[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>

1 files changed, 10 insertions, 3 deletions

diff --git a/fs/fcntl.c b/fs/fcntl.c
index bfecc6238083..d2f3ed8acd93 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -16,6 +16,7 @@
 #include <linux/security.h>
 #include <linux/ptrace.h>
 #include <linux/signal.h>
+#include <linux/rcupdate.h>
 
 #include <asm/poll.h>
 #include <asm/siginfo.h>
@@ -64,8 +65,8 @@ static int locate_fd(struct files_struct *files,
         if (orig_start >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)
                 goto out;
 
-        fdt = files_fdtable(files);
 repeat:
+        fdt = files_fdtable(files);
         /*
          * Someone might have closed fd's in the range
          * orig_start..fdt->next_fd
@@ -95,9 +96,15 @@ repeat:
         if (error)
                 goto repeat;
 
+        /*
+         * We reacquired files_lock, so we are safe as long as
+         * we reacquire the fdtable pointer and use it while holding
+         * the lock, no one can free it during that time.
+         */
+        fdt = files_fdtable(files);
         if (start <= fdt->next_fd)
                 fdt->next_fd = newfd + 1;
-        
+
         error = newfd;
         
 out:
@@ -163,7 +170,7 @@ asmlinkage long sys_dup2(unsigned int oldfd, unsigned int newfd)
         if (!tofree && FD_ISSET(newfd, fdt->open_fds))
                 goto out_fput;
 
-        fdt->fd[newfd] = file;
+        rcu_assign_pointer(fdt->fd[newfd], file);
         FD_SET(newfd, fdt->open_fds);
         FD_CLR(newfd, fdt->close_on_exec);
         spin_unlock(&files->file_lock);