fs/netfs/locking.c

// 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);