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

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

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;
   /*




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