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    SubjectRe: [PATCH 08/17] proc/fd: In proc_fd_link use fcheck_task
    On Mon, Aug 17, 2020 at 05:04:16PM -0500, Eric W. Biederman wrote:
    > 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.
    >
    > Using fcheck_task instead of get_files_struct simplifies proc_fd_link by
    > removing unnecessary locking, and reference counting.
    >
    > [1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
    > Suggested-by: Oleg Nesterov <oleg@redhat.com>
    > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
    > ---

    Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

    > fs/proc/fd.c | 14 ++++----------
    > 1 file changed, 4 insertions(+), 10 deletions(-)
    >
    > diff --git a/fs/proc/fd.c b/fs/proc/fd.c
    > index 4048a87c51ee..abfdcb21cc79 100644
    > --- a/fs/proc/fd.c
    > +++ b/fs/proc/fd.c
    > @@ -141,29 +141,23 @@ static const struct dentry_operations tid_fd_dentry_operations = {
    >
    > static int proc_fd_link(struct dentry *dentry, struct path *path)
    > {
    > - struct files_struct *files = NULL;
    > struct task_struct *task;
    > int ret = -ENOENT;
    >
    > task = get_proc_task(d_inode(dentry));
    > if (task) {
    > - files = get_files_struct(task);
    > - put_task_struct(task);
    > - }
    > -
    > - if (files) {
    > unsigned int fd = proc_fd(d_inode(dentry));
    > struct file *fd_file;
    >
    > - spin_lock(&files->file_lock);
    > - fd_file = fcheck_files(files, fd);
    > + rcu_read_lock();
    > + fd_file = fcheck_task(task, fd);
    > if (fd_file) {
    > *path = fd_file->f_path;
    > path_get(&fd_file->f_path);
    > ret = 0;
    > }
    > - spin_unlock(&files->file_lock);
    > - put_files_struct(files);
    > + rcu_read_unlock();
    > + put_task_struct(task);
    > }
    >
    > return ret;
    > --
    > 2.25.0
    >

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