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* Thanks to Janito I had a closer look to cache_detach_sub_transaction(), and it 

turned out it's pretty much broken. Not only did it potentially leak memory,
  it would also potentially replace the original data with a NULL pointer,
  making the current transaction non revertable. The code should now be much
  clearer. This fixes bug #6378.
* Changed a few "if (... != NULL)" into ASSERTs, since the code should bail out
  earlier in these cases already.
* Added a TODO comment to cache_start_sub_transaction() about its broken discard
  handling. This can cause FS corruptions in case the parent transaction is ever
  going to be aborted (which shouldn't happen in real life, though).
* Added a bit more and better comments.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@37899 a95241bf-73f2-0310-859d-f6bbb57e9c96
This commit is contained in:
Axel Dörfler 2010-08-04 13:19:20 +00:00
parent c6247544ed
commit 3426ce1404
1 changed files with 51 additions and 27 deletions
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68
src/system/kernel/cache/block_cache.cpp vendored
View File

@ -60,8 +60,19 @@ struct cached_block {
block_link link;
off_t block_number;
void* current_data;
// The data that is seen by everyone using the API; this one is always
// present.
void* original_data;
// When in a transaction, this contains the original data from before
// the transaction.
void* parent_data;
// This is a lazily alloced buffer that represents the contents of the
// block in the parent transaction. It may point to current_data if the
// contents have been changed only in the parent transaction, or, if the
// block has been changed in the current sub transaction already, to a
// new block containing the contents changed in the parent transaction.
// If this is NULL, the block has not been changed in the parent
// transaction at all.
#if BLOCK_CACHE_DEBUG_CHANGED
void* compare;
#endif
@ -2838,9 +2849,10 @@ cache_abort_transaction(void* _cache, int32 id)
next = block->transaction_next;
if (block->original_data != NULL) {
TRACE(("cache_abort_transaction(id = %ld): restored contents of block %Ld\n",
transaction->id, block->block_number));
memcpy(block->current_data, block->original_data, cache->block_size);
TRACE(("cache_abort_transaction(id = %ld): restored contents of "
"block %Ld\n", transaction->id, block->block_number));
memcpy(block->current_data, block->original_data,
cache->block_size);
cache->Free(block->original_data);
block->original_data = NULL;
}
@ -2925,16 +2937,31 @@ cache_detach_sub_transaction(void* _cache, int32 id,
continue;
}
if (block->original_data != NULL && block->parent_data != NULL) {
// free the original data if the parent data of the transaction
// will be made current - but keep them otherwise
if (block->parent_data != NULL) {
// The block changed in the parent - free the original data, since
// they will be replaced by what is in current.
ASSERT(block->original_data != NULL);
cache->Free(block->original_data);
if (block->parent_data != block->current_data) {
// The block had been changed in both transactions
block->original_data = block->parent_data;
} else {
// The block has only been changed in the parent
block->original_data = NULL;
}
if (block->parent_data == NULL
|| block->parent_data != block->current_data) {
// we need to move this block over to the new transaction
block->original_data = block->parent_data;
// move the block to the previous transaction list
transaction->blocks.Add(block);
block->previous_transaction = transaction;
block->parent_data = NULL;
}
if (block->original_data != NULL) {
// This block had been changed in the current sub transaction,
// we need to move this block over to the new transaction.
ASSERT(block->parent_data == NULL);
if (last == NULL)
newTransaction->first_block = block;
else
@ -2945,13 +2972,6 @@ cache_detach_sub_transaction(void* _cache, int32 id,
} else
block->transaction = NULL;
if (block->parent_data != NULL) {
// move the block to the previous transaction list
transaction->blocks.Add(block);
block->previous_transaction = transaction;
block->parent_data = NULL;
}
block->transaction_next = NULL;
}
@ -2996,16 +3016,15 @@ cache_abort_sub_transaction(void* _cache, int32 id)
next = block->transaction_next;
if (block->parent_data == NULL) {
if (block->original_data != NULL) {
// the parent transaction didn't change the block, but the sub
// transaction did - we need to revert from the original data
ASSERT(block->original_data != NULL);
memcpy(block->current_data, block->original_data,
cache->block_size);
}
} else if (block->parent_data != block->current_data) {
// the block has been changed and must be restored
TRACE(("cache_abort_sub_transaction(id = %ld): restored contents of block %Ld\n",
transaction->id, block->block_number));
TRACE(("cache_abort_sub_transaction(id = %ld): restored contents "
"of block %Ld\n", transaction->id, block->block_number));
memcpy(block->current_data, block->parent_data, cache->block_size);
cache->Free(block->parent_data);
}
@ -3032,7 +3051,8 @@ cache_start_sub_transaction(void* _cache, int32 id)
cache_transaction* transaction = lookup_transaction(cache, id);
if (transaction == NULL) {
panic("cache_start_sub_transaction(): invalid transaction ID %ld\n", id);
panic("cache_start_sub_transaction(): invalid transaction ID %ld\n",
id);
return B_BAD_VALUE;
}
@ -3048,6 +3068,9 @@ cache_start_sub_transaction(void* _cache, int32 id)
if (block->discard) {
// This block has been discarded in the parent transaction
// TODO: this is wrong: since the parent transaction is not
// finished here (just extended), this block cannot be reverted
// anymore in case the parent transaction is aborted!!!
if (last != NULL)
last->transaction_next = next;
else
@ -3057,6 +3080,7 @@ cache_start_sub_transaction(void* _cache, int32 id)
transaction->num_blocks--;
continue;
}
if (transaction->has_sub_transaction
&& block->parent_data != NULL
&& block->parent_data != block->current_data) {