Linux Kernel Architecture

Chapter 10: Filesystems without Persistent Storage

sysfs_write_file

fill_write_file

Something to write? flush_write_buffer

Figure 10-15: Code flow diagram forsysfs_write_file.

Directory Traversal

Thelookupmethod ofsysfs_dir_inode_operationsis the basic building block for directory traversal.

We therefore need to have a closer look atsysfs_lookup. Figure 10-16 provides the code flow diagram.

sysfs_lookup

sysfs_find_dirent

sysfs_get_inode

Update and rehash dentry

Figure 10-16: Code flow diagram

forsysfs_lookup.

Attributes constitute the entries of a directory, and the function tries to find an attribute with a specific

name that belongs to an instance ofstruct sysfs_dirent. By iterating over them and comparing names,

the desired entry can be found. Recall that all attributes associated with akobjectare stored in a linked

list whose head issysfs_dirent.s_dir.children. This data structure is now brought to good use:

fs/sysfs/dir.c

struct sysfs_dirent *sysfs_find_dirent(struct sysfs_dirent *parent_sd,

const unsigned char *name)

{

struct sysfs_dirent *sd;

for (sd = parent_sd->s_dir.children; sd; sd = sd->s_sibling)

if (!strcmp(sd->s_name, name))

return sd;

return NULL;

}

sysfs_find_direntis used bysysfs_lookupto find the desiredsysfs_direntinstance for a given

filename. With this in hand, the kernel then needs to establish the connection between sysfs, kernel

subsystem, and the filesystem representation by attaching thesysfs_direntinstance of the attribute

with thedentryinstance of the attribute file.

Dentry and inode are then connected withsysfs_get_inode. The method resorts tosysfs_init_inode;

this function is discussed in Section 10.3.3.

The final steps are not sysfs-specific: The inode information is filled into thedentry.Thisalsorequires

rehashing thedentryon the global dentry hash.