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

Eric Dumazet
2015-04-20T20:50:01Z

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.

diff --git a/fs/file.c b/fs/file.c
index 93c5f89c248b..d72bdcacd4df 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -145,17 +145,23 @@ static int expand_fdtable(struct files_struct *files, int nr)
{
struct fdtable *new_fdt, *cur_fdt;

+ files->resize_in_progress++;
spin_unlock(&files->file_lock);
new_fdt = alloc_fdtable(nr);
+ /* make sure all __fd_install() have seen resize_in_progress */
+ synchronize_sched();
spin_lock(&files->file_lock);
- if (!new_fdt)
+ if (!new_fdt) {
+ files->resize_in_progress--;
return -ENOMEM;
+ }
/*
* extremely unlikely race - sysctl_nr_open decreased between the check in
* caller and alloc_fdtable(). Cheaper to catch it here...
*/
if (unlikely(new_fdt->max_fds <= nr)) {
__free_fdtable(new_fdt);
+ files->resize_in_progress--;
return -EMFILE;
}
/*
@@ -173,6 +179,9 @@ static int expand_fdtable(struct files_struct *files, int nr)
/* Somebody else expanded, so undo our attempt */
__free_fdtable(new_fdt);
}
+ /* coupled with smp_rmb() in __fd_install() */
+ smp_wmb();
+ files->resize_in_progress--;
return 1;
}

@@ -256,6 +265,7 @@ 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;
newf->next_fd = 0;
new_fdt = &newf->fdtab;
new_fdt->max_fds = NR_OPEN_DEFAULT;
@@ -553,11 +563,21 @@ void __fd_install(struct files_struct *files, unsigned int fd,
struct file *file)
{
struct fdtable *fdt;
- spin_lock(&files->file_lock);
+
+ rcu_read_lock_sched();
+
+ while (unlikely(READ_ONCE(files->resize_in_progress))) {
+ rcu_read_unlock_sched();
+ yield();
+ rcu_read_lock_sched();
+ }
+ /* coupled with smp_wmb() in expand_fdtable() */
+ smp_rmb();
fdt = files_fdtable(files);
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..7496a0d73a7c 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -47,6 +47,7 @@ struct files_struct {
* read mostly part
*/
atomic_t count;
+ int resize_in_progress;
struct fdtable __rcu *fdt;
struct fdtable fdtab;
/*