[CRIU] Re: [PATCH] reg-files: Serialize linking of ghost files

[Pavel Emelyanov]

On 09/17/2012 12:50 PM, Cyrill Gorcunov wrote:
> In case if there at least two tasks with same ghost
> file we've a race in open_path -- both task may enter
> the open_path at same time and try to link ghost file
> to same name leading to
> 
>  | 26445: Error (files-reg.c:363): Can't link
>  | /home/crtools/test/zdtm/live/static/write_read10.test (deleted).cr.1.ghost ->
>  | /home/crtools/test/zdtm/live/static/write_read10.test (deleted)
> 
> The both tasks have ghost files as
> 
>  | id:  0x3 flags: 0x8002 pos: 0000000000000000 ... name: "/home/crtools/test/zdtm/live/static/write_read10.test (deleted)"
> 
> and
> 
>  | id: 0x10 flags: 0x8002 pos: 0000000000000000 ... name: "/home/crtools/test/zdtm/live/static/write_read10.test (deleted)"
> 
> Thus, to serialize link/unlink procedure we use per-ghost-file mutex.

In that case users can be just unsigned int

> Signed-off-by: Cyrill Gorcunov <gorcunov at openvz.org>
> ---
>  files-reg.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/files-reg.c b/files-reg.c
> index b459130..3620d23 100644
> --- a/files-reg.c
> +++ b/files-reg.c
> @@ -38,6 +38,7 @@ struct ghost_file {
>  		};
>  	};
>  	atomic_t users;
> +	mutex_t lock;
>  };
>  
>  static u32 ghost_file_ids = 1;
> @@ -115,6 +116,7 @@ static int open_remap_ghost(struct reg_file_info *rfi,
>  
>  	gf->id = rfe->remap_id;
>  	atomic_set(&gf->users, 0);
> +	mutex_init(&gf->lock);
>  	list_add_tail(&gf->list, &ghost_files);
>  gf_found:
>  	atomic_inc(&gf->users);
> @@ -351,12 +353,14 @@ static int open_path(struct file_desc *d,
>  
>  	rfi = container_of(d, struct reg_file_info, d);
>  
> -	if (rfi->ghost)
> +	if (rfi->ghost) {
> +		mutex_lock(&rfi->ghost->lock);
>  		if (link(rfi->ghost->path, rfi->path) < 0) {
>  			pr_perror("Can't link %s -> %s\n",
>  					rfi->ghost->path, rfi->path);
>  			return -1;
>  		}
> +	}
>  
>  	tmp = open_cb(rfi, arg);
>  	if (tmp < 0) {
> @@ -370,6 +374,7 @@ static int open_path(struct file_desc *d,
>  			pr_info("Unlink the ghost %s\n", rfi->ghost->path);
>  			unlink(rfi->ghost->path);
>  		}
> +		mutex_unlock(&rfi->ghost->lock);
>  	}
>  
>  	if (restore_fown(tmp, rfi->rfe->fown))