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proc/fd: In fdinfo seq_show don't use get_files_struct
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@redhat.com Suggested-by: Oleg Nesterov <oleg@redhat.com> Acked-by: Christian Brauner <christian.brauner@ubuntu.com> v1: https://lkml.kernel.org/r/20200817220425.9389-12-ebiederm@xmission.com Link: https://lkml.kernel.org/r/20201120231441.29911-17-ebiederm@xmission.com Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
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@ -28,9 +28,8 @@ static int seq_show(struct seq_file *m, void *v)
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if (!task)
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return -ENOENT;
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files = get_files_struct(task);
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put_task_struct(task);
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task_lock(task);
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files = task->files;
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if (files) {
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unsigned int fd = proc_fd(m->private);
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@ -47,8 +46,9 @@ static int seq_show(struct seq_file *m, void *v)
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ret = 0;
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}
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spin_unlock(&files->file_lock);
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put_files_struct(files);
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}
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task_unlock(task);
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put_task_struct(task);
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if (ret)
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return ret;
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@ -57,6 +57,7 @@ static int seq_show(struct seq_file *m, void *v)
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(long long)file->f_pos, f_flags,
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real_mount(file->f_path.mnt)->mnt_id);
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/* show_fd_locks() never deferences files so a stale value is safe */
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show_fd_locks(m, file, files);
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if (seq_has_overflowed(m))
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goto out;
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