Re: [PATCH] Implement libc_once_retry for atomic initialization with allocation

[Florian Weimer <fweimer at redhat dot com>]

On 01/04/2018 06:15 PM, Torvald Riegel wrote:
> On Thu, 2018-01-04 at 16:09 +0100, Florian Weimer wrote:
> > Subject: [PATCH] Implement libc_once_retry for atomic initialization with allocation
>
> Can you find a different name for it?  It has similarities to a generic
> libc_once, but the pattern is more special.

libc_concurrent_init?

> Do you need it often enough to make it worth breaking it out?

I think it's worthwhile to separate the concurrency review from the 
application logic.

> > diff --git a/include/atomic.h b/include/atomic.h
> > index 6af07dba58..91e176b040 100644
> > --- a/include/atomic.h
> >
> > +++ b/include/atomic.h
>
> I don't think this should go into atomic.h because this is where we
> really handle the basics of synchronization.  This is a synchronization
> helper, and doesn't need to be touched by someone porting glibc to
> another arch.

What would be a good place instead?  A new header file?

> > +/* Perform an acquire MO load on *PLACE.  If *PLACE is not NULL,
> >
> > +   return *PLACE.  Otherwise, call ALLOCATE (CLOSURE).  If that
> >
> > +   returns NULL, return NULL.  Otherwise, atomically replace *PLACE
> >
> > +   with PTR, the result of the ALLOCATE call (with acquire-release
>
> That doesn't quite match what you implement.  First, you don't replace
> it unconditionally, but use a CAS (you can't just throw in an
> "atomically" here because the allocations are mixed in and they aren't).
> Second, you use the weak CAS but the loop includes alloc/dealloc; see
> below.

Hmm, right, that is unnecessary.  I have updated the comment in the 
attached patch.

/* Perform an acquire MO load on *PLACE.  If *PLACE is not NULL,
   return *PLACE.  Otherwise, call ALLOCATE (CLOSURE), yielding
   RESULT.  If RESULT equals NULL, return NULL.  Otherwise, atomically
   replace the NULL value in *PLACE with the RESULT value.  If it
   turns out that *PLACE was updated concurrently, call DEALLOCATE
   (CLOSURE, RESULT) to undo the effect of ALLOCATE, and return the
   new value of *PLACE (after an acquire MO load).  If DEALLOCATE is
   NULL, call free (RESULT) instead.

   The net effect is that if libc_once_retry returns a non-NULL value,
   the caller can assume that pointed-to data has been initialized
   according to the ALLOCATE function.

> > +  do
> >
> > +    {
> >
> > +      result = allocate (closure);
> >
> > +      if (result == NULL)
> >
> > +        return NULL;
> >
> > +
> >
> > +      /* Synchronizes with the acquire MO load in
> >
> > +         __libc_once_retry.  */
> >
> > +      void *expected = NULL;
> >
> > +      if (atomic_compare_exchange_weak_release (place, &expected, result))
> >
> > +        return result;
>
> It seems you're looking for a strong CAS here, so why don't you make a
> loop around the weak CAS?  Using an acq_rel CAS, this is simpler.

It's the only CAS we have: weak with relaxed MO on failure.  I need 
strong with acquire MO on failure, and currently, the only way to get 
that is with the loop and the additional load.  Sorry.

> You'd avoid having more than one allocate/deallocate pair, which might
> be unexpected given that you're documentation of the function's
> semantics doesn't make that clear (and it might perhaps matter for
> custom alloc/dealloc functions).

Oh, good point.  I changed the implementation in the attached patch.

Thanks,
Florian

Subject: [PATCH] Implement libc_once_retry for atomic initialization with allocation
To: libc-alpha@sourceware.org

2018-01-04  Florian Weimer  <fweimer@redhat.com>

	* include/atomic.h (__libc_once_retry_slow): Declare.
	(libc_once_retry): Define.
	* misc/libc_once_retry.c: New file.
	* misc/tst-libc_once_retry.c: Likewise.
	* misc/Makefile (routines): Add libc_once_retry.
	(tests-internal): Add tst-libc_once_retry.
	* misc/Versions (GLIBC_PRIVATE): Add __libc_once_retry_slow.

diff --git a/include/atomic.h b/include/atomic.h
index 6af07dba58..ec996cd224 100644
--- a/include/atomic.h
+++ b/include/atomic.h
@@ -826,4 +826,63 @@ void __atomic_link_error (void);
 # error ATOMIC_EXCHANGE_USES_CAS has to be defined.
 #endif
 
+/* Slow path for libc_once_retry; see below.  */
+void *__libc_once_retry_slow (void **__place,
+			      void *(*__allocate) (void *__closure),
+			      void (*__deallocate) (void *__closure,
+						    void *__ptr),
+			      void *__closure);
+
+/* Perform an acquire MO load on *PLACE.  If *PLACE is not NULL,
+   return *PLACE.  Otherwise, call ALLOCATE (CLOSURE), yielding
+   RESULT.  If RESULT equals NULL, return NULL.  Otherwise, atomically
+   replace the NULL value in *PLACE with the RESULT value.  If it
+   turns out that *PLACE was updated concurrently, call DEALLOCATE
+   (CLOSURE, RESULT) to undo the effect of ALLOCATE, and return the
+   new value of *PLACE (after an acquire MO load).  If DEALLOCATE is
+   NULL, call free (RESULT) instead.
+
+   The net effect is that if libc_once_retry returns a non-NULL value,
+   the caller can assume that pointed-to data has been initialized
+   according to the ALLOCATE function.
+
+   It is expected that callers define an inline helper function which
+   adds type safety, like this.
+
+   struct foo { ... };
+   struct foo *global_foo;
+   static void *allocate_foo (void *closure);
+   static void *deallocate_foo (void *closure, void *ptr);
+
+   static inline struct foo *
+   get_foo (void)
+   {
+     return libc_once_retry (&global_foo, allocate_foo, free_foo, NULL);
+   }
+
+   Usage of this function looks like this:
+
+   struct foo *local_foo = get_foo ();
+   if (local_foo == NULL)
+      report_allocation_failure ();
+
+   Compared to __libc_once, __libc_once_retry has the advantage that
+   it does not need separate space for a control variable, and that it
+   is safe with regards to cancellation and other forms of exception
+   handling if the provided callback functions are safe.  */
+static inline void *
+libc_once_retry (void **__place, void *(*__allocate) (void *__closure),
+		 void (*__deallocate) (void *__closure, void *__ptr),
+		 void *__closure)
+{
+  /* Synchronizes with the release MO CAS in
+     __libc_once_retry_slow.  */
+  void *__result = atomic_load_acquire (__place);
+  if (__result != NULL)
+    return __result;
+  else
+    return __libc_once_retry_slow (__place, __allocate, __deallocate,
+				   __closure);
+}
+
 #endif	/* atomic.h */
diff --git a/misc/Makefile b/misc/Makefile
index a5076b3672..7b1314d01b 100644
--- a/misc/Makefile
+++ b/misc/Makefile
@@ -70,7 +70,8 @@ routines := brk sbrk sstk ioctl \
 	    getloadavg getclktck \
 	    fgetxattr flistxattr fremovexattr fsetxattr getxattr \
 	    listxattr lgetxattr llistxattr lremovexattr lsetxattr \
-	    removexattr setxattr getauxval ifunc-impl-list makedev
+	    removexattr setxattr getauxval ifunc-impl-list makedev \
+	    libc_once_retry
 
 generated += tst-error1.mtrace tst-error1-mem.out
 
@@ -84,7 +85,7 @@ tests := tst-dirname tst-tsearch tst-fdset tst-efgcvt tst-mntent tst-hsearch \
 	 tst-preadvwritev tst-preadvwritev64 tst-makedev tst-empty \
 	 tst-preadvwritev2 tst-preadvwritev64v2
 
-tests-internal := tst-atomic tst-atomic-long
+tests-internal := tst-atomic tst-atomic-long tst-libc_once_retry
 tests-static := tst-empty
 
 ifeq ($(run-built-tests),yes)
diff --git a/misc/Versions b/misc/Versions
index bfbda505e4..a129e90fc0 100644
--- a/misc/Versions
+++ b/misc/Versions
@@ -165,5 +165,6 @@ libc {
     __tdelete; __tfind; __tsearch; __twalk;
     __mmap; __munmap; __mprotect;
     __sched_get_priority_min; __sched_get_priority_max;
+    __libc_once_retry_slow;
   }
 }
diff --git a/misc/libc_once_retry.c b/misc/libc_once_retry.c
new file mode 100644
index 0000000000..91c486975a
--- /dev/null
+++ b/misc/libc_once_retry.c
@@ -0,0 +1,57 @@
+/* Concurrent initialization of a pointer.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <atomic.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+void *
+__libc_once_retry_slow (void **place, void *(*allocate) (void *closure),
+                        void (*deallocate) (void *closure, void *ptr),
+                        void *closure)
+{
+  void *result = allocate (closure);
+  if (result == NULL)
+    return NULL;
+
+  while (true)
+    {
+      /* Synchronizes with the acquire MO load in
+         __libc_once_retry.  */
+      void *expected = NULL;
+      if (atomic_compare_exchange_weak_release (place, &expected, result))
+        return result;
+
+      /* The failed CAS has relaxed MO semantics, so perform another
+         acquire MO load.  */
+      void *other_result = atomic_load_acquire (place);
+      if (other_result == NULL)
+        /* Spurious failure.  Try again.  */
+        continue;
+
+      /* We lost the race.  Free what we allocated and return the
+         other result.  */
+      if (deallocate == NULL)
+        free (result);
+      else
+        deallocate (closure, result);
+      return other_result;
+    }
+
+  return result;
+}
diff --git a/misc/tst-libc_once_retry.c b/misc/tst-libc_once_retry.c
new file mode 100644
index 0000000000..9b2bbd187c
--- /dev/null
+++ b/misc/tst-libc_once_retry.c
@@ -0,0 +1,175 @@
+/* Test the libc_once_retry function.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <atomic.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+
+/* Allocate a new string.  */
+static void *
+allocate_string (void *closure)
+{
+  return xstrdup (closure);
+}
+
+/* Allocation and deallocation functions which are not expected to be
+   called.  */
+
+static void *
+allocate_not_called (void *closure)
+{
+  FAIL_EXIT1 ("allocation function called unexpectedly (%p)", closure);
+}
+
+static void
+deallocate_not_called (void *closure, void *ptr)
+{
+  FAIL_EXIT1 ("deallocate function called unexpectedly (%p, %p)",
+              closure, ptr);
+}
+
+/* Counter for various function calls.  */
+static int function_called;
+
+/* An allocation function which returns NULL and records that it has
+   been called.  */
+static void *
+allocate_return_null (void *closure)
+{
+  /* The function should only be called once.  */
+  TEST_COMPARE (function_called, 0);
+  ++function_called;
+  return NULL;
+}
+
+
+/* The following is used to check the retry logic, by causing a fake
+   race condition.  */
+static void *fake_race_place;
+static char fake_race_region[3]; /* To obtain unique addresses.  */
+
+static void *
+fake_race_allocate (void *closure)
+{
+  TEST_VERIFY (closure == &fake_race_region[0]);
+  TEST_COMPARE (function_called, 0);
+  ++function_called;
+  /* Fake allocation by another thread.  */
+  fake_race_place = &fake_race_region[1];
+  return &fake_race_region[2];
+}
+
+static void
+fake_race_deallocate (void *closure, void *ptr)
+{
+  /* Check that the pointer returned from fake_race_allocate is
+     deallocated (and not the one stored in fake_race_place).  */
+  TEST_VERIFY (ptr == &fake_race_region[2]);
+
+  TEST_VERIFY (fake_race_place == &fake_race_region[1]);
+  TEST_VERIFY (closure == &fake_race_region[0]);
+  TEST_COMPARE (function_called, 1);
+  ++function_called;
+}
+
+/* Similar to fake_race_allocate, but expects to be paired with free
+   as the deallocation function.  */
+static void *
+fake_race_allocate_for_free (void *closure)
+{
+  TEST_VERIFY (closure == &fake_race_region[0]);
+  TEST_COMPARE (function_called, 0);
+  ++function_called;
+  /* Fake allocation by another thread.  */
+  fake_race_place = &fake_race_region[1];
+  return xstrdup ("to be freed");
+}
+
+static int
+do_test (void)
+{
+  /* Simple allocation.  */
+  void *place1 = NULL;
+  char *string1 = libc_once_retry (&place1, allocate_string,
+                                   deallocate_not_called,
+                                   (char *) "test string 1");
+  TEST_VERIFY_EXIT (string1 != NULL);
+  TEST_VERIFY (strcmp ("test string 1", string1) == 0);
+  /* Second call returns the first pointer, without calling any
+     callbacks.  */
+  TEST_VERIFY (string1
+               == libc_once_retry (&place1, allocate_not_called,
+                                   deallocate_not_called,
+                                   (char *) "test string 1a"));
+
+  /* Difference place should result in another call.  */
+  void *place2 = NULL;
+  char *string2 = libc_once_retry (&place2, allocate_string,
+                                   deallocate_not_called,
+                                   (char *) "test string 2");
+  TEST_VERIFY_EXIT (string2 != NULL);
+  TEST_VERIFY (strcmp ("test string 2", string2) == 0);
+  TEST_VERIFY (string1 != string2);
+
+  /* Check error reporting (NULL return value from the allocation
+     function).  */
+  void *place3 = NULL;
+  char *string3 = libc_once_retry (&place3, allocate_return_null,
+                                   deallocate_not_called, NULL);
+  TEST_VERIFY (string3 == NULL);
+  TEST_COMPARE (function_called, 1);
+
+  /* Check that the deallocation function is called if the race is
+     lost.  */
+  function_called = 0;
+  TEST_VERIFY (libc_once_retry (&fake_race_place,
+                                fake_race_allocate,
+                                fake_race_deallocate,
+                                &fake_race_region[0])
+               == &fake_race_region[1]);
+  TEST_COMPARE (function_called, 2);
+  function_called = 3;
+  TEST_VERIFY (libc_once_retry (&fake_race_place,
+                                fake_race_allocate,
+                                fake_race_deallocate,
+                                &fake_race_region[0])
+               == &fake_race_region[1]);
+  TEST_COMPARE (function_called, 3);
+
+  /* Similar, but this time rely on that free is called.  */
+  function_called = 0;
+  fake_race_place = NULL;
+  TEST_VERIFY (libc_once_retry (&fake_race_place,
+                                fake_race_allocate_for_free,
+                                NULL,
+                                &fake_race_region[0])
+               == &fake_race_region[1]);
+  TEST_COMPARE (function_called, 1);
+  function_called = 3;
+  TEST_VERIFY (libc_once_retry (&fake_race_place,
+                                fake_race_allocate_for_free,
+                                NULL,
+                                &fake_race_region[0])
+               == &fake_race_region[1]);
+  TEST_COMPARE (function_called, 3);
+
+  return 0;
+}
+
+#include <support/test-driver.c>