On Thu, 5 Mar 2015 18:37:18 +0300 Andrey Vagin <avagin@xxxxxxxxxx> wrote:
> Let's show locks which are associated with a file descriptor in
> its fdinfo file.
>
> Currently we don't have a reliable way to determine who holds a lock.
> We can find some information in /proc/locks, but PID which is reported
> there can be wrong. For example, a process takes a lock, then forks a
> child and dies. In this case /proc/locks contains the parent pid, which
> can be reused by another process.
>
> $ cat /proc/locks
> ...
> 6: FLOCK ADVISORY WRITE 324 00:13:13431 0 EOF
> ...
>
> $ ps -C rpcbind
> PID TTY TIME CMD
> 332 ? 00:00:00 rpcbind
>
> $ cat /proc/332/fdinfo/4
> pos: 0
> flags: 0100000
> mnt_id: 22
> lock: 1: FLOCK ADVISORY WRITE 324 00:13:13431 0 EOF
>
> $ ls -l /proc/332/fd/4
> lr-x------ 1 root root 64 Mar 5 14:43 /proc/332/fd/4 -> /run/rpcbind.lock
>
> $ ls -l /proc/324/fd/
> total 0
> lrwx------ 1 root root 64 Feb 27 14:50 0 -> /dev/pts/0
> lrwx------ 1 root root 64 Feb 27 14:50 1 -> /dev/pts/0
> lrwx------ 1 root root 64 Feb 27 14:49 2 -> /dev/pts/0
>
> You can see that the process with the 324 pid doesn't hold the lock.
>
> This information is required for proper dumping and restoring file
> locks.
>
> ...
>
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -8,6 +8,7 @@
> #include <linux/security.h>
> #include <linux/file.h>
> #include <linux/seq_file.h>
> +#include <linux/fs.h>
>
> #include <linux/proc_fs.h>
>
> @@ -48,16 +49,24 @@ static int seq_show(struct seq_file *m, void *v)
> put_files_struct(files);
> }
>
> - if (!ret) {
> - seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\n",
> - (long long)file->f_pos, f_flags,
> - real_mount(file->f_path.mnt)->mnt_id);
> - if (file->f_op->show_fdinfo)
> - file->f_op->show_fdinfo(m, file);
> - ret = seq_has_overflowed(m);
> - fput(file);
> - }
> + if (ret)
> + return ret;
> +
> + seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\n",
> + (long long)file->f_pos, f_flags,
> + real_mount(file->f_path.mnt)->mnt_id);
> +
> + show_fd_locks(m, file, files);
> + ret = seq_has_overflowed(m);
> + if (ret)
> + goto out;
seq_has_overflowed() returns a boolean, but fs/seq_file.c:traverse() is
looking for a -ve errno from ->show().
Documentation/filesystems/seq_file.txt says
If all is well, the show() function should return zero. A negative error
code in the usual manner indicates that something went wrong; it will be
passed back to user space. This function can also return SEQ_SKIP, which
causes the current item to be skipped; if the show() function has already
generated output before returning SEQ_SKIP, that output will be dropped.
> + if (file->f_op->show_fdinfo)
> + file->f_op->show_fdinfo(m, file);
> + ret = seq_has_overflowed(m);
>
> +out:
> + fput(file);
> return ret;
> }
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