protobuf/upb/mem/arena.c

/*
 * Copyright (c) 2009-2021, Google LLC
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *     * Neither the name of Google LLC nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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 */

#include "upb/mem/arena_internal.h"
#include "upb/port/atomic.h"

// Must be last.
#include "upb/port/def.inc"

static uintptr_t upb_cleanup_metadata(uint32_t* cleanup,
                                      bool has_initial_block) {
  return (uintptr_t)cleanup | has_initial_block;
}

struct _upb_MemBlock {
  // Atomic only for the benefit of SpaceAllocated().
  UPB_ATOMIC(_upb_MemBlock*) next;
  uint32_t size;
  // Data follows.
};

static const size_t memblock_reserve =
    UPB_ALIGN_UP(sizeof(_upb_MemBlock), UPB_MALLOC_ALIGN);

static upb_Arena* _upb_Arena_FindRoot(upb_Arena* a) {
  uintptr_t poc = upb_Atomic_Load(&a->parent_or_count, memory_order_acquire);
  while (_upb_Arena_IsTaggedPointer(poc)) {
    upb_Arena* next = _upb_Arena_PointerFromTagged(poc);
    uintptr_t next_poc =
        upb_Atomic_Load(&next->parent_or_count, memory_order_acquire);

    if (_upb_Arena_IsTaggedPointer(next_poc)) {
      // To keep complexity down, we lazily collapse levels of the tree.  This
      // keeps it flat in the final case, but doesn't cost much incrementally.
      //
      // Path splitting keeps time complexity down, see:
      //   https://en.wikipedia.org/wiki/Disjoint-set_data_structure
      //
      // We can safely use a relaxed atomic here because all threads doing this
      // will converge on the same value and we don't need memory orderings to
      // be visible.
      //
      // This is true because:
      // - If no fuses occur, this will eventually become the root.
      // - If fuses are actively occuring, the root may change, but the
      //   invariant is that `parent_or_count` merely points to *a* parent.
      //
      // In other words, it is moving towards "the" root, and that root may move
      // further away over time, but the path towards that root will continue to
      // be valid and the creation of the path carries all the memory orderings
      // required.
      upb_Atomic_Store(&a->parent_or_count, next_poc, memory_order_relaxed);
    }
    a = next;
    poc = next_poc;
  }
  return a;
}

size_t upb_Arena_SpaceAllocated(upb_Arena* arena) {
  arena = _upb_Arena_FindRoot(arena);
  size_t memsize = 0;

  while (arena != NULL) {
    _upb_MemBlock* block = arena->blocks;
    while (block != NULL) {
      memsize += sizeof(_upb_MemBlock) + block->size;
      block = upb_Atomic_Load(&block->next, memory_order_relaxed);
    }
    arena = upb_Atomic_Load(&arena->next, memory_order_relaxed);
  }

  return memsize;
}

uint32_t upb_Arena_DebugRefCount(upb_Arena* a) {
  // These loads could probably be relaxed, but given that this is debug-only,
  // it's not worth introducing a new variant for it.
  uintptr_t poc = upb_Atomic_Load(&a->parent_or_count, memory_order_acquire);
  while (_upb_Arena_IsTaggedPointer(poc)) {
    a = _upb_Arena_PointerFromTagged(poc);
    poc = upb_Atomic_Load(&a->parent_or_count, memory_order_acquire);
  }
  return _upb_Arena_RefCountFromTagged(poc);
}

static void upb_Arena_AddBlock(upb_Arena* a, void* ptr, size_t size) {
  _upb_MemBlock* block = ptr;

  // Insert into linked list.
  upb_Atomic_Init(&block->next, a->blocks);
  block->size = (uint32_t)size;
  upb_Atomic_Store(&a->blocks, block, memory_order_relaxed);

  a->head.ptr = UPB_PTR_AT(block, memblock_reserve, char);
  a->head.end = UPB_PTR_AT(block, size, char);

  UPB_POISON_MEMORY_REGION(a->head.ptr, a->head.end - a->head.ptr);
}

static bool upb_Arena_AllocBlock(upb_Arena* a, size_t size) {
  if (!a->block_alloc) return false;
  _upb_MemBlock* last_block = upb_Atomic_Load(&a->blocks, memory_order_relaxed);
  size_t last_size = last_block != NULL ? last_block->size : 128;
  size_t block_size = UPB_MAX(size, last_size * 2) + memblock_reserve;
  _upb_MemBlock* block = upb_malloc(upb_Arena_BlockAlloc(a), block_size);

  if (!block) return false;
  upb_Arena_AddBlock(a, block, block_size);
  return true;
}

void* _upb_Arena_SlowMalloc(upb_Arena* a, size_t size) {
  if (!upb_Arena_AllocBlock(a, size)) return NULL; /* Out of memory. */
  UPB_ASSERT(_upb_ArenaHas(a) >= size);
  return upb_Arena_Malloc(a, size);
}

/* Public Arena API ***********************************************************/

static upb_Arena* upb_Arena_InitSlow(upb_alloc* alloc) {
  const size_t first_block_overhead = sizeof(upb_Arena) + memblock_reserve;
  upb_Arena* a;

  /* We need to malloc the initial block. */
  char* mem;
  size_t n = first_block_overhead + 256;
  if (!alloc || !(mem = upb_malloc(alloc, n))) {
    return NULL;
  }

  a = UPB_PTR_AT(mem, n - sizeof(*a), upb_Arena);
  n -= sizeof(*a);

  a->block_alloc = upb_Arena_MakeBlockAlloc(alloc, 0);
  upb_Atomic_Init(&a->parent_or_count, _upb_Arena_TaggedFromRefcount(1));
  upb_Atomic_Init(&a->next, NULL);
  upb_Atomic_Init(&a->tail, a);
  upb_Atomic_Init(&a->blocks, NULL);

  upb_Arena_AddBlock(a, mem, n);

  return a;
}

upb_Arena* upb_Arena_Init(void* mem, size_t n, upb_alloc* alloc) {
  upb_Arena* a;

  if (n) {
    /* Align initial pointer up so that we return properly-aligned pointers. */
    void* aligned = (void*)UPB_ALIGN_UP((uintptr_t)mem, UPB_MALLOC_ALIGN);
    size_t delta = (uintptr_t)aligned - (uintptr_t)mem;
    n = delta <= n ? n - delta : 0;
    mem = aligned;
  }

  /* Round block size down to alignof(*a) since we will allocate the arena
   * itself at the end. */
  n = UPB_ALIGN_DOWN(n, UPB_ALIGN_OF(upb_Arena));

  if (UPB_UNLIKELY(n < sizeof(upb_Arena))) {
    return upb_Arena_InitSlow(alloc);
  }

  a = UPB_PTR_AT(mem, n - sizeof(*a), upb_Arena);

  upb_Atomic_Init(&a->parent_or_count, _upb_Arena_TaggedFromRefcount(1));
  upb_Atomic_Init(&a->next, NULL);
  upb_Atomic_Init(&a->tail, a);
  upb_Atomic_Init(&a->blocks, NULL);
  a->block_alloc = upb_Arena_MakeBlockAlloc(alloc, 1);
  a->head.ptr = mem;
  a->head.end = UPB_PTR_AT(mem, n - sizeof(*a), char);

  return a;
}

static void arena_dofree(upb_Arena* a) {
  UPB_ASSERT(_upb_Arena_RefCountFromTagged(a->parent_or_count) == 1);

  while (a != NULL) {
    // Load first since arena itself is likely from one of its blocks.
    upb_Arena* next_arena =
        (upb_Arena*)upb_Atomic_Load(&a->next, memory_order_acquire);
    upb_alloc* block_alloc = upb_Arena_BlockAlloc(a);
    _upb_MemBlock* block = upb_Atomic_Load(&a->blocks, memory_order_relaxed);
    while (block != NULL) {
      // Load first since we are deleting block.
      _upb_MemBlock* next_block =
          upb_Atomic_Load(&block->next, memory_order_relaxed);
      upb_free(block_alloc, block);
      block = next_block;
    }
    a = next_arena;
  }
}

void upb_Arena_Free(upb_Arena* a) {
  uintptr_t poc = upb_Atomic_Load(&a->parent_or_count, memory_order_acquire);
retry:
  while (_upb_Arena_IsTaggedPointer(poc)) {
    a = _upb_Arena_PointerFromTagged(poc);
    poc = upb_Atomic_Load(&a->parent_or_count, memory_order_acquire);
  }

  // compare_exchange or fetch_sub are RMW operations, which are more
  // expensive then direct loads.  As an optimization, we only do RMW ops
  // when we need to update things for other threads to see.
  if (poc == _upb_Arena_TaggedFromRefcount(1)) {
    arena_dofree(a);
    return;
  }

  if (upb_Atomic_CompareExchangeStrong(
          &a->parent_or_count, &poc,
          _upb_Arena_TaggedFromRefcount(_upb_Arena_RefCountFromTagged(poc) - 1),
          memory_order_release, memory_order_acquire)) {
    // We were >1 and we decremented it successfully, so we are done.
    return;
  }

  // We failed our update, so someone has done something, retry the whole
  // process, but the failed exchange reloaded `poc` for us.
  goto retry;
}

bool upb_Arena_Fuse(upb_Arena* a1, upb_Arena* a2) {
  // SAFE IN THE PRESENCE OF FUSE/FREE RACES BUT NOT IN THE
  // PRESENCE OF FUSE/FUSE RACES!!!
  //
  // `parent_or_count` has two disctint modes
  // -  parent pointer mode
  // -  refcount mode
  //
  // In parent pointer mode, it may change what pointer it refers to in the
  // tree, but it will always approach a root.  Any operation that walks the
  // tree to the root may collapse levels of the tree concurrently.
  //
  // In refcount mode, any free operation may lower the refcount.
  //
  // Only a fuse operation may increase the refcount.
  // Only a fuse operation may switch `parent_or_count` from parent mode to
  // refcount mode.
  //
  // Given that we do not allow fuse/fuse races, we may rely on the invariant
  // that only refcounts can change once we have found the root.  Because the
  // threads doing the fuse must hold references, we can guarantee that no
  // refcounts will reach zero concurrently.

  upb_Arena* r1 = _upb_Arena_FindRoot(a1);
  upb_Arena* r2 = _upb_Arena_FindRoot(a2);

  if (r1 == r2) return true;  // Already fused.

  // Do not fuse initial blocks since we cannot lifetime extend them.
  // Any other fuse scenario is allowed.
  if (upb_Arena_HasInitialBlock(r1)) return false;
  if (upb_Arena_HasInitialBlock(r2)) return false;

  uintptr_t r1_poc =
      upb_Atomic_Load(&r1->parent_or_count, memory_order_acquire);
  uintptr_t r2_poc =
      upb_Atomic_Load(&r2->parent_or_count, memory_order_acquire);
  UPB_ASSERT(_upb_Arena_IsTaggedRefcount(r1_poc));
  UPB_ASSERT(_upb_Arena_IsTaggedRefcount(r2_poc));

  // Keep the tree shallow by joining the smaller tree to the larger.
  if (_upb_Arena_RefCountFromTagged(r1_poc) <
      _upb_Arena_RefCountFromTagged(r2_poc)) {
    upb_Arena* tmp = r1;
    r1 = r2;
    r2 = tmp;

    uintptr_t tmp_poc = r1_poc;
    r1_poc = r2_poc;
    r2_poc = tmp_poc;
  }

  // The moment we install `r1` as the parent for `r2` all racing frees may
  // immediately begin decrementing `r1`'s refcount.  So we must install all the
  // refcounts that we know about first to prevent a premature unref to zero.
  uint32_t r2_refcount = _upb_Arena_RefCountFromTagged(r2_poc);
  upb_Atomic_Add(&r1->parent_or_count, ((uintptr_t)r2_refcount) << 1,
                 memory_order_release);

  // When installing `r1` as the parent for `r2` racing frees may have changed
  // the refcount for `r2` so we need to capture the old value to fix up `r1`'s
  // refcount based on the delta from what we saw the first time.
  r2_poc = upb_Atomic_Exchange(&r2->parent_or_count,
                               _upb_Arena_TaggedFromPointer(r1),
                               memory_order_acq_rel);
  UPB_ASSERT(_upb_Arena_IsTaggedRefcount(r2_poc));
  uint32_t delta_refcount = r2_refcount - _upb_Arena_RefCountFromTagged(r2_poc);
  if (delta_refcount != 0) {
    upb_Atomic_Sub(&r1->parent_or_count, ((uintptr_t)delta_refcount) << 1,
                   memory_order_release);
  }

  // Now append r2's linked list of arenas to r1's.
  upb_Arena* r2_tail = upb_Atomic_Load(&r2->tail, memory_order_relaxed);
  upb_Arena* r1_tail = upb_Atomic_Load(&r1->tail, memory_order_relaxed);
  upb_Arena* r1_next = upb_Atomic_Load(&r1_tail->next, memory_order_relaxed);
  while (r1_next != NULL) {
    // r1->tail was stale.  This can happen, but tail should always converge on
    // the true tail.
    r1_tail = r1_next;
    r1_next = upb_Atomic_Load(&r1_tail->next, memory_order_relaxed);
  }

  upb_Arena* old_next =
      upb_Atomic_Exchange(&r1_tail->next, r2, memory_order_relaxed);

  // Once fuse/fuse races are allowed, it will need to be a CAS instead that
  // handles this mismatch gracefully.
  UPB_ASSERT(old_next == NULL);

  upb_Atomic_Store(&r1->tail, r2_tail, memory_order_relaxed);

  return true;
}