text: /*
   * Copyright (c) 2016 Trond Myklebust
   *
   * I/O and data path helper functionality.
   */
  
  #include <linux/types.h>
  #include <linux/kernel.h>
  #include <linux/bitops.h>
  #include <linux/rwsem.h>
  #include <linux/fs.h>
  #include <linux/nfs_fs.h>
  
  #include "internal.h"
  
  /* Call with exclusively locked inode->i_rwsem */
  static void nfs_block_o_direct(struct nfs_inode *nfsi, struct inode *inode)
  {
  	if (test_bit(NFS_INO_ODIRECT, &nfsi->flags)) {
  		clear_bit(NFS_INO_ODIRECT, &nfsi->flags);
  		inode_dio_wait(inode);
  	}
  }
  
  /**
   * nfs_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 NFS_INO_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
   * NFS_INO_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.
   */
  void
  nfs_start_io_read(struct inode *inode)
  {
  	struct nfs_inode *nfsi = NFS_I(inode);
  	/* Be an optimist! */
  	down_read(&inode->i_rwsem);
  	if (test_bit(NFS_INO_ODIRECT, &nfsi->flags) == 0)
  		return;
  	up_read(&inode->i_rwsem);
  	/* Slow path.... */
  	down_write(&inode->i_rwsem);
  	nfs_block_o_direct(nfsi, inode);
  	downgrade_write(&inode->i_rwsem);
  }
  
  /**
   * nfs_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
  nfs_end_io_read(struct inode *inode)
  {
  	up_read(&inode->i_rwsem);
  }
  
  /**
   * nfs_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.
   */
  void
  nfs_start_io_write(struct inode *inode)
  {
  	down_write(&inode->i_rwsem);
  	nfs_block_o_direct(NFS_I(inode), inode);
  }
  
  /**
   * nfs_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
  nfs_end_io_write(struct inode *inode)
  {
  	up_write(&inode->i_rwsem);
  }
  
  /* Call with exclusively locked inode->i_rwsem */
  static void nfs_block_buffered(struct nfs_inode *nfsi, struct inode *inode)
  {
  	if (!test_bit(NFS_INO_ODIRECT, &nfsi->flags)) {
  		set_bit(NFS_INO_ODIRECT, &nfsi->flags);
  		nfs_sync_mapping(inode->i_mapping);
  	}
  }
  
  /**
   * nfs_end_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 NFS_INO_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
   * NFS_INO_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.
   */
  void
  nfs_start_io_direct(struct inode *inode)
  {
  	struct nfs_inode *nfsi = NFS_I(inode);
  	/* Be an optimist! */
  	down_read(&inode->i_rwsem);
  	if (test_bit(NFS_INO_ODIRECT, &nfsi->flags) != 0)
  		return;
  	up_read(&inode->i_rwsem);
  	/* Slow path.... */
  	down_write(&inode->i_rwsem);
  	nfs_block_buffered(nfsi, inode);
  	downgrade_write(&inode->i_rwsem);
  }
  
  /**
   * nfs_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
  nfs_end_io_direct(struct inode *inode)
  {
  	up_read(&inode->i_rwsem);
  }
  