When discussing[1] exec and posix file locks it was realized that none of the callers of get_files_struct fundamentally needed to call get_files_struct, and that by switching them to helper functions instead it will both simplify their code and remove unnecessary increments of files_struct.count. Those unnecessary increments can result in exec unnecessarily unsharing files_struct which breaking posix locks, and it can result in fget_light having to fallback to fget reducing system performance. Instead hold task_lock for the duration that task->files needs to be stable in seq_show. The task_lock was already taken in get_files_struct, and so skipping get_files_struct performs less work overall, and avoids the problems with the files_struct reference count. [1] https://lkml.kernel.org/r/20180915160423.GA31461@xxxxxxxxxx Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx> Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx> v1: https://lkml.kernel.org/r/20200817220425.9389-12-ebiederm@xxxxxxxxxxxx Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> --- fs/proc/fd.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/fs/proc/fd.c b/fs/proc/fd.c index 72c1525b4b3e..cb51763ed554 100644 --- a/fs/proc/fd.c +++ b/fs/proc/fd.c @@ -28,9 +28,8 @@ static int seq_show(struct seq_file *m, void *v) if (!task) return -ENOENT; - files = get_files_struct(task); - put_task_struct(task); - + task_lock(task); + files = task->files; if (files) { unsigned int fd = proc_fd(m->private); @@ -47,8 +46,9 @@ static int seq_show(struct seq_file *m, void *v) ret = 0; } spin_unlock(&files->file_lock); - put_files_struct(files); } + task_unlock(task); + put_task_struct(task); if (ret) return ret; @@ -57,6 +57,7 @@ static int seq_show(struct seq_file *m, void *v) (long long)file->f_pos, f_flags, real_mount(file->f_path.mnt)->mnt_id); + /* show_fd_locks() never deferences files so a stale value is safe */ show_fd_locks(m, file, files); if (seq_has_overflowed(m)) goto out; -- 2.25.0
