[PATCH, libfortran] PR 48587 Newunit allocator

Janne Blomqvist blomqvist.janne@gmail.com
Thu Oct 13 15:16:00 GMT 2016


Currently GFortran newer reuses unit numbers allocated with NEWUNIT=,
instead having a simple counter that is decremented each time such a
unit is opened.  For a long running program which repeatedly opens
files with NEWUNIT= and closes them, the counter can wrap around and
cause an abort.  This patch replaces the counter with an allocator
that keeps track of which units numbers are allocated, and can reuse
them once they have been deallocated.  Since operating systems tend to
limit the number of simultaneous open files for a process to a
relatively modest number, a relatively simple approach with a linear
scan through an array suffices.  Though as a small optimization there
is a low water indicator keeping track of the index for which all unit
numbers below are already allocated.  This linear scan also ensures
that we always allocate the smallest available unit number.

2016-10-13  Janne Blomqvist  <jb@gcc.gnu.org>

	PR libfortran/48587
	* io/io.h (get_unique_unit_number): Remove prototype.
	(newunit_alloc): New prototype.
	* io/open.c (st_open): Call newunit_alloc.
	* io/unit.c (newunits,newunit_size,newunit_lwi): New static
	variables.
	(GFC_FIRST_NEWUNIT): Rename to NEWUNIT_START.
	(next_available_newunit): Remove variable.
	(get_unit): Call newunit_alloc.
	(close_unit_1): Call newunit_free.
	(close_units): Free newunits array.
	(get_unique_number): Remove function.
	(newunit_alloc): New function.
	(newunit_free): New function.

Regtested on x86_64-pc-linux-gnu. Ok for trunk?
---
 libgfortran/io/io.h   |   5 ++-
 libgfortran/io/open.c |   2 +-
 libgfortran/io/unit.c | 103 ++++++++++++++++++++++++++++++++++++++++----------
 3 files changed, 86 insertions(+), 24 deletions(-)

diff --git a/libgfortran/io/io.h b/libgfortran/io/io.h
index ea93fba..aaacc08 100644
--- a/libgfortran/io/io.h
+++ b/libgfortran/io/io.h
@@ -715,8 +715,9 @@ internal_proto (finish_last_advance_record);
 extern int unit_truncate (gfc_unit *, gfc_offset, st_parameter_common *);
 internal_proto (unit_truncate);
 
-extern GFC_INTEGER_4 get_unique_unit_number (st_parameter_common *);
-internal_proto(get_unique_unit_number);
+extern int newunit_alloc (void);
+internal_proto(newunit_alloc);
+
 
 /* open.c */
 
diff --git a/libgfortran/io/open.c b/libgfortran/io/open.c
index d074b02..2e7163d 100644
--- a/libgfortran/io/open.c
+++ b/libgfortran/io/open.c
@@ -812,7 +812,7 @@ st_open (st_parameter_open *opp)
   if ((opp->common.flags & IOPARM_LIBRETURN_MASK) == IOPARM_LIBRETURN_OK)
     {
       if ((opp->common.flags & IOPARM_OPEN_HAS_NEWUNIT))
-	opp->common.unit = get_unique_unit_number(&opp->common);
+	opp->common.unit = newunit_alloc ();
       else if (opp->common.unit < 0)
 	{
 	  u = find_unit (opp->common.unit);
diff --git a/libgfortran/io/unit.c b/libgfortran/io/unit.c
index 274b24b..cc24ca7 100644
--- a/libgfortran/io/unit.c
+++ b/libgfortran/io/unit.c
@@ -29,6 +29,7 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 #include "unix.h"
 #include <stdlib.h>
 #include <string.h>
+#include <assert.h>
 
 
 /* IO locking rules:
@@ -68,12 +69,34 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    on it.  unlock_unit or close_unit must be always called only with the
    private lock held.  */
 
-/* Subroutines related to units */
 
-/* Unit number to be assigned when NEWUNIT is used in an OPEN statement.  */
-#define GFC_FIRST_NEWUNIT -10
+
+/* Table of allocated newunit values.  A simple solution would be to
+   map OS file descriptors (fd's) to unit numbers, e.g. with newunit =
+   -fd - 2, however that doesn't work since Fortran allows an existing
+   unit number to be reassociated with a new file. Thus the simple
+   approach may lead to a situation where we'd try to assign a
+   (negative) unit number which already exists. Hence we must keep
+   track of allocated newunit values ourselves. This is the purpose of
+   the newunits array. The indices map to newunit values as newunit =
+   -index + NEWUNIT_FIRST. E.g. newunits[0] having the value true
+   means that a unit with number NEWUNIT_FIRST exists. Similar to
+   POSIX file descriptors, we always allocate the lowest (in absolute
+   value) available unit number.
+ */
+static bool *newunits;
+static int newunit_size; /* Total number of elements in the newunits array.  */
+/* Low water indicator for the newunits array. Below the LWI all the
+   units are allocated, above and equal to the LWI there may be both
+   allocated and free units. */
+static int newunit_lwi;
+static void newunit_free (int);
+
+/* Unit numbers assigned with NEWUNIT start from here.  */
+#define NEWUNIT_START -10
+
+
 #define NEWUNIT_STACK_SIZE 16
-static GFC_INTEGER_4 next_available_newunit = GFC_FIRST_NEWUNIT;
 
 /* A stack to save previously used newunit-assigned unit numbers to
    allow them to be reused without reallocating the gfc_unit structure
@@ -81,6 +104,7 @@ static GFC_INTEGER_4 next_available_newunit = GFC_FIRST_NEWUNIT;
 static gfc_saved_unit newunit_stack[NEWUNIT_STACK_SIZE];
 static int newunit_tos = 0; /* Index to Top of Stack.  */
 
+
 #define CACHE_SIZE 3
 static gfc_unit *unit_cache[CACHE_SIZE];
 gfc_offset max_offset;
@@ -551,7 +575,7 @@ get_unit (st_parameter_dt *dtp, int do_create)
       if ((dtp->common.flags & IOPARM_DT_HAS_UDTIO) != 0)
 	{
 	  dtp->u.p.unit_is_internal = 1;
-	  dtp->common.unit = get_unique_unit_number (&dtp->common);
+	  dtp->common.unit = newunit_alloc ();
 	  unit = get_gfc_unit (dtp->common.unit, do_create);
 	  set_internal_unit (dtp, unit, kind);
 	  fbuf_init (unit, 128);
@@ -567,7 +591,7 @@ get_unit (st_parameter_dt *dtp, int do_create)
 	    }
 	  else
 	    {
-	      dtp->common.unit = get_unique_unit_number (&dtp->common);
+	      dtp->common.unit = newunit_alloc ();
 	      unit = xcalloc (1, sizeof (gfc_unit));
 	      fbuf_init (unit, 128);
 	    }
@@ -734,6 +758,9 @@ close_unit_1 (gfc_unit *u, int locked)
   free_format_hash_table (u);
   fbuf_destroy (u);
 
+  if (u->unit_number <= NEWUNIT_START)
+    newunit_free (u->unit_number);
+
   if (!locked)
     __gthread_mutex_unlock (&u->lock);
 
@@ -788,6 +815,9 @@ close_units (void)
 	free (newunit_stack[newunit_tos].unit->s);
 	free (newunit_stack[newunit_tos--].unit);
       }
+
+  free (newunits);
+
 #ifdef HAVE_FREELOCALE
   freelocale (c_locale);
 #endif
@@ -885,25 +915,56 @@ finish_last_advance_record (gfc_unit *u)
   fbuf_flush (u, u->mode);
 }
 
+
 /* Assign a negative number for NEWUNIT in OPEN statements or for
    internal units.  */
-GFC_INTEGER_4
-get_unique_unit_number (st_parameter_common *common)
+int
+newunit_alloc (void)
 {
-  GFC_INTEGER_4 num;
-
-#ifdef HAVE_SYNC_FETCH_AND_ADD
-  num = __sync_fetch_and_add (&next_available_newunit, -1);
-#else
   __gthread_mutex_lock (&unit_lock);
-  num = next_available_newunit--;
-  __gthread_mutex_unlock (&unit_lock);
-#endif
-  /* Do not allow NEWUNIT numbers to wrap.  */
-  if (num > GFC_FIRST_NEWUNIT)
+  if (!newunits)
     {
-      generate_error (common, LIBERROR_INTERNAL, "NEWUNIT exhausted");
-      return 0;
+      newunits = xcalloc (32, 1);
+      newunit_size = 32;
     }
-  return num;
+
+  /* Search for the next available newunit.  */
+  for (int ii = newunit_lwi; ii < newunit_size; ii++)
+    {
+      if (!newunits[ii])
+        {
+          newunits[ii] = true;
+          newunit_lwi = ii + 1;
+	  __gthread_mutex_unlock (&unit_lock);
+          return -ii + NEWUNIT_START;
+        }
+    }
+
+  /* Search failed, bump size of array and allocate the first
+     available unit.  */
+  int old_size = newunit_size;
+  newunit_size *= 2;
+  void *p = realloc (newunits, newunit_size);
+  if (!p)
+    abort ();
+  newunits = p;
+  memset (newunits + old_size, 0, old_size);
+  newunits[old_size] = true;
+  newunit_lwi = old_size + 1;
+    __gthread_mutex_unlock (&unit_lock);
+  return -old_size + NEWUNIT_START;
+}
+
+
+/* Free a previously allocated newunit= unit number.  unit_lock must
+   be held when calling.  */
+
+static void
+newunit_free (int unit)
+{
+  int ind = -unit + NEWUNIT_START;
+  assert(ind >= 0 && ind < newunit_size);
+  newunits[ind] = false;
+  if (ind < newunit_lwi)
+    newunit_lwi = ind;
 }
-- 
2.7.4



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