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File management in the Linux kernel
-----------------------------------
This document describes how locking for files (struct file)
and file descriptor table (struct files) works.
Up until 2.6.12, the file descriptor table has been protected
with a lock (files->file_lock) and reference count (files->count).
->file_lock protected accesses to all the file related fields
of the table. ->count was used for sharing the file descriptor
table between tasks cloned with CLONE_FILES flag. Typically
this would be the case for posix threads. As with the common
refcounting model in the kernel, the last task doing
a put_files_struct() frees the file descriptor (fd) table.
The files (struct file) themselves are protected using
reference count (->f_count).
In the new lock-free model of file descriptor management,
the reference counting is similar, but the locking is
based on RCU. The file descriptor table contains multiple
elements - the fd sets (open_fds and close_on_exec, the
array of file pointers, the sizes of the sets and the array
etc.). In order for the updates to appear atomic to
a lock-free reader, all the elements of the file descriptor
table are in a separate structure - struct fdtable.
files_struct contains a pointer to struct fdtable through
which the actual fd table is accessed. Initially the
fdtable is embedded in files_struct itself. On a subsequent
expansion of fdtable, a new fdtable structure is allocated
and files->fdtab points to the new structure. The fdtable
structure is freed with RCU and lock-free readers either
see the old fdtable or the new fdtable making the update
appear atomic. Here are the locking rules for
the fdtable structure -
1. All references to the fdtable must be done through
the files_fdtable() macro :
struct fdtable *fdt;
rcu_read_lock();
fdt = files_fdtable(files);
....
if (n <= fdt->max_fds)
....
...
rcu_read_unlock();
files_fdtable() uses rcu_dereference() macro which takes care of
the memory barrier requirements for lock-free dereference.
The fdtable pointer must be read within the read-side
critical section.
2. Reading of the fdtable as described above must be protected
by rcu_read_lock()/rcu_read_unlock().
3. For any update to the fd table, files->file_lock must
be held.
4. To look up the file structure given an fd, a reader
must use either fcheck() or fcheck_files() APIs. These
take care of barrier requirements due to lock-free lookup.
An example :
struct file *file;
rcu_read_lock();
file = fcheck(fd);
if (file) {
...
}
....
rcu_read_unlock();
5. Handling of the file structures is special. Since the look-up
of the fd (fget()/fget_light()) are lock-free, it is possible
that look-up may race with the last put() operation on the
file structure. This is avoided using atomic_inc_not_zero()
on ->f_count :
rcu_read_lock();
file = fcheck_files(files, fd);
if (file) {
if (atomic_inc_not_zero(&file->f_count))
*fput_needed = 1;
else
/* Didn't get the reference, someone's freed */
file = NULL;
}
rcu_read_unlock();
....
return file;
atomic_inc_not_zero() detects if refcounts is already zero or
goes to zero during increment. If it does, we fail
fget()/fget_light().
6. Since both fdtable and file structures can be looked up
lock-free, they must be installed using rcu_assign_pointer()
API. If they are looked up lock-free, rcu_dereference()
must be used. However it is advisable to use files_fdtable()
and fcheck()/fcheck_files() which take care of these issues.
7. While updating, the fdtable pointer must be looked up while
holding files->file_lock. If ->file_lock is dropped, then
another thread expand the files thereby creating a new
fdtable and making the earlier fdtable pointer stale.
For example :
spin_lock(&files->file_lock);
fd = locate_fd(files, file, start);
if (fd >= 0) {
/* locate_fd() may have expanded fdtable, load the ptr */
fdt = files_fdtable(files);
FD_SET(fd, fdt->open_fds);
FD_CLR(fd, fdt->close_on_exec);
spin_unlock(&files->file_lock);
.....
Since locate_fd() can drop ->file_lock (and reacquire ->file_lock),
the fdtable pointer (fdt) must be loaded after locate_fd().
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