Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install

Eric Dumazet
2015-04-21T18:05:48Z

On Mon, 2015-04-20 at 13:49 -0700, Eric Dumazet wrote:
> On Mon, 2015-04-20 at 10:15 -0700, Eric Dumazet wrote:
> > On Mon, 2015-04-20 at 17:10 +0200, Mateusz Guzik wrote:
> >
> > > Sorry for spam but I came up with another hack. :)
> > >
> > > The idea is that we can have a variable which would signify the that
> > > given thread is playing with fd table in fd_install (kind of a lock
> > > embedded into task_struct). We would also have a flag in files struct
> > > indicating that a thread would like to resize it.
> > >
> > > expand_fdtable would set the flag and iterate over all threads waiting
> > > for all of them to have the var set to 0.
> >
> > The opposite : you have to block them in some way and add a rcu_sched()
> > or something.
>
> Here is the patch I cooked here but not yet tested.

In following version :

1) I replaced the yield() hack by a proper wait queue.

2) I do not invoke synchronize_sched() for mono threaded programs.

3) I avoid multiple threads doing a resize and then only one wins the
deal.

(copying/clearing big amount of memory only to discover another guy did
the same is a big waste of resources)

This seems to run properly on my hosts.

Your comments/tests are most welcomed, thanks !

fs/file.c | 41 +++++++++++++++++++++++++++++++++-----
include/linux/fdtable.h | 3 ++
2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 93c5f89c248b..e0e113a56444 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -147,6 +147,9 @@ static int expand_fdtable(struct files_struct *files, int nr)

spin_unlock(&files->file_lock);
new_fdt = alloc_fdtable(nr);
+ /* make sure no __fd_install() are still updating fdt */
+ if (atomic_read(&files->count) > 1)
+ synchronize_sched();
spin_lock(&files->file_lock);
if (!new_fdt)
return -ENOMEM;
@@ -170,9 +173,12 @@ static int expand_fdtable(struct files_struct *files, int nr)
if (cur_fdt != &files->fdtab)
call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
} else {
+ WARN_ON_ONCE(1);
/* Somebody else expanded, so undo our attempt */
__free_fdtable(new_fdt);
}
+ /* coupled with smp_rmb() in __fd_install() */
+ smp_wmb();
return 1;
}

@@ -187,19 +193,33 @@ static int expand_fdtable(struct files_struct *files, int nr)
static int expand_files(struct files_struct *files, int nr)
{
struct fdtable *fdt;
+ int expanded = 0;

+begin:
fdt = files_fdtable(files);

/* Do we need to expand? */
if (nr < fdt->max_fds)
- return 0;
+ return expanded;

/* Can we expand? */
if (nr >= sysctl_nr_open)
return -EMFILE;

+ while (unlikely(files->resize_in_progress)) {
+ spin_unlock(&files->file_lock);
+ expanded = 1;
+ wait_event(files->resize_wait, !files->resize_in_progress);
+ spin_lock(&files->file_lock);
+ goto begin;
+ }
+
/* All good, so we try */
- return expand_fdtable(files, nr);
+ files->resize_in_progress = true;
+ expanded = expand_fdtable(files, nr);
+ files->resize_in_progress = false;
+ wake_up_all(&files->resize_wait);
+ return expanded;
}

static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
@@ -256,6 +276,8 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
atomic_set(&newf->count, 1);

spin_lock_init(&newf->file_lock);
+ newf->resize_in_progress = 0;
+ init_waitqueue_head(&newf->resize_wait);
newf->next_fd = 0;
new_fdt = &newf->fdtab;
new_fdt->max_fds = NR_OPEN_DEFAULT;
@@ -553,11 +575,20 @@ void __fd_install(struct files_struct *files, unsigned int fd,
struct file *file)
{
struct fdtable *fdt;
- spin_lock(&files->file_lock);
- fdt = files_fdtable(files);
+
+ rcu_read_lock_sched();
+
+ while (unlikely(files->resize_in_progress)) {
+ rcu_read_unlock_sched();
+ wait_event(files->resize_wait, !files->resize_in_progress);
+ rcu_read_lock_sched();
+ }
+ /* coupled with smp_wmb() in expand_fdtable() */
+ smp_rmb();
+ fdt = READ_ONCE(files->fdt);
BUG_ON(fdt->fd[fd] != NULL);
rcu_assign_pointer(fdt->fd[fd], file);
- spin_unlock(&files->file_lock);
+ rcu_read_unlock_sched();
}

void fd_install(unsigned int fd, struct file *file)
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 230f87bdf5ad..fbb88740634a 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -47,6 +47,9 @@ struct files_struct {
* read mostly part
*/
atomic_t count;
+ bool resize_in_progress;
+ wait_queue_head_t resize_wait;
+
struct fdtable __rcu *fdt;
struct fdtable fdtab;
/*