__libdw_dieabbrev: Replace rwlock with __atomic builtins

__libdw_dieabbrev uses the abbrev_lock rwlock to synchronize access to the
Dwarf_Die abbrev field as well as its lazy loading.  Calls to rwlock_wrlock
and unlock incur significant performance overhead even in single-threaded
cases.

This patch implements thread safety in __libdw_dieabbrev with GCC __atomic
builtins instead of an rwlock, improving performance.  abbrev_lock has been
changed to a mutex and now synchronizes access to the Dwarf_CU field
last_abbrev_offset in __libdw_findabbrev.

libdw/
	* dwarf_begin_elf.c (valid_p): Change type of abbrev_lock from
	rwlock to mutex.
	* dwarf_end.c (cu_free): Ditto.
	* dwarf_tag.c (__libdw_findabbrev): Protect
	cu->last_abbrev_offset with mutex.
	* libdwP.h (struct Dwarf_CU): Change type of abbrev_lock from
	rwlock to mutex.
	(__libdw_dieabbrev): Use __atomic builtins instead of rwlock.
	* libdw_findcu.c (__libdw_intern_next_unit): Change type of
	abbrev_lock from rwlock to mutex.

Signed-off-by: Aaron Merey <amerey@redhat.com>

diff --git a/libdw/dwarf_begin_elf.c b/libdw/dwarf_begin_elf.c
index 63e2805d3439627c2f60d7229384eb761c5b806d..f5cabde0711135a9e1cca529a8a12716ca8250ee 100644 (file)
--- a/libdw/dwarf_begin_elf.c
+++ b/libdw/dwarf_begin_elf.c
@@ -359,8 +359,8 @@ valid_p (Dwarf *result)
          result->fake_loc_cu->version = 4;
          result->fake_loc_cu->split = NULL;
          eu_search_tree_init (&result->fake_loc_cu->locs_tree);
-         rwlock_init (result->fake_loc_cu->abbrev_lock);
          rwlock_init (result->fake_loc_cu->split_lock);
+         mutex_init (result->fake_loc_cu->abbrev_lock);
          mutex_init (result->fake_loc_cu->src_lock);
          mutex_init (result->fake_loc_cu->str_off_base_lock);
          mutex_init (result->fake_loc_cu->intern_lock);
@@ -392,8 +392,8 @@ valid_p (Dwarf *result)
          result->fake_loclists_cu->version = 5;
          result->fake_loclists_cu->split = NULL;
          eu_search_tree_init (&result->fake_loclists_cu->locs_tree);
-         rwlock_init (result->fake_loclists_cu->abbrev_lock);
          rwlock_init (result->fake_loclists_cu->split_lock);
+         mutex_init (result->fake_loclists_cu->abbrev_lock);
          mutex_init (result->fake_loclists_cu->src_lock);
          mutex_init (result->fake_loclists_cu->str_off_base_lock);
          mutex_init (result->fake_loclists_cu->intern_lock);
@@ -430,8 +430,8 @@ valid_p (Dwarf *result)
          result->fake_addr_cu->version = 5;
          result->fake_addr_cu->split = NULL;
          eu_search_tree_init (&result->fake_addr_cu->locs_tree);
-         rwlock_init (result->fake_addr_cu->abbrev_lock);
          rwlock_init (result->fake_addr_cu->split_lock);
+         mutex_init (result->fake_addr_cu->abbrev_lock);
          mutex_init (result->fake_addr_cu->src_lock);
          mutex_init (result->fake_addr_cu->str_off_base_lock);
          mutex_init (result->fake_addr_cu->intern_lock);

diff --git a/libdw/dwarf_end.c b/libdw/dwarf_end.c
index 979b11689edac09c466f1604333d7abab797a70e..fa27b19e1ba4fb2aec18973044510da7240259ca 100644 (file)
--- a/libdw/dwarf_end.c
+++ b/libdw/dwarf_end.c
@@ -63,8 +63,8 @@ cu_free (void *arg)
   struct Dwarf_CU *p = (struct Dwarf_CU *) arg;
 
   eu_search_tree_fini (&p->locs_tree, noop_free);
-  rwlock_fini (p->abbrev_lock);
   rwlock_fini (p->split_lock);
+  mutex_fini (p->abbrev_lock);
   mutex_fini (p->src_lock);
   mutex_fini (p->str_off_base_lock);
   mutex_fini (p->intern_lock);

diff --git a/libdw/dwarf_tag.c b/libdw/dwarf_tag.c
index 218382a173386dbb5739c13228ac7f2a922b3a48..7daa48134601131f6bf55e73d6f45b0e48159a00 100644 (file)
--- a/libdw/dwarf_tag.c
+++ b/libdw/dwarf_tag.c
@@ -53,15 +53,19 @@ __libdw_findabbrev (struct Dwarf_CU *cu, unsigned int code)
 
        /* Find the next entry.  It gets automatically added to the
           hash table.  */
+       mutex_lock (cu->abbrev_lock);
        abb = __libdw_getabbrev (cu->dbg, cu, cu->last_abbrev_offset, &length);
+
        if (abb == NULL || abb == DWARF_END_ABBREV)
          {
            /* Make sure we do not try to search for it again.  */
            cu->last_abbrev_offset = (size_t) -1l;
+           mutex_unlock (cu->abbrev_lock);
            return DWARF_END_ABBREV;
          }
 
        cu->last_abbrev_offset += length;
+       mutex_unlock (cu->abbrev_lock);
 
        /* Is this the code we are looking for?  */
        if (abb->code == code)

diff --git a/libdw/libdwP.h b/libdw/libdwP.h
index b77db1423fe0bb68e4634b9f3b1be6ebe50f0e81..628777b069da4b68bc032460c5928fc62ef68e30 100644 (file)
--- a/libdw/libdwP.h
+++ b/libdw/libdwP.h
@@ -455,14 +455,14 @@ struct Dwarf_CU
      Don't access directly, call __libdw_cu_locs_base.  */
   Dwarf_Off locs_base;
 
-  /* Synchronize access to the abbrev member of a Dwarf_Die that
-     refers to this Dwarf_CU.  Covers __libdw_die_abbrev. */
-  rwlock_define(, abbrev_lock);
-
   /* Synchronize access to the split member of this Dwarf_CU.
      Covers __libdw_find_split_unit.  */
   rwlock_define(, split_lock);
 
+  /* Synchronize access to the last_abbrev_offset member of a Dwarf_Die
+     that refers to this Dwarf_CU.  */
+  mutex_define(, abbrev_lock);
+
   /* Synchronize access to the lines and files members.
      Covers dwarf_getsrclines and dwarf_getsrcfiles.  */
   mutex_define(, src_lock);
@@ -823,41 +823,49 @@ static inline Dwarf_Abbrev *
 __nonnull_attribute__ (1)
 __libdw_dieabbrev (Dwarf_Die *die, const unsigned char **readp)
 {
+  Dwarf_Abbrev *end_abbrev = DWARF_END_ABBREV;
+  Dwarf_Abbrev *expected = (Dwarf_Abbrev *) NULL;
+
   if (unlikely (die->cu == NULL))
     {
-      die->abbrev = DWARF_END_ABBREV;
-      return DWARF_END_ABBREV;
+      /* __atomic_* compiler builtin functions are used instead of <stdatomic.h>
+        because the builtins can operate on non-_Atomic types.
+        Dwarf_Die.abbrev cannot be made _Atomic without possibly breaking ABI
+        compatibility.  */
+      __atomic_compare_exchange_n (&die->abbrev, &expected, end_abbrev, false,
+                                  __ATOMIC_RELEASE, __ATOMIC_ACQUIRE);
+      return end_abbrev;
     }
 
-  rwlock_wrlock (die->cu->abbrev_lock);
-
-  /* Do we need to get the abbreviation, or need to read after the code?  */
-  if (die->abbrev == NULL || readp != NULL)
+  Dwarf_Abbrev *abbrev = __atomic_load_n (&die->abbrev, __ATOMIC_ACQUIRE);
+  if (abbrev == NULL || readp != NULL)
     {
-      /* Get the abbreviation code.  */
+      /* We need to get the abbreviation or need to read after the code.  */
       unsigned int code;
       const unsigned char *addr = die->addr;
-
       if (addr >= (const unsigned char *) die->cu->endp)
        {
-         die->abbrev = DWARF_END_ABBREV;
-         rwlock_unlock (die->cu->abbrev_lock);
-         return DWARF_END_ABBREV;
+         __atomic_compare_exchange_n (&die->abbrev, &expected,
+                                      end_abbrev, false,
+                                      __ATOMIC_RELEASE, __ATOMIC_ACQUIRE);
+         return end_abbrev;
        }
 
+      /* Get the abbreviation code.  */
       get_uleb128 (code, addr, die->cu->endp);
       if (readp != NULL)
        *readp = addr;
 
       /* Find the abbreviation.  */
-      if (die->abbrev == NULL)
-       die->abbrev = __libdw_findabbrev (die->cu, code);
+      if (abbrev == NULL)
+       {
+         abbrev = __libdw_findabbrev (die->cu, code);
+         __atomic_compare_exchange_n (&die->abbrev, &expected, abbrev, false,
+                                      __ATOMIC_RELEASE, __ATOMIC_ACQUIRE);
+       }
     }
 
-  Dwarf_Abbrev *result = die->abbrev;
-  rwlock_unlock (die->cu->abbrev_lock);
-
-  return result;
+  return abbrev;
 }
 
 /* Helper functions for form handling.  */

diff --git a/libdw/libdw_findcu.c b/libdw/libdw_findcu.c
index 592673439c488dbb20ad13b7203827017180ef8a..a1f700d38970f5c96e976029a22df00c1e9a9e8d 100644 (file)
--- a/libdw/libdw_findcu.c
+++ b/libdw/libdw_findcu.c
@@ -177,8 +177,8 @@ __libdw_intern_next_unit (Dwarf *dbg, bool debug_types)
   newp->startp = data->d_buf + newp->start;
   newp->endp = data->d_buf + newp->end;
   eu_search_tree_init (&newp->locs_tree);
-  rwlock_init (newp->abbrev_lock);
   rwlock_init (newp->split_lock);
+  mutex_init (newp->abbrev_lock);
   mutex_init (newp->src_lock);
   mutex_init (newp->str_off_base_lock);
   mutex_init (newp->intern_lock);