write semaphores).</rant>

Yeah.

> Anyhow, the following is a simple implementation of semaphores designed
> to satisfy the needs of those I/O subsystems that want to support
> asynchronous behaviour too. Please comment.
> 

So I've been staring at this code for a while and I Just Don't Get It.  If
I want some custom callback function to be called when someone does an
iosem_unlock(), how do I do it?

Or have I misunderstood the intent?  Some /* comments */ would be appropriate..

> +struct iosem {
> +	unsigned long state;
> +	wait_queue_head_t wait;
> +};
> +
> +#define IOSEM_LOCK_EXCLUSIVE (24)
> +/* #define IOSEM_LOCK_SHARED (25) */
> +
> +struct iosem_wait {
> +	struct iosem *lock;
> +	wait_queue_t wait;
> +};
> +
> +struct iosem_work {
> +	struct work_struct work;
> +	struct iosem_wait waiter;
> +};

Commenting the data structures is particularly helpful.

> +extern void FASTCALL(iosem_lock(struct iosem *lk));
> +extern void FASTCALL(iosem_unlock(struct iosem *lk));
> +extern int iosem_lock_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
> +
> +static inline void init_iosem(struct iosem *lk)
> +{
> +	lk->state = 0;
> +	init_waitqueue_head(&lk->wait);
> +}
> +
> +static inline void init_iosem_waiter(struct iosem_wait *waiter)
> +{
> +	waiter->lock = NULL;
> +	init_waitqueue_entry(&waiter->wait, current);
> +	INIT_LIST_HEAD(&waiter->wait.task_list);
> +}
> +
> +static inline void init_iosem_work(struct iosem_work *wk, void (*func)(void *), void *data)
> +{
> +	INIT_WORK(&wk->work, func, data);
> +}

I'd be inclined to call these iosem_init, iosem_waiter_init and
iosem_work_init.

> --- /dev/null
> +++ linux-2.6.12-rc1/lib/iosem.c
> @@ -0,0 +1,103 @@
> +/*
> + * linux/fs/nfs/iosem.c

This filename is stale.

> +	spin_lock(&lk->wait.lock);

I wonder if this lock should be irq-safe everywhere.  Is it not possible
that someone might want to do an unlock from irq context?

> +	if (lk->state != 0) {
> +		waiter->lock = lk;
> +		add_wait_queue_exclusive_locked(&lk->wait, &waiter->wait);
> +		ret = -EINPROGRESS;
> +	} else {
> +		lk->state |= 1 << IOSEM_LOCK_EXCLUSIVE;
> +		ret = 0;
> +	}
> +	spin_unlock(&lk->wait.lock);
> +	return ret;
> +}

Again, some commentary would be needed to help the poor reader understand
what a -EINPROGRESS return means.

> +	struct iosem_wait waiter;
> +
> +	might_sleep();
> +
> +	init_iosem_waiter(&waiter);
> +	waiter.wait.func = iosem_lock_wake_function;
> +
> +	set_current_state(TASK_UNINTERRUPTIBLE);
> +	if (__iosem_lock(lk, &waiter))
> +		schedule();
> +	__set_current_state(TASK_RUNNING);
> +
> +	BUG_ON(!list_empty(&waiter.wait.task_list));
> +}

Is this BUG_ON() safe?  No locks are held, so couldn't another object get
added by some other thread of control?

> +int iosem_lock_and_schedule_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
> +{
> +	struct iosem_wait *waiter = container_of(wait, struct iosem_wait, wait);
> +	struct iosem_work *wk = container_of(waiter, struct iosem_work, waiter);
> +	unsigned long *lk_state = &waiter->lock->state;
> +	int ret = 0;
> +
> +	if (*lk_state == 0) {
> +		ret = schedule_work(&wk->work);
> +		if (ret) {
> +			*lk_state |= 1 << IOSEM_LOCK_EXCLUSIVE;
> +			list_del_init(&wait->task_list);
> +		}
> +	}
> +	return ret;
> +}

Again, I don't understand why this function was created.  I think it means
that there are restrictions upon what keventd can do with iosems, to avoid
deadlocking.  If correct, they should be spelled out.

> +int fastcall iosem_lock_and_schedule_work(struct iosem *lk, struct iosem_work *wk)
> +{
> +	int ret;
> +
> +	init_iosem_waiter(&wk->waiter);
> +	wk->waiter.wait.func = iosem_lock_and_schedule_function;
> +	ret = __iosem_lock(lk, &wk->waiter);
> +	if (ret == 0)
> +		ret = schedule_work(&wk->work);
> +	return ret;
> +}
> +EXPORT_SYMBOL(iosem_lock_and_schedule_work);

Ditto.

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

on den 30.03.2005 Klokka 14:34 (-0800) skreiv Andrew Morton:

> > Anyhow, the following is a simple implementation of semaphores designed
> > to satisfy the needs of those I/O subsystems that want to support
> > asynchronous behaviour too. Please comment.
> > 
> 
> So I've been staring at this code for a while and I Just Don't Get It.  If
> I want some custom callback function to be called when someone does an
> iosem_unlock(), how do I do it?

I haven't added support for arbitrary callback functions. It is quite
possible to expand the interfaces to do so should someone need that
functionality, however my current needs only dictate that I be able to
grant the iosem token to a workqueue item, then schedule that work for
execution by keventd.

This is required in order to allow threads such as rpciod or keventd
itself (for which sleeping may cause deadlocks) to ask the iosem manager
code to simply queue the work that need to run once the iosem has been
granted. That work function is then, of course, responsible for
releasing the iosem when it is done.

> Or have I misunderstood the intent?  Some /* comments */ would be appropriate..

Will do.

Cheers,
  Trond
-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Andrew Morton]

Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
>
> This is required in order to allow threads such as rpciod or keventd
> itself (for which sleeping may cause deadlocks) to ask the iosem manager
> code to simply queue the work that need to run once the iosem has been
> granted. That work function is then, of course, responsible for
> releasing the iosem when it is done.

I see.  I think.  Should we be using those aio/N threads for this?  They
don't seem to do much else...

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

on den 30.03.2005 Klokka 15:44 (-0800) skreiv Andrew Morton:
> Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> >
> > This is required in order to allow threads such as rpciod or keventd
> > itself (for which sleeping may cause deadlocks) to ask the iosem manager
> > code to simply queue the work that need to run once the iosem has been
> > granted. That work function is then, of course, responsible for
> > releasing the iosem when it is done.
> 
> I see.  I think.  Should we be using those aio/N threads for this?  They
> don't seem to do much else...

That would be quite OK by me if nobody objects.

Cheers,
  Trond
-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

on den 30.03.2005 Klokka 18:17 (-0500) skreiv Trond Myklebust:
> > Or have I misunderstood the intent?  Some /* comments */ would be appropriate..
> 
> Will do.

OK. Plenty of comments added that will hopefully clarify what is going
on and how to use the API. Also some cleanups of the code.

I haven't changed the name "iosem" as Nikita suggested. That's not
because I'm opposed to doing so, but I haven't yet heard something that
I like. Suggestions welcome...

Cheers,
 Trond

--------------
NFS: Add support for iosems.

 These act rather like semaphores, but also have support for asynchronous
 I/O, using the wait_queue_t callback features.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
---
 include/linux/iosem.h |  110 +++++++++++++++++++++++++++++++
 lib/Makefile          |    2 
 lib/iosem.c           |  177 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 288 insertions(+), 1 deletion(-)

Index: linux-2.6.12-rc1/include/linux/iosem.h
===================================================================
--- /dev/null
+++ linux-2.6.12-rc1/include/linux/iosem.h
@@ -0,0 +1,110 @@
+/*
+ * include/linux/iosem.h
+ *
+ * Copyright (C) 2005 Trond Myklebust <Trond.Myklebust@netapp.com>
+ *
+ * Definitions for iosems. These can act as mutexes, but unlike
+ * semaphores, their code is 100% arch-independent, and can therefore
+ * easily be expanded in order to provide for things like
+ * asynchronous I/O.
+ */
+
+#ifndef __LINUX_SEM_LOCK_H
+#define __LINUX_SEM_LOCK_H
+
+#ifdef __KERNEL__
+#include <linux/wait.h>
+#include <linux/workqueue.h>
+
+/*
+ * struct iosem: iosem mutex
+ *      state: bitmask - currently only signals whether or not an exclusive
+ *      		 lock has been taken
+ *      wait: FIFO wait queue
+ */
+struct iosem {
+#define IOSEM_LOCK_EXCLUSIVE (31)
+/* #define IOSEM_LOCK_SHARED (30) */
+	unsigned long state;
+	wait_queue_head_t wait;
+};
+
+
+
+/*
+ * struct iosem_wait: acts as a request for a lock on the iosem
+ *      lock: backpointer to the iosem
+ *      wait: wait queue entry. note that the callback function
+ *            defines what to do when the lock has been granted
+ */
+struct iosem_wait {
+	struct iosem *lock;
+	wait_queue_t wait;
+};
+
+/*
+ * struct iosem_work: used by asynchronous waiters.
+ *
+ * 	work: work to schedule once the iosem has been granted. The
+ * 	      function containing the critical code that needs to
+ * 	      run under the protection of the lock should be placed here.
+ * 	      The same function is responsible for calling iosem_unlock()
+ * 	      when done.
+ * 	waiter: iosem waitqueue entry
+ */
+struct iosem_work {
+	struct work_struct work;
+	struct iosem_wait waiter;
+};
+
+/*
+ * Functions for synchronous i/o
+ */
+
+/* Synchronously grab an iosem.
+ * These functions act in pretty much the same way down()/up()
+ * do for semaphores.
+ */
+extern void FASTCALL(iosem_lock(struct iosem *lk));
+extern void FASTCALL(iosem_unlock(struct iosem *lk));
+
+/*
+ * Callback function to wake up the sleeping task once
+ * it has been granted an exclusive lock
+ */
+extern int iosem_lock_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
+
+/* Initialize a struct iosem in the "unlocked" state */
+static inline void iosem_init(struct iosem *lk)
+{
+	lk->state = 0;
+	init_waitqueue_head(&lk->wait);
+}
+
+/* Initializes a lock request */
+static inline void iosem_waiter_init(struct iosem_wait *waiter)
+{
+	waiter->lock = NULL;
+	init_waitqueue_entry(&waiter->wait, current);
+	INIT_LIST_HEAD(&waiter->wait.task_list);
+}
+
+/* 
+ * Functions for asynchronous I/O.
+ */
+
+/* Requests an exclusive lock on the iosem on behalf of a workqueue entry "wk".
+ * Schedule wk->work for execution as soon as the lock is granted. */
+extern int FASTCALL(iosem_lock_and_schedule_work(struct iosem *lk, struct iosem_work *wk));
+
+/* Waitqueue notifier that schedules work once the exclusive lock has
+ * been granted */
+extern int iosem_lock_and_schedule_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
+
+static inline void iosem_work_init(struct iosem_work *wk, void (*func)(void *), void *data)
+{
+	INIT_WORK(&wk->work, func, data);
+}
+
+#endif /* __KERNEL__ */
+#endif /* __LINUX_SEM_LOCK_H */
Index: linux-2.6.12-rc1/lib/iosem.c
===================================================================
--- /dev/null
+++ linux-2.6.12-rc1/lib/iosem.c
@@ -0,0 +1,177 @@
+/*
+ * linux/lib/iosem.c
+ *
+ * Copyright (C) 2005 Trond Myklebust <Trond.Myklebust@netapp.com>
+ *
+ * A set of primitives for semaphore-like locks that also support notification
+ * callbacks for waiters.
+ */
+#include <linux/config.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/wait.h>
+#include <linux/iosem.h>
+
+/*
+ * Common function for requesting an exclusive lock on an iosem
+ *
+ * Note: should be called while holding the non-irqsafe spinlock
+ * lk->wait.lock. The spinlock is non-irqsafe as we have no reason (yet) to
+ * expect anyone to take/release iosems from within an interrupt
+ * context (and 'cos it is a _bug_ to attempt to wake up the waitqueue
+ * lk->wait using anything other than iosem_unlock()).
+ */
+static inline int __iosem_lock(struct iosem *lk, struct iosem_wait *waiter)
+{
+	int ret;
+
+	if (lk->state != 0) {
+		/* The lock cannot be immediately granted: queue waiter */
+		waiter->lock = lk;
+		add_wait_queue_exclusive_locked(&lk->wait, &waiter->wait);
+		ret = -EINPROGRESS;
+	} else {
+		lk->state |= 1 << IOSEM_LOCK_EXCLUSIVE;
+		ret = 0;
+	}
+	return ret;
+}
+
+/**
+ * iosem_unlock - release an exclusive lock
+ * @iosem - the iosem on which we hold an exclusive lock
+ */
+void fastcall iosem_unlock(struct iosem *lk)
+{
+	spin_lock(&lk->wait.lock);
+	lk->state &= ~(1 << IOSEM_LOCK_EXCLUSIVE);
+	wake_up_locked(&lk->wait);
+	spin_unlock(&lk->wait.lock);
+}
+EXPORT_SYMBOL(iosem_unlock);
+
+/**
+ * iosem_lock_wake_function - take an exclusive lock and wake up sleeping task
+ * @wait: waitqueue entry. Must be part of an initialized struct iosem_wait
+ * @mode:
+ * @sync:
+ * @key:
+ *
+ * Standard wait_queue_func_t callback function used by iosem_lock(). When
+ * called, it will attempt to wake up the sleeping task, and set an
+ * exclusive lock on the iosem.
+ * On success, @wait is automatically removed from the iosem's waitqueue,
+ * and a non-zero value is returned.
+ *
+ * This function will in practice *always* be called from within iosem_unlock()
+ */
+int iosem_lock_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+	struct iosem_wait *waiter = container_of(wait, struct iosem_wait, wait);
+	unsigned long *lk_state = &waiter->lock->state;
+	int ret = 0;
+
+	if (*lk_state == 0) {
+		ret = default_wake_function(wait, mode, sync, key);
+		if (ret) {
+			*lk_state |= 1 << IOSEM_LOCK_EXCLUSIVE;
+			list_del_init(&wait->task_list);
+		}
+	}
+	return ret;
+}
+
+/**
+ * iosem_lock - synchronously take an exclusive lock
+ * @iosem - the iosem to take an exclusive lock
+ *
+ * If the exclusive lock cannot be immediately granted, put the current task
+ * to uninterruptible sleep until it can.
+ */
+void fastcall iosem_lock(struct iosem *lk)
+{
+	struct iosem_wait waiter;
+
+	might_sleep();
+
+	iosem_waiter_init(&waiter);
+	waiter.wait.func = iosem_lock_wake_function;
+
+	spin_lock(&lk->wait.lock);
+	if (__iosem_lock(lk, &waiter) != 0) {
+		/* Must wait for lock... */
+		for (;;) {
+			set_current_state(TASK_UNINTERRUPTIBLE);
+			if (list_empty(&waiter.wait.task_list))
+				break;
+			spin_unlock(&lk->wait.lock);
+			schedule();
+			spin_lock(&lk->wait.lock);
+		}
+		__set_current_state(TASK_RUNNING);
+	}
+	spin_unlock(&lk->wait.lock);
+}
+EXPORT_SYMBOL(iosem_lock);
+
+/**
+ * iosem_lock_and_schedule_function - take an exclusive lock and schedule work
+ * @wait: waitqueue entry. Must be part of an initialized struct iosem_work
+ * @mode: unused
+ * @sync: unused
+ * @key: unused
+ *
+ * Standard wait_queue_func_t callback function used by
+ * iosem_lock_and_schedule_work. When called, it will attempt to queue the
+ * work function and set the exclusive lock on the iosem.
+ * On success, @wait is removed from the iosem's waitqueue, and a non-zero
+ * value is returned.
+ *
+ * This function will in practice *always* be called from within iosem_unlock()
+ */
+int iosem_lock_and_schedule_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+	struct iosem_wait *waiter = container_of(wait, struct iosem_wait, wait);
+	struct iosem_work *wk = container_of(waiter, struct iosem_work, waiter);
+	unsigned long *lk_state = &waiter->lock->state;
+	int ret = 0;
+
+	if (*lk_state == 0) {
+		ret = schedule_work(&wk->work);
+		if (ret) {
+			*lk_state |= 1 << IOSEM_LOCK_EXCLUSIVE;
+			list_del_init(&wait->task_list);
+		}
+	}
+	return ret;
+}
+
+/**
+ * iosem_lock_and_schedule_work - request an exclusive lock and schedule work
+ * @lk: pointer to iosem
+ * @wk: pointer to iosem_work
+ *
+ * Request an exclusive lock on the iosem. If the lock cannot be immediately
+ * granted, place wk->waiter on the iosem's waitqueue, and return, else
+ * immediately queue the work function wk->work.
+ *
+ * Once the exclusive lock has been granted, the work function described by
+ * wk->work is queued in keventd. It is then the responsibility of that work
+ * function to release the exclusive lock once it has been granted.
+ *
+ * returns -EINPROGRESS if the lock could not be immediately granted.
+ */
+int fastcall iosem_lock_and_schedule_work(struct iosem *lk, struct iosem_work *wk)
+{
+	int ret;
+
+	iosem_waiter_init(&wk->waiter);
+	wk->waiter.wait.func = iosem_lock_and_schedule_function;
+	spin_lock(&lk->wait.lock);
+	ret = __iosem_lock(lk, &wk->waiter);
+	spin_unlock(&lk->wait.lock);
+	if (ret == 0)
+		ret = schedule_work(&wk->work);
+	return ret;
+}
+EXPORT_SYMBOL(iosem_lock_and_schedule_work);
Index: linux-2.6.12-rc1/lib/Makefile
===================================================================
--- linux-2.6.12-rc1.orig/lib/Makefile
+++ linux-2.6.12-rc1/lib/Makefile
@@ -8,7 +8,7 @@ lib-y := errno.o ctype.o string.o vsprin
 	 bitmap.o extable.o kobject_uevent.o prio_tree.o sha1.o \
 	 halfmd4.o
 
-obj-y += sort.o parser.o
+obj-y += sort.o parser.o iosem.o
 
 ifeq ($(CONFIG_DEBUG_KOBJECT),y)
 CFLAGS_kobject.o += -DDEBUG

-- 
Trond Myklebust <trondmy@trondhjem.org>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Andrew Morton]

Trond Myklebust <trondmy@trondhjem.org> wrote:
>
>  on den 30.03.2005 Klokka 18:17 (-0500) skreiv Trond Myklebust:
>  > > Or have I misunderstood the intent?  Some /* comments */ would be appropriate..
>  > 
>  > Will do.
> 
>  OK. Plenty of comments added that will hopefully clarify what is going
>  on and how to use the API. Also some cleanups of the code.

Ah, so that's what it does ;)

I guess once we have a caller in-tree we could merge this.  I wonder if
there's other existing code which should be converted to iosems.

You chose to not use the aio kernel threads?

Does iosem_lock_and_schedule_function() need locking?  It nonatomically
alters *lk_state.

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

to den 31.03.2005 Klokka 16:13 (-0800) skreiv Andrew Morton:
> Trond Myklebust <trondmy@trondhjem.org> wrote:
> >
> >  on den 30.03.2005 Klokka 18:17 (-0500) skreiv Trond Myklebust:
> >  > > Or have I misunderstood the intent?  Some /* comments */ would be appropriate..
> >  > 
> >  > Will do.
> > 
> >  OK. Plenty of comments added that will hopefully clarify what is going
> >  on and how to use the API. Also some cleanups of the code.
> 
> Ah, so that's what it does ;)
> 
> I guess once we have a caller in-tree we could merge this.  I wonder if
> there's other existing code which should be converted to iosems.

I can put it into the NFS client stream which feeds into the -mm kernel.
That will enable me to queue up the NFSv4 patches that depend on it
too...

> You chose to not use the aio kernel threads?

I thought I'd do that in a separate patch since the aio workqueue is
currently statically defined in aio.c.

> Does iosem_lock_and_schedule_function() need locking?  It nonatomically
> alters *lk_state.

iosem_lock_and_schedule_function() will always be called with the
iosem->wait.lock held, since it is a waitqueue notification function.

In practice it is called by iosem_unlock(). The call to wake_up_locked()
will trigger a call to __wake_up_common() which again tries the
notification function of each waiter on the queue until it finds one
that succeeds.

Cheers,
  Trond

-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Suparna Bhattacharya]

On Thu, Mar 31, 2005 at 08:22:17PM -0500, Trond Myklebust wrote:
> to den 31.03.2005 Klokka 16:13 (-0800) skreiv Andrew Morton:
> > Trond Myklebust <trondmy@trondhjem.org> wrote:
> > >
> > >  on den 30.03.2005 Klokka 18:17 (-0500) skreiv Trond Myklebust:
> > >  > > Or have I misunderstood the intent?  Some /* comments */ would be appropriate..
> > >  > 
> > >  > Will do.
> > > 
> > >  OK. Plenty of comments added that will hopefully clarify what is going
> > >  on and how to use the API. Also some cleanups of the code.
> > 
> > Ah, so that's what it does ;)
> > 
> > I guess once we have a caller in-tree we could merge this.  I wonder if
> > there's other existing code which should be converted to iosems.
> 
> I can put it into the NFS client stream which feeds into the -mm kernel.
> That will enable me to queue up the NFSv4 patches that depend on it
> too...
> 
> > You chose to not use the aio kernel threads?
> 
> I thought I'd do that in a separate patch since the aio workqueue is
> currently statically defined in aio.c.

I'll take a look at the patch over the weekend. I had a patch
for aio semaphores a long while back, but I need to spend some time
to understand how different this is.

Regards
Suparna

> 
> > Does iosem_lock_and_schedule_function() need locking?  It nonatomically
> > alters *lk_state.
> 
> iosem_lock_and_schedule_function() will always be called with the
> iosem->wait.lock held, since it is a waitqueue notification function.
> 
> In practice it is called by iosem_unlock(). The call to wake_up_locked()
> will trigger a call to __wake_up_common() which again tries the
> notification function of each waiter on the queue until it finds one
> that succeeds.
> 
> Cheers,
>   Trond
> 
> -- 
> Trond Myklebust <trond.myklebust@fys.uio.no>
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

må den 04.04.2005 Klokka 12:22 (-0400) skreiv Benjamin LaHaise:

> > Your approach looks reasonable and simple enough. It'd be useful if I
> > could visualize the caller side of things as in the NFS client stream
> > as you mention - do you plan to post that soon ? 
> > I'm tempted to think about the possibility of using your iosems
> > with retry-based AIO.
> 
> I'm not sure I see the point in adding yet another type of lock to the 
> kernel that has different infrastructure.  This will end up with much 
> more code changing to adapt to the new iosems, rather than just using a 
> new primative on the existing semaphores.

The point is to avoid having to develop and test 23 different and highly
arch-dependent (and hence non-maintainable) versions of the same code.
In particular note that many of those architectures appear to be heavily
over-optimized to directly inline assembly code as well as to use
non-standard calling conventions. They all also manage somehow to differ
w.r.t. spinlocking conventions and even w.r.t. the internal structures
and algorithms used.

IOW: the current semaphore implementations really all need to die, and
be replaced by a single generic version to which it is actually
practical to add new functionality.

Failing that, it is _much_ easier to convert the generic code that needs
to support aio to use a new locking implementation and then test that.
It is not as if conversion to aio won't involve changes to the code in
the area surrounding those locks anyway.

Cheers,
  Trond
-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Benjamin LaHaise]

On Mon, Apr 04, 2005 at 01:56:35PM -0400, Trond Myklebust wrote:
> IOW: the current semaphore implementations really all need to die, and
> be replaced by a single generic version to which it is actually
> practical to add new functionality.

I can see that goal, but I don't think introducing iosems is the right 
way to acheive it.  Instead (and I'll start tackling this), how about 
factoring out the existing semaphore implementations to use a common 
lib/semaphore.c, much like lib/rwsem.c?  The iosems can be used as a 
basis for the implementation, but we can avoid having to do a giant 
s/semaphore/iosem/g over the kernel tree.

> Failing that, it is _much_ easier to convert the generic code that needs
> to support aio to use a new locking implementation and then test that.
> It is not as if conversion to aio won't involve changes to the code in
> the area surrounding those locks anyway.

Quite true.  There's a lot more work to do in this area, and these common 
primatives are needed to make progress.  Someone at netapp sent me an 
email yesterday asking about aio support in NFS, so there is some demand 
out there.  Cheers,

		-ben
-- 
"Time is what keeps everything from happening all at once." -- John Wheeler

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

ty den 05.04.2005 Klokka 11:46 (-0400) skreiv Benjamin LaHaise:

> I can see that goal, but I don't think introducing iosems is the right 
> way to acheive it.  Instead (and I'll start tackling this), how about 
> factoring out the existing semaphore implementations to use a common 
> lib/semaphore.c, much like lib/rwsem.c?  The iosems can be used as a 
> basis for the implementation, but we can avoid having to do a giant 
> s/semaphore/iosem/g over the kernel tree.

If you're willing to take this on then you have my full support and I'd
be happy to lend a hand.

Cheers,
  Trond
-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Bill Huey]

On Tue, Apr 05, 2005 at 09:20:57PM -0400, Trond Myklebust wrote:
> ty den 05.04.2005 Klokka 11:46 (-0400) skreiv Benjamin LaHaise:
> 
> > I can see that goal, but I don't think introducing iosems is the right 
> > way to acheive it.  Instead (and I'll start tackling this), how about 
> > factoring out the existing semaphore implementations to use a common 
> > lib/semaphore.c, much like lib/rwsem.c?  The iosems can be used as a 
> > basis for the implementation, but we can avoid having to do a giant 
> > s/semaphore/iosem/g over the kernel tree.
> 
> If you're willing to take this on then you have my full support and I'd
> be happy to lend a hand.

I would expect also that some RT subgroups would be highly interested in
getting it to respect priority for reworking parts of softirq.

bill

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Suparna Bhattacharya]

On Tue, Apr 05, 2005 at 11:46:41AM -0400, Benjamin LaHaise wrote:
> On Mon, Apr 04, 2005 at 01:56:35PM -0400, Trond Myklebust wrote:
> > IOW: the current semaphore implementations really all need to die, and
> > be replaced by a single generic version to which it is actually
> > practical to add new functionality.
> 
> I can see that goal, but I don't think introducing iosems is the right 
> way to acheive it.  Instead (and I'll start tackling this), how about 
> factoring out the existing semaphore implementations to use a common 
> lib/semaphore.c, much like lib/rwsem.c?  The iosems can be used as a 
> basis for the implementation, but we can avoid having to do a giant 
> s/semaphore/iosem/g over the kernel tree.

That would be really neat, if you can get to it.

Regards
Suparna

> 
> > Failing that, it is _much_ easier to convert the generic code that needs
> > to support aio to use a new locking implementation and then test that.
> > It is not as if conversion to aio won't involve changes to the code in
> > the area surrounding those locks anyway.
> 
> Quite true.  There's a lot more work to do in this area, and these common 
> primatives are needed to make progress.  Someone at netapp sent me an 
> email yesterday asking about aio support in NFS, so there is some demand 
> out there.  Cheers,
> 
> 		-ben
> -- 
> "Time is what keeps everything from happening all at once." -- John Wheeler
> --
> To unsubscribe, send a message with 'unsubscribe linux-aio' in
> the body to majordomo@kvack.org.  For more info on Linux AIO,
> see: http://www.kvack.org/aio/
> Don't email: <a href=mailto:"aart@kvack.org">aart@kvack.org</a>

-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Christoph Hellwig]

On Tue, Apr 05, 2005 at 11:46:41AM -0400, Benjamin LaHaise wrote:
> On Mon, Apr 04, 2005 at 01:56:35PM -0400, Trond Myklebust wrote:
> > IOW: the current semaphore implementations really all need to die, and
> > be replaced by a single generic version to which it is actually
> > practical to add new functionality.
> 
> I can see that goal, but I don't think introducing iosems is the right 
> way to acheive it.  Instead (and I'll start tackling this), how about 
> factoring out the existing semaphore implementations to use a common 
> lib/semaphore.c, much like lib/rwsem.c?  The iosems can be used as a 
> basis for the implementation, but we can avoid having to do a giant 
> s/semaphore/iosem/g over the kernel tree.

Note that iosem is also a total misowner, it's not a counting semaphore
but a sleeping mutex with some special features.

Now if someone wants my two cent on how to resolve the two gazillion different
implementations mess:

 - switch all current semaphore users that don't need counting semaphores
   over to use a mutex_t type.  For now it can map to struct semaphore.
 - rip out all existing complicated struct semaphore implementations and
   replace it with a portable C implementation.  There's not a lot of users
   anyway.  Add a mutex_t implementation that allows sensible assembly hooks
   for architectures instead of reimplementing all of it
 - add more features to mutex_t where nessecary

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Benjamin LaHaise]

On Thu, Apr 07, 2005 at 12:43:02PM +0100, Christoph Hellwig wrote:
>  - switch all current semaphore users that don't need counting semaphores
>    over to use a mutex_t type.  For now it can map to struct semaphore.
>  - rip out all existing complicated struct semaphore implementations and
>    replace it with a portable C implementation.  There's not a lot of users
>    anyway.  Add a mutex_t implementation that allows sensible assembly hooks
>    for architectures instead of reimplementing all of it
>  - add more features to mutex_t where nessecary

Oh dear, this is going to take a while.  In any case, here is such a 
first step in creating such a sequence of patches.  Located at 
http://www.kvack.org/~bcrl/patches/mutex-A0/ are the following patches:

	00_mutex.diff	- Introduces the basic mutex abstraction on top 
			  of the existing semaphore implementation.
	01_i_sem.diff	- Converts all users of i_sem to use the mutex 
			  abstraction.
	10_new_mutex.diff - Replaces the semphore mutex with a new mutex 
			    derrived from Trond's iosem patch.  Note that 
			    this fixes a serious bug in iosems: see the 
			    change in mutex_lock_wake_function that ignores 
			    the return value of default_wake_function, as 
			    on SMP a process might still be running while 
			    we actually made progress.
	sem-test.c	- A basic stress tester for the mutex / semaphore.

I'm still not convinced that introducing the mutex type is the best 
approach, especially given the history of the up()/down() implementation.

On the aio side of things, I introduced the owner field in the mutex (as 
opposed to the flag in Trond's iosem) for the next patch in the series to 
enable something like the following api:

	int aio_lock_mutex(struct mutex *lock, struct iocb *iocb);

	...generic_file_read....
	{
		ret = mutex_lock_aio(&inode->i_sem, iocb);
		if (ret)
			return ret; /* aio_lock_mutex can return -EIOCBQUEUED */
		...
		mutex_unlock(&inode->i_sem);
	}

mutex_lock_aio will attempt to take the lock if the iocb is not the owner, 
otherwise it returns immediately (ie ->owner == iocb).  This will allow for 
code paths that support aio to follow a fairly similar coding style to the 
synchronous io path.

More next week...

		-ben
-- 
"Time is what keeps everything from happening all at once." -- John Wheeler

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

fr den 08.04.2005 Klokka 18:39 (-0400) skreiv Benjamin LaHaise:

> On the aio side of things, I introduced the owner field in the mutex (as 
> opposed to the flag in Trond's iosem) for the next patch in the series to 
> enable something like the following api:
> 
> 	int aio_lock_mutex(struct mutex *lock, struct iocb *iocb);

Any chance of a more generic interface too?

iocbs are fairly high level objects, and so I do not see them helping to
resolve low level filesystem problems such as the NFSv4 state cleanup.

Cheers,
  Trond

-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Suparna Bhattacharya]

On Fri, Apr 08, 2005 at 07:31:46PM -0400, Trond Myklebust wrote:
> fr den 08.04.2005 Klokka 18:39 (-0400) skreiv Benjamin LaHaise:
> 
> > On the aio side of things, I introduced the owner field in the mutex (as 
> > opposed to the flag in Trond's iosem) for the next patch in the series to 
> > enable something like the following api:
> > 
> > 	int aio_lock_mutex(struct mutex *lock, struct iocb *iocb);
> 
> Any chance of a more generic interface too?
> 
> iocbs are fairly high level objects, and so I do not see them helping to
> resolve low level filesystem problems such as the NFSv4 state cleanup.

My preferred approach would be to make the wait queue element the
primitive, rather than the iocb, precisely for this reason.

Guess its time for me to repost my aio-wait-bit based patch set - it
doesn't cover the async semaphores bit, but should indicate the general
direction of thinking.

I still need to look at Ben's patches though.

Regards
Suparna

> 
> Cheers,
>   Trond
> 
> -- 
> Trond Myklebust <trond.myklebust@fys.uio.no>
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-aio' in
> the body to majordomo@kvack.org.  For more info on Linux AIO,
> see: http://www.kvack.org/aio/
> Don't email: <a href=mailto:"aart@kvack.org">aart@kvack.org</a>

-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[David Howells]

Benjamin LaHaise <bcrl@kvack.org> wrote:

> Oh dear, this is going to take a while.  In any case, here is such a 
> first step in creating such a sequence of patches.  Located at 
> http://www.kvack.org/~bcrl/patches/mutex-A0/ are the following patches:
> ...
> 	10_new_mutex.diff - Replaces the semphore mutex with a new mutex 
> 			    derrived from Trond's iosem patch.  Note that 
> 			    this fixes a serious bug in iosems: see the 
> 			    change in mutex_lock_wake_function that ignores 
> 			    the return value of default_wake_function, as 
> 			    on SMP a process might still be running while 
> 			    we actually made progress.

Can I suggest you don't use a wait_queue_t in your mutex? The way you've
implemented your mutex you appear to have to take spinlocks and disable
interrupts a lot more than is necessary.

You might want to look at how I've implemented semaphores for the frv arch:

	include/asm-frv/semaphore.h
	arch/frv/kernel/semaphore.c

On FRV disabling interrupts is quite expensive, so I've made my own simple
semaphores that don't need to take spinlocks and disable interrupts in the
down() path once the sleeping task has been queued[*]. These work in exactly
the same way as rwsems.

  [*] except for the case where down_interruptible() is interrupted.

The point being that since up() needs to take the semaphore's spinlock anyway
so that it can access the list, up() does all the necessary dequeuing of tasks
before waking them up.

David

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Trond Myklebust]

fr den 15.04.2005 Klokka 17:13 (+0100) skreiv David Howells:
> Can I suggest you don't use a wait_queue_t in your mutex? The way you've
> implemented your mutex you appear to have to take spinlocks and disable
> interrupts a lot more than is necessary.

> You might want to look at how I've implemented semaphores for the frv arch:
> 
> 	include/asm-frv/semaphore.h
> 	arch/frv/kernel/semaphore.c
> 
> On FRV disabling interrupts is quite expensive, so I've made my own simple
> semaphores that don't need to take spinlocks and disable interrupts in the
> down() path once the sleeping task has been queued[*]. These work in exactly
> the same way as rwsems.

> The point being that since up() needs to take the semaphore's spinlock
> anyway
> so that it can access the list, up() does all the necessary dequeuing
> of tasks
> before waking them up.

I'm looking at the code, and I'm completely failing to see your point.
Exactly how is that scheme incompatible with use of wait_queue_t?

AFAICS You grab the wait_queue_t lock once in down()/__mutex_lock()
order to try to take the lock (or queue the waiter if that fails), then
once more in order to pass the mutex on to the next waiter on
up()/mutex_unlock(). That is more or less the exact same thing I was
doing with iosems using bog-standard waitqueues, and which Ben has
adapted to his mutexes. What am I missing?

One of the motivations for doing that as far as the iosems were
concerned, BTW, was to avoid adding to this ecology of functionally
identical structures that we have for semaphores, rwsems, and wait
queues.

Cheers,
  Trond

-- 
Trond Myklebust <trond.myklebust@fys.uio.no>

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[Benjamin LaHaise]

On Fri, Apr 15, 2005 at 03:42:54PM -0700, Trond Myklebust wrote:
> AFAICS You grab the wait_queue_t lock once in down()/__mutex_lock()
> order to try to take the lock (or queue the waiter if that fails), then
> once more in order to pass the mutex on to the next waiter on
> up()/mutex_unlock(). That is more or less the exact same thing I was
> doing with iosems using bog-standard waitqueues, and which Ben has
> adapted to his mutexes. What am I missing?

I didn't quite see that either.

What about the use of atomic operations on frv?  Are they more lightweight 
than a semaphore, making for a better fastpath?

		-ben
-- 
"Time is what keeps everything from happening all at once." -- John Wheeler

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[David Howells]

Benjamin LaHaise <bcrl@kvack.org> wrote:

> 
> What about the use of atomic operations on frv?  Are they more lightweight 
> than a semaphore, making for a better fastpath?

What do you mean? Atomic ops don't compare to semaphores.

On FRV atomic ops don't disable interrupts; they reserve one of the eight
conditional execution flags at compile time and that flag is cleared by entry
to an exception, thus aborting the write instruction. See:

	Documentation/fujitsu/frv/atomic-ops.txt

I could try and improve the fastpath on the semaphores for FRV - and perhaps
should - but I implemented the semaphores before I'd thought of the clever way
to do atomic ops.

David

Re: [RFC] Add support for semaphore-like structure with support for asynchronous I/O

[David Howells]

Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> 
> AFAICS You grab the wait_queue_t lock once in down()/__mutex_lock()
> order to try to take the lock (or queue the waiter if that fails), then
> once more in order to pass the mutex on to the next waiter on
> up()/mutex_unlock(). That is more or less the exact same thing I was
> doing with iosems using bog-standard waitqueues, and which Ben has
> adapted to his mutexes. What am I missing?

In Ben's patch it looks like the down() grabs the spinlock twice. Once to
queue yourself and one to dequeue yourself. The up() grabs the spinlock once
to wake you up, but it wasn't obvious that it actually dequeues you.

David