Re: [PATCH v5] Fix #27777 - now use a doubly-linked list for _IO_list_all

["Cristian Rodríguez" <cristian@rodriguez.im>]

On Mon, May 13, 2024 at 9:50 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> From: Alexandre Ferrieux <alexandre.ferrieux@orange.com>
>
> This patch fixes BZ #27777 "fclose does a linear search, takes ages when
> many FILE* are opened".  Simply put, the master list of opened (FILE*),
> namely _IO_list_all, is a singly-linked list.  As a consequence, the
> removal of a single element is in O(N), which cripples the performance
> of fclose().  The patch switches to a doubly-linked list, yielding O(1)
> removal.  The one padding field in struct _IO_FILE, __pad5, is renamed
> to _prevchain for a doubly-linked list.  Since fields in struct _IO_FILE
> after the _lock field are internal to glibc and opaque to applications.
> We can change them as long as the size of struct _IO_FILE is unchanged,
> which is checked as the part of glibc ABI with sizes of _IO_2_1_stdin_,
> _IO_2_1_stdout_ and _IO_2_1_stderr_.
>
> NB: When _IO_vtable_offset (fp) == 0, copy relocation will cover the
> whole struct _IO_FILE.  Otherwise, only fields up to the _lock field
> will be copied to applications at run-time.  It is used to check if
> the _prevchain field can be safely accessed.
>
> After opening 2 million (FILE*), the fclose() of 100 of them takes quite
> a few seconds without the patch, and under 2 seconds with it on a loaded
> machine.
>
> No test is added since there are no functional changes.
>
> Co-Authored-By: H.J. Lu <hjl.tools@gmail.com>
> Signed-off-by: Alexandre Ferrieux <alexandre.ferrieux@orange.com>
> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> ---
>  libio/bits/types/struct_FILE.h |  4 ++--
>  libio/genops.c                 | 26 ++++++++++++++++++++++++++
>  libio/stdfiles.c               | 15 +++++++++++++++
>  3 files changed, 43 insertions(+), 2 deletions(-)
>
> diff --git a/libio/bits/types/struct_FILE.h b/libio/bits/types/struct_FILE.h
> index 7cdaae86f8..d8d26639d1 100644
> --- a/libio/bits/types/struct_FILE.h
> +++ b/libio/bits/types/struct_FILE.h
> @@ -92,10 +92,10 @@ struct _IO_FILE_complete
>    struct _IO_wide_data *_wide_data;
>    struct _IO_FILE *_freeres_list;
>    void *_freeres_buf;
> -  size_t __pad5;
> +  struct _IO_FILE **_prevchain;
>    int _mode;
>    /* Make sure we don't get into trouble again.  */
> -  char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
> +  char _unused2[15 * sizeof (int) - 5 * sizeof (void *)];
>  };
>
>  /* These macros are used by bits/stdio.h and internal headers.  */
> diff --git a/libio/genops.c b/libio/genops.c
> index bc45e60a09..994ee9c0b1 100644
> --- a/libio/genops.c
> +++ b/libio/genops.c
> @@ -48,6 +48,19 @@ flush_cleanup (void *not_used)
>  }
>  #endif
>
> +/* Fields in struct _IO_FILE after the _lock field are internal to
> +   glibc and opaque to applications.  We can change them as long as
> +   the size of struct _IO_FILE is unchanged, which is checked as the
> +   part of glibc ABI with sizes of _IO_2_1_stdin_, _IO_2_1_stdout_
> +   and _IO_2_1_stderr_.
> +
> +   NB: When _IO_vtable_offset (fp) == 0, copy relocation will cover the
> +   whole struct _IO_FILE.  Otherwise, only fields up to the _lock field
> +   will be copied.  */
> +_Static_assert (offsetof (struct _IO_FILE, _prevchain)
> +               > offsetof (struct _IO_FILE, _lock),
> +               "offset of _prevchain > offset of _lock");
> +
>  void
>  _IO_un_link (struct _IO_FILE_plus *fp)
>  {
> @@ -62,6 +75,14 @@ _IO_un_link (struct _IO_FILE_plus *fp)
>  #endif
>        if (_IO_list_all == NULL)
>         ;
> +      else if (_IO_vtable_offset ((FILE *) fp) == 0)
> +       {
> +         FILE **pr = fp->file._prevchain;
> +         FILE *nx = fp->file._chain;
> +         *pr = nx;
> +         if (nx != NULL)
> +           nx->_prevchain = pr;
> +       }
>        else if (fp == _IO_list_all)
>         _IO_list_all = (struct _IO_FILE_plus *) _IO_list_all->file._chain;
>        else
> @@ -95,6 +116,11 @@ _IO_link_in (struct _IO_FILE_plus *fp)
>        _IO_flockfile ((FILE *) fp);
>  #endif
>        fp->file._chain = (FILE *) _IO_list_all;
> +      if (_IO_vtable_offset ((FILE *) fp) == 0)
> +       {
> +         fp->file._prevchain = (FILE **) &_IO_list_all;
> +         _IO_list_all->file._prevchain = &fp->file._chain;
> +       }
>        _IO_list_all = fp;
>  #ifdef _IO_MTSAFE_IO
>        _IO_funlockfile ((FILE *) fp);
> diff --git a/libio/stdfiles.c b/libio/stdfiles.c
> index cd8eca8bf3..d607fa02e0 100644
> --- a/libio/stdfiles.c
> +++ b/libio/stdfiles.c
> @@ -54,4 +54,19 @@ DEF_STDFILE(_IO_2_1_stdout_, 1, &_IO_2_1_stdin_, _IO_NO_READS);
>  DEF_STDFILE(_IO_2_1_stderr_, 2, &_IO_2_1_stdout_, _IO_NO_READS+_IO_UNBUFFERED);
>
>  struct _IO_FILE_plus *_IO_list_all = &_IO_2_1_stderr_;
> +
> +/* Finish the double-linking for stdfiles as static initialization
> +   cannot.  */
> +
> +__THROW __attribute__ ((constructor))
> +static void
> +_IO_stdfiles_init (void)
> +{
> +  struct _IO_FILE **f;
> +  for (f = (struct _IO_FILE **) &_IO_list_all;
> +       *f != NULL;
> +       f = &(*f)->_chain)
> +    (*f)->_prevchain = f;
> +}
> +
>  libc_hidden_data_def (_IO_list_all)
> --
> 2.45.0
>

Looks as OK as it can be to me given the constraints and need for
backward compatibility.
Ping so it does not get forgotten.