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Subject[patch 14/22] pollfs: pollable futex
Asynchronously wait for FUTEX_WAKE operation on a futex if it still contains
a given value. There can be only one futex wait per file descriptor. However,
it can be rearmed (possibly at a different address) anytime.

The pollable futex approach is far superior (send and receive events from
userspace or kernel) to eventfd and fixes (supercedes) FUTEX_FD at the same time.

Building block for pollable semaphores and user-defined events.

Signed-off-by: Davi E. M. Arnaut <davi@haxent.com.br>

---
fs/pollfs/Makefile | 1
fs/pollfs/futex.c | 154 +++++++++++++++++++++++++++++++++++++++++++++++++++++
init/Kconfig | 7 ++
3 files changed, 162 insertions(+)

Index: linux-2.6/fs/pollfs/Makefile
===================================================================
--- linux-2.6.orig/fs/pollfs/Makefile
+++ linux-2.6/fs/pollfs/Makefile
@@ -3,3 +3,4 @@ pollfs-y := file.o

pollfs-$(CONFIG_POLLFS_SIGNAL) += signal.o
pollfs-$(CONFIG_POLLFS_TIMER) += timer.o
+pollfs-$(CONFIG_POLLFS_FUTEX) += futex.o
Index: linux-2.6/fs/pollfs/futex.c
===================================================================
--- /dev/null
+++ linux-2.6/fs/pollfs/futex.c
@@ -0,0 +1,154 @@
+/*
+ * pollable futex
+ *
+ * Copyright (C) 2007 Davi E. M. Arnaut
+ *
+ * Licensed under the GNU GPL. See the file COPYING for details.
+ */
+
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/err.h>
+#include <linux/wait.h>
+#include <linux/poll.h>
+#include <linux/pollfs_fs.h>
+#include <linux/futex.h>
+
+struct futex_event {
+ union {
+ void __user *addr;
+ u64 padding;
+ };
+ int val;
+};
+
+struct pfs_futex {
+ struct futex_q q;
+ struct futex_event fevt;
+ struct mutex mutex;
+ unsigned volatile queued;
+ struct pfs_file file;
+};
+
+static ssize_t read(struct pfs_futex *evs, struct futex_event __user *ufevt)
+{
+ int ret;
+ struct futex_event fevt;
+
+ mutex_lock(&evs->mutex);
+
+ fevt = evs->fevt;
+
+ ret = -EAGAIN;
+
+ if (!evs->queued)
+ ret = -EINVAL;
+ else if (list_empty(&evs->q.list))
+ ret = futex_wait_unqueue(&evs->q);
+
+ switch (ret) {
+ case 1:
+ ret = -EAGAIN;
+ case 0:
+ evs->queued = 0;
+ }
+
+ mutex_unlock(&evs->mutex);
+
+ if (ret < 0)
+ return ret;
+
+ if (copy_to_user(ufevt, &fevt, sizeof(fevt)))
+ return -EFAULT;
+
+ return 0;
+}
+
+static ssize_t write(struct pfs_futex *evs,
+ const struct futex_event __user *ufevt)
+{
+ int ret;
+ struct futex_event fevt;
+
+ if (copy_from_user(&fevt, ufevt, sizeof(fevt)))
+ return -EFAULT;
+
+ mutex_lock(&evs->mutex);
+
+ if (evs->queued)
+ futex_wait_unqueue(&evs->q);
+
+ ret = futex_wait_queue(&evs->q, fevt.addr, fevt.val);
+
+ if (ret)
+ evs->queued = 0;
+ else {
+ evs->queued = 1;
+ evs->fevt = fevt;
+ }
+
+ mutex_unlock(&evs->mutex);
+
+ return ret;
+}
+
+static int poll(struct pfs_futex *evs)
+{
+ int ret;
+
+ while (!mutex_trylock(&evs->mutex))
+ cpu_relax();
+
+ ret = evs->queued && list_empty(&evs->q.list) ? POLLIN : 0;
+
+ mutex_unlock(&evs->mutex);
+
+ return ret;
+}
+
+static int release(struct pfs_futex *evs)
+{
+ if (evs->queued)
+ futex_wait_unqueue(&evs->q);
+
+ mutex_destroy(&evs->mutex);
+
+ kfree(evs);
+
+ return 0;
+}
+
+static const struct pfs_operations futex_ops = {
+ .read = PFS_READ(read, struct pfs_futex, struct futex_event),
+ .write = PFS_WRITE(write, struct pfs_futex, struct futex_event),
+ .poll = PFS_POLL(poll, struct pfs_futex),
+ .release = PFS_RELEASE(release, struct pfs_futex),
+ .rsize = sizeof(struct futex_event),
+ .wsize = sizeof(struct futex_event),
+};
+
+asmlinkage long sys_plfutex(void)
+{
+ long error;
+ struct pfs_futex *evs;
+
+ evs = kzalloc(sizeof(*evs), GFP_KERNEL);
+ if (!evs)
+ return -ENOMEM;
+
+ mutex_init(&evs->mutex);
+ init_waitqueue_head(&evs->q.waiters);
+
+ evs->file.data = evs;
+ evs->file.fops = &futex_ops;
+ evs->file.wait = &evs->q.waiters;
+
+ error = pfs_open(&evs->file);
+
+ if (error < 0)
+ release(evs);
+
+ return error;
+}
Index: linux-2.6/init/Kconfig
===================================================================
--- linux-2.6.orig/init/Kconfig
+++ linux-2.6/init/Kconfig
@@ -483,6 +483,13 @@ config POLLFS_TIMER
help
Pollable timer support

+config POLLFS_FUTEX
+ bool "Enable pollfs futex" if EMBEDDED
+ default y
+ depends on POLLFS && FUTEX
+ help
+ Pollable futex support
+
config SHMEM
bool "Use full shmem filesystem" if EMBEDDED
default y
--
-
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