1 files changed, 300 insertions, 0 deletions
diff --git a/fs/netfs/locking.c b/fs/netfs/locking.c
new file mode 100644
index 000000000..4e3be2b81
--- /dev/null
+++ b/fs/netfs/locking.c
@@ -0,0 +1,300 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * I/O and data path helper functionality.
+ *
+ * Borrowed from NFS Copyright (c) 2016 Trond Myklebust
+ */
+
+#include <linux/kernel.h>
+#include <linux/netfs.h>
+#include "internal.h"
+
+struct netfs_wb_waiter {
+	struct list_head	link;		/* Link in ictx->wb_queue */
+	struct task_struct	*waiter;	/* Waiter task; cleared when lock granted */
+};
+
+/*
+ * inode_dio_wait_interruptible - wait for outstanding DIO requests to finish
+ * @inode: inode to wait for
+ *
+ * Waits for all pending direct I/O requests to finish so that we can
+ * proceed with a truncate or equivalent operation.
+ *
+ * Must be called under a lock that serializes taking new references
+ * to i_dio_count, usually by inode->i_mutex.
+ */
+static int netfs_inode_dio_wait_interruptible(struct inode *inode)
+{
+	if (inode_dio_finished(inode))
+		return 0;
+
+	inode_dio_wait_interruptible(inode);
+	return !inode_dio_finished(inode) ? -ERESTARTSYS : 0;
+}
+
+/* Call with exclusively locked inode->i_rwsem */
+static int netfs_block_o_direct(struct netfs_inode *ictx)
+{
+	if (!test_bit(NETFS_ICTX_ODIRECT, &ictx->flags))
+		return 0;
+	clear_bit(NETFS_ICTX_ODIRECT, &ictx->flags);
+	return netfs_inode_dio_wait_interruptible(&ictx->inode);
+}
+
+/**
+ * netfs_start_io_read - declare the file is being used for buffered reads
+ * @inode: file inode
+ *
+ * Declare that a buffered read operation is about to start, and ensure
+ * that we block all direct I/O.
+ * On exit, the function ensures that the NETFS_ICTX_ODIRECT flag is unset,
+ * and holds a shared lock on inode->i_rwsem to ensure that the flag
+ * cannot be changed.
+ * In practice, this means that buffered read operations are allowed to
+ * execute in parallel, thanks to the shared lock, whereas direct I/O
+ * operations need to wait to grab an exclusive lock in order to set
+ * NETFS_ICTX_ODIRECT.
+ * Note that buffered writes and truncates both take a write lock on
+ * inode->i_rwsem, meaning that those are serialised w.r.t. the reads.
+ */
+int netfs_start_io_read(struct inode *inode)
+	__acquires(inode->i_rwsem)
+{
+	struct netfs_inode *ictx = netfs_inode(inode);
+
+	/* Be an optimist! */
+	if (down_read_interruptible(&inode->i_rwsem) < 0)
+		return -ERESTARTSYS;
+	if (test_bit(NETFS_ICTX_ODIRECT, &ictx->flags) == 0)
+		return 0;
+	up_read(&inode->i_rwsem);
+
+	/* Slow path.... */
+	if (down_write_killable(&inode->i_rwsem) < 0)
+		return -ERESTARTSYS;
+	if (netfs_block_o_direct(ictx) < 0) {
+		up_write(&inode->i_rwsem);
+		return -ERESTARTSYS;
+	}
+	downgrade_write(&inode->i_rwsem);
+	return 0;
+}
+EXPORT_SYMBOL(netfs_start_io_read);
+
+/**
+ * netfs_end_io_read - declare that the buffered read operation is done
+ * @inode: file inode
+ *
+ * Declare that a buffered read operation is done, and release the shared
+ * lock on inode->i_rwsem.
+ */
+void netfs_end_io_read(struct inode *inode)
+	__releases(inode->i_rwsem)
+{
+	up_read(&inode->i_rwsem);
+}
+EXPORT_SYMBOL(netfs_end_io_read);
+
+/**
+ * netfs_start_io_write - declare the file is being used for buffered writes
+ * @inode: file inode
+ *
+ * Declare that a buffered read operation is about to start, and ensure
+ * that we block all direct I/O.
+ */
+int netfs_start_io_write(struct inode *inode)
+	__acquires(inode->i_rwsem)
+{
+	struct netfs_inode *ictx = netfs_inode(inode);
+
+	if (down_write_killable(&inode->i_rwsem) < 0)
+		return -ERESTARTSYS;
+	if (netfs_block_o_direct(ictx) < 0) {
+		up_write(&inode->i_rwsem);
+		return -ERESTARTSYS;
+	}
+	downgrade_write(&inode->i_rwsem);
+	return 0;
+}
+EXPORT_SYMBOL(netfs_start_io_write);
+
+/**
+ * netfs_end_io_write - declare that the buffered write operation is done
+ * @inode: file inode
+ *
+ * Declare that a buffered write operation is done, and release the
+ * lock on inode->i_rwsem.
+ */
+void netfs_end_io_write(struct inode *inode)
+	__releases(inode->i_rwsem)
+{
+	up_read(&inode->i_rwsem);
+}
+EXPORT_SYMBOL(netfs_end_io_write);
+
+/* Call with exclusively locked inode->i_rwsem */
+static int netfs_block_buffered(struct inode *inode)
+{
+	struct netfs_inode *ictx = netfs_inode(inode);
+	int ret;
+
+	if (!test_bit(NETFS_ICTX_ODIRECT, &ictx->flags)) {
+		set_bit(NETFS_ICTX_ODIRECT, &ictx->flags);
+		if (inode->i_mapping->nrpages != 0) {
+			unmap_mapping_range(inode->i_mapping, 0, 0, 0);
+			ret = filemap_fdatawait(inode->i_mapping);
+			if (ret < 0) {
+				clear_bit(NETFS_ICTX_ODIRECT, &ictx->flags);
+				return ret;
+			}
+		}
+	}
+	return 0;
+}
+
+/**
+ * netfs_start_io_direct - declare the file is being used for direct i/o
+ * @inode: file inode
+ *
+ * Declare that a direct I/O operation is about to start, and ensure
+ * that we block all buffered I/O.
+ * On exit, the function ensures that the NETFS_ICTX_ODIRECT flag is set,
+ * and holds a shared lock on inode->i_rwsem to ensure that the flag
+ * cannot be changed.
+ * In practice, this means that direct I/O operations are allowed to
+ * execute in parallel, thanks to the shared lock, whereas buffered I/O
+ * operations need to wait to grab an exclusive lock in order to clear
+ * NETFS_ICTX_ODIRECT.
+ * Note that buffered writes and truncates both take a write lock on
+ * inode->i_rwsem, meaning that those are serialised w.r.t. O_DIRECT.
+ */
+int netfs_start_io_direct(struct inode *inode)
+	__acquires(inode->i_rwsem)
+{
+	struct netfs_inode *ictx = netfs_inode(inode);
+	int ret;
+
+	/* Be an optimist! */
+	if (down_read_interruptible(&inode->i_rwsem) < 0)
+		return -ERESTARTSYS;
+	if (test_bit(NETFS_ICTX_ODIRECT, &ictx->flags) != 0)
+		return 0;
+	up_read(&inode->i_rwsem);
+
+	/* Slow path.... */
+	if (down_write_killable(&inode->i_rwsem) < 0)
+		return -ERESTARTSYS;
+	ret = netfs_block_buffered(inode);
+	if (ret < 0) {
+		up_write(&inode->i_rwsem);
+		return ret;
+	}
+	downgrade_write(&inode->i_rwsem);
+	return 0;
+}
+EXPORT_SYMBOL(netfs_start_io_direct);
+
+/**
+ * netfs_end_io_direct - declare that the direct i/o operation is done
+ * @inode: file inode
+ *
+ * Declare that a direct I/O operation is done, and release the shared
+ * lock on inode->i_rwsem.
+ */
+void netfs_end_io_direct(struct inode *inode)
+	__releases(inode->i_rwsem)
+{
+	up_read(&inode->i_rwsem);
+}
+EXPORT_SYMBOL(netfs_end_io_direct);
+
+/*
+ * Wait to have exclusive access to writeback.
+ */
+static bool netfs_wb_begin_wait(struct netfs_inode *ictx)
+{
+	struct netfs_wb_waiter waiter = {};
+	struct task_struct *tsk = current;
+	bool got = false;
+
+	spin_lock(&ictx->lock);
+
+	if (test_and_set_bit_lock(NETFS_ICTX_WB_LOCK, &ictx->flags)) {
+		get_task_struct(tsk);
+		waiter.waiter = tsk;
+		list_add_tail(&waiter.link, &ictx->wb_queue);
+	} else {
+		got = true;
+	}
+	spin_unlock(&ictx->lock);
+
+	if (!got) {
+		for (;;) {
+			set_current_state(TASK_UNINTERRUPTIBLE);
+			/* Read waiter before accessing inode state. */
+			if (smp_load_acquire(&waiter.waiter) == NULL)
+				break;
+			schedule();
+		}
+	}
+	__set_current_state(TASK_RUNNING);
+	return true;
+}
+
+/**
+ * netfs_wb_begin - Begin writeback, waiting if need be
+ * @ictx: The inode to get writeback access on
+ * @nowait: Return failure immediately rather than waiting if true
+ *
+ * Begin writeback to an inode, waiting for exclusive access if @nowait is
+ * false.  This prevents collection from being done out of order with respect
+ * to the issuance of write subrequests.
+ *
+ * Note that writeback may be ended in a different process (e.g. the collection
+ * function on a workqueue) than started it.
+ *
+ * Return: True if can proceed, false if denied.
+ */
+bool netfs_wb_begin(struct netfs_inode *ictx, bool nowait)
+{
+	if (!test_and_set_bit_lock(NETFS_ICTX_WB_LOCK, &ictx->flags))
+		return true;
+	if (nowait) {
+		netfs_stat(&netfs_n_wb_lock_skip);
+		return false;
+	}
+	netfs_stat(&netfs_n_wb_lock_wait);
+	return netfs_wb_begin_wait(ictx);
+}
+EXPORT_SYMBOL(netfs_wb_begin);
+
+/* netfs_wb_end - End writeback
+ * @ictx: The inode we have writeback access to
+ *
+ * End writeback access on an inode, waking up the next writeback request.
+ */
+void netfs_wb_end(struct netfs_inode *ictx)
+{
+	struct netfs_wb_waiter *waiter;
+	struct task_struct *tsk;
+
+	WARN_ON_ONCE(!test_bit(NETFS_ICTX_WB_LOCK, &ictx->flags));
+
+	spin_lock(&ictx->lock);
+
+	waiter = list_first_entry_or_null(&ictx->wb_queue, struct netfs_wb_waiter, link);
+	if (waiter) {
+		list_del(&waiter->link);
+		tsk = waiter->waiter;
+		/* Write inode state before clearing waiter. */
+		smp_store_release(&waiter->waiter, NULL);
+		wake_up_process(tsk);
+		put_task_struct(tsk);
+	} else {
+		clear_bit_unlock(NETFS_ICTX_WB_LOCK, &ictx->flags);
+	}
+
+	spin_unlock(&ictx->lock);
+}
+EXPORT_SYMBOL(netfs_wb_end);