[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/fcntl.c b/fs/fcntl.c
index bfecc62..d2f3ed8 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 @@
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 @@
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 @@
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);