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

[Eric Dumazet]

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;
   /*




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