[PATCH] files: files struct with RCU
Patch to eliminate struct files_struct.file_lock spinlock on the reader side
and use rcu refcounting rcuref_xxx api for the f_count refcounter. The
updates to the fdtable are done by allocating a new fdtable structure and
setting files->fdt to point to the new structure. The fdtable structure is
protected by RCU thereby allowing lock-free lookup. For fd arrays/sets that
are vmalloced, we use keventd to free them since RCU callbacks can't sleep. A
global list of fdtable to be freed is not scalable, so we use a per-cpu list.
If keventd is already handling the current cpu's work, we use a timer to defer
queueing of that work.
Since the last publication, this patch has been re-written to avoid using
explicit memory barriers and use rcu_assign_pointer(), rcu_dereference()
premitives instead. This required that the fd information is kept in a
separate structure (fdtable) and updated atomically.
Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/fs/open.c b/fs/open.c
index b654251..2fac58c 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -24,6 +24,7 @@
#include <linux/personality.h>
#include <linux/pagemap.h>
#include <linux/syscalls.h>
+#include <linux/rcupdate.h>
#include <asm/unistd.h>
@@ -930,9 +931,8 @@
struct fdtable *fdt;
spin_lock(&files->file_lock);
fdt = files_fdtable(files);
- if (unlikely(fdt->fd[fd] != NULL))
- BUG();
- fdt->fd[fd] = file;
+ BUG_ON(fdt->fd[fd] != NULL);
+ rcu_assign_pointer(fdt->fd[fd], file);
spin_unlock(&files->file_lock);
}
@@ -1024,7 +1024,7 @@
filp = fdt->fd[fd];
if (!filp)
goto out_unlock;
- fdt->fd[fd] = NULL;
+ rcu_assign_pointer(fdt->fd[fd], NULL);
FD_CLR(fd, fdt->close_on_exec);
__put_unused_fd(files, fd);
spin_unlock(&files->file_lock);
