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

Mateusz Guzik
2015-04-21T20:06:46Z

On Tue, Apr 21, 2015 at 11:05:43AM -0700, Eric Dumazet wrote:
> 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.
> >

What I described would block them, although it was a crappy approach
(iterating threads vs cpus). I was wondering if RCU could be abused for
this feature and apparently it can.

> > 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.
>

One could argue this last bit could be committed separately (a different
logical change).

As I read up about synchronize_sched and rcu_read_lock_sched, the code
should be correct.

Also see nits below.

> (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);

The reader may be left confused why there is a warning while the comment
does not indicate anything is wrong.

> }
> + /* 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;
> + }

This does not loop anymore, so s/while/if/ ?

> +
> /* 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;
> /*
>
>
>
>

--
Mateusz Guzik