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Use a doubly-linked list for _IO_list_all (bug 27777)
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> Reviewed-by: Carlos O'Donell <carlos@redhat.com>
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@ -92,10 +92,10 @@ struct _IO_FILE_complete
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struct _IO_wide_data *_wide_data;
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struct _IO_FILE *_freeres_list;
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void *_freeres_buf;
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size_t __pad5;
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struct _IO_FILE **_prevchain;
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int _mode;
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/* Make sure we don't get into trouble again. */
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char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
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char _unused2[15 * sizeof (int) - 5 * sizeof (void *)];
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};
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/* These macros are used by bits/stdio.h and internal headers. */
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@ -48,6 +48,19 @@ flush_cleanup (void *not_used)
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}
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#endif
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/* Fields in struct _IO_FILE after the _lock field are internal to
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glibc and opaque to applications. We can change them as long as
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the size of struct _IO_FILE is unchanged, which is checked as the
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part of glibc ABI with sizes of _IO_2_1_stdin_, _IO_2_1_stdout_
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and _IO_2_1_stderr_.
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NB: When _IO_vtable_offset (fp) == 0, copy relocation will cover the
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whole struct _IO_FILE. Otherwise, only fields up to the _lock field
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will be copied. */
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_Static_assert (offsetof (struct _IO_FILE, _prevchain)
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> offsetof (struct _IO_FILE, _lock),
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"offset of _prevchain > offset of _lock");
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void
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_IO_un_link (struct _IO_FILE_plus *fp)
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{
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@ -62,6 +75,14 @@ _IO_un_link (struct _IO_FILE_plus *fp)
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#endif
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if (_IO_list_all == NULL)
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;
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else if (_IO_vtable_offset ((FILE *) fp) == 0)
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{
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FILE **pr = fp->file._prevchain;
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FILE *nx = fp->file._chain;
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*pr = nx;
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if (nx != NULL)
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nx->_prevchain = pr;
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}
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else if (fp == _IO_list_all)
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_IO_list_all = (struct _IO_FILE_plus *) _IO_list_all->file._chain;
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else
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@ -95,6 +116,11 @@ _IO_link_in (struct _IO_FILE_plus *fp)
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_IO_flockfile ((FILE *) fp);
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#endif
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fp->file._chain = (FILE *) _IO_list_all;
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if (_IO_vtable_offset ((FILE *) fp) == 0)
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{
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fp->file._prevchain = (FILE **) &_IO_list_all;
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_IO_list_all->file._prevchain = &fp->file._chain;
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}
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_IO_list_all = fp;
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#ifdef _IO_MTSAFE_IO
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_IO_funlockfile ((FILE *) fp);
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@ -54,4 +54,19 @@ DEF_STDFILE(_IO_2_1_stdout_, 1, &_IO_2_1_stdin_, _IO_NO_READS);
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DEF_STDFILE(_IO_2_1_stderr_, 2, &_IO_2_1_stdout_, _IO_NO_READS+_IO_UNBUFFERED);
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struct _IO_FILE_plus *_IO_list_all = &_IO_2_1_stderr_;
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/* Finish double-linking stdin, stdout, and stderr in a constructor.
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Static initialization cannot complete the _prevchain setup. */
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__THROW __attribute__ ((constructor))
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static void
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_IO_stdfiles_init (void)
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{
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struct _IO_FILE **f;
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for (f = (struct _IO_FILE **) &_IO_list_all;
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*f != NULL;
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f = &(*f)->_chain)
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(*f)->_prevchain = f;
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}
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libc_hidden_data_def (_IO_list_all)
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