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Make async rehash behave with snapshots (thread safety issues)

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Former-commit-id: 372adf39a80252b8035e3c948fcaf7d5ef6f928f
This commit is contained in:
John Sully 2021-03-16 02:38:41 +00:00
parent 25f56c59b0
commit 4f06fb2b4f
4 changed files with 82 additions and 44 deletions
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54
src/dict.cpp
View File

@ -129,6 +129,7 @@ int _dictInit(dict *d, dictType *type,
d->rehashidx = -1;
d->iterators = 0;
d->asyncdata = nullptr;
d->refcount = 1;
return DICT_OK;
}
@ -369,8 +370,14 @@ int dictRehash(dict *d, int n) {
return 1;
}
dictAsyncRehashCtl::dictAsyncRehashCtl(struct dict *d, dictAsyncRehashCtl *next) : dict(d), next(next) {
queue.reserve(c_targetQueueSize);
__atomic_fetch_add(&d->refcount, 1, __ATOMIC_RELEASE);
}
dictAsyncRehashCtl *dictRehashAsyncStart(dict *d, int buckets) {
if (!dictIsRehashing(d)) return 0;
assert(d->type->asyncfree != nullptr);
if (!dictIsRehashing(d) || d->iterators != 0) return nullptr;
d->asyncdata = new dictAsyncRehashCtl(d, d->asyncdata);
@ -454,7 +461,7 @@ void dictCompleteRehashAsync(dictAsyncRehashCtl *ctl, bool fFree) {
}
}
if (fUnlinked && !ctl->release) {
if (fUnlinked && !ctl->abondon) {
if (d->ht[0].table != nullptr) { // can be null if we're cleared during the rehash
for (auto &wi : ctl->queue) {
// We need to remove it from the source hash table, and store it in the dest.
@ -487,23 +494,10 @@ void dictCompleteRehashAsync(dictAsyncRehashCtl *ctl, bool fFree) {
}
if (fFree) {
while (ctl->deGCList != nullptr) {
auto next = ctl->deGCList->next;
dictFreeKey(d, ctl->deGCList);
dictFreeVal(d, ctl->deGCList);
zfree(ctl->deGCList);
ctl->deGCList = next;
}
d->type->asyncfree(ctl);
// Was the dictionary free'd while we were in flight?
if (ctl->release) {
if (d->asyncdata != nullptr)
d->asyncdata->release = true;
else
dictRelease(d);
}
delete ctl;
// Remove our reference
dictRelease(d);
}
}
@ -514,6 +508,16 @@ long long timeInMilliseconds(void) {
return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000);
}
dictAsyncRehashCtl::~dictAsyncRehashCtl() {
while (deGCList != nullptr) {
auto next = deGCList->next;
dictFreeKey(dict, deGCList);
dictFreeVal(dict, deGCList);
zfree(deGCList);
deGCList = next;
}
}
/* Rehash in ms+"delta" milliseconds. The value of "delta" is larger
* than 0, and is smaller than 1 in most cases. The exact upper bound
* depends on the running time of dictRehash(d,100).*/
@ -537,7 +541,7 @@ int dictRehashMilliseconds(dict *d, int ms) {
* dictionary so that the hash table automatically migrates from H1 to H2
* while it is actively used. */
static void _dictRehashStep(dict *d) {
unsigned long iterators;
unsigned iterators;
__atomic_load(&d->iterators, &iterators, __ATOMIC_RELAXED);
if (iterators == 0) dictRehash(d,2);
}
@ -766,13 +770,11 @@ int _dictClear(dict *d, dictht *ht, void(callback)(void *)) {
/* Clear & Release the hash table */
void dictRelease(dict *d)
{
if (d->asyncdata) {
d->asyncdata->release = true;
return;
if (__atomic_sub_fetch(&d->refcount, 1, __ATOMIC_ACQ_REL) == 0) {
_dictClear(d,&d->ht[0],NULL);
_dictClear(d,&d->ht[1],NULL);
zfree(d);
}
_dictClear(d,&d->ht[0],NULL);
_dictClear(d,&d->ht[1],NULL);
zfree(d);
}
dictEntry *dictFindWithPrev(dict *d, const void *key, uint64_t h, dictEntry ***dePrevPtr, dictht **pht, bool fShallowCompare)
@ -1460,7 +1462,7 @@ void dictGetStats(char *buf, size_t bufsize, dict *d) {
void dictForceRehash(dict *d)
{
unsigned long iterators;
unsigned iterators;
__atomic_load(&d->iterators, &iterators, __ATOMIC_RELAXED);
while (iterators == 0 && dictIsRehashing(d)) _dictRehashStep(d);
}

12
src/dict.h
View File

@ -53,6 +53,7 @@ extern "C" {
/* Unused arguments generate annoying warnings... */
#define DICT_NOTUSED(V) ((void) V)
struct dictAsyncRehashCtl;
typedef struct dictEntry {
void *key;
@ -72,6 +73,7 @@ typedef struct dictType {
int (*keyCompare)(void *privdata, const void *key1, const void *key2);
void (*keyDestructor)(void *privdata, void *key);
void (*valDestructor)(void *privdata, void *obj);
void (*asyncfree)(dictAsyncRehashCtl *);
} dictType;
/* This is our hash table structure. Every dictionary has two of this as we
@ -98,13 +100,12 @@ struct dictAsyncRehashCtl {
struct dict *dict = nullptr;
std::vector<workItem> queue;
size_t hashIdx = 0;
bool release = false;
dictAsyncRehashCtl *next = nullptr;
std::atomic<bool> done { false };
std::atomic<bool> abondon { false };
dictAsyncRehashCtl(struct dict *d, dictAsyncRehashCtl *next) : dict(d), next(next) {
queue.reserve(c_targetQueueSize);
}
dictAsyncRehashCtl(struct dict *d, dictAsyncRehashCtl *next);
~dictAsyncRehashCtl();
};
#else
struct dictAsyncRehashCtl;
@ -115,7 +116,8 @@ typedef struct dict {
void *privdata;
dictht ht[2];
long rehashidx; /* rehashing not in progress if rehashidx == -1 */
unsigned long iterators; /* number of iterators currently running */
unsigned iterators; /* number of iterators currently running */
unsigned refcount;
dictAsyncRehashCtl *asyncdata;
} dict;

7
src/server.cpp
View File

@ -1359,6 +1359,8 @@ uint64_t dictEncObjHash(const void *key) {
}
}
void dictGCAsyncFree(dictAsyncRehashCtl *async);
/* Generic hash table type where keys are Redis Objects, Values
* dummy pointers. */
dictType objectKeyPointerValueDictType = {
@ -1407,8 +1409,9 @@ dictType dbDictType = {
NULL, /* key dup */
NULL, /* val dup */
dictSdsKeyCompare, /* key compare */
dictDbKeyDestructor, /* key destructor */
dictObjectDestructor /* val destructor */
dictDbKeyDestructor, /* key destructor */
dictObjectDestructor, /* val destructor */
dictGCAsyncFree /* async free destructor */
};
/* db->pdict, keys are sds strings, vals are Redis objects. */

53
src/snapshot.cpp
View File

@ -26,6 +26,17 @@ public:
std::vector<dictEntry*> vecde;
};
void discontinueAsyncRehash(dict *d) {
if (d->asyncdata != nullptr) {
auto adata = d->asyncdata;
while (adata != nullptr) {
adata->abondon = true;
adata = adata->next;
}
d->rehashidx = 0;
}
}
const redisDbPersistentDataSnapshot *redisDbPersistentData::createSnapshot(uint64_t mvccCheckpoint, bool fOptional)
{
serverAssert(GlobalLocksAcquired());
@ -67,14 +78,8 @@ const redisDbPersistentDataSnapshot *redisDbPersistentData::createSnapshot(uint6
// We can't have async rehash modifying these. Setting the asyncdata list to null
// will cause us to throw away the async work rather than modify the tables in flight
if (m_pdict->asyncdata != nullptr) {
m_pdict->asyncdata = nullptr;
m_pdict->rehashidx = 0;
}
if (m_pdictTombstone->asyncdata != nullptr) {
m_pdictTombstone->rehashidx = 0;
m_pdictTombstone->asyncdata = nullptr;
}
discontinueAsyncRehash(m_pdict);
discontinueAsyncRehash(m_pdictTombstone);
spdb->m_fAllChanged = false;
spdb->m_fTrackingChanges = 0;
@ -124,6 +129,7 @@ const redisDbPersistentDataSnapshot *redisDbPersistentData::createSnapshot(uint6
m_pdbSnapshotASYNC = nullptr;
}
std::atomic_thread_fence(std::memory_order_seq_cst);
return m_pdbSnapshot;
}
@ -318,6 +324,12 @@ bool redisDbPersistentDataSnapshot::freeTombstoneObjects(int depth)
dictForceRehash(dictTombstoneNew);
aeAcquireLock();
if (m_pdbSnapshot->m_pdict->asyncdata != nullptr) {
// In this case we use the asyncdata to free us, not our own lazy free
for (auto de : splazy->vecde)
dictFreeUnlinkedEntry(m_pdbSnapshot->m_pdict, de);
splazy->vecde.clear();
}
dict *dT = m_pdbSnapshot->m_pdict;
splazy->vecdictLazyFree.push_back(m_pdictTombstone);
__atomic_store(&m_pdictTombstone, &dictTombstoneNew, __ATOMIC_RELEASE);
@ -402,9 +414,14 @@ void redisDbPersistentData::endSnapshot(const redisDbPersistentDataSnapshot *psn
}
// Delete the object from the source dict, we don't use dictDelete to avoid a second search
if (deSnapshot != nullptr)
splazy->vecde.push_back(deSnapshot);
*dePrev = deSnapshot->next;
*dePrev = deSnapshot->next; // Unlink it first
if (deSnapshot != nullptr) {
if (m_spdbSnapshotHOLDER->m_pdict->asyncdata != nullptr) {
dictFreeUnlinkedEntry(m_spdbSnapshotHOLDER->m_pdict, deSnapshot);
} else {
splazy->vecde.push_back(deSnapshot);
}
}
ht->used--;
}
@ -641,3 +658,17 @@ bool redisDbPersistentDataSnapshot::FStale() const
static const uint64_t msStale = 500;
return ((getMvccTstamp() - m_mvccCheckpoint) >> MVCC_MS_SHIFT) >= msStale;
}
void dictGCAsyncFree(dictAsyncRehashCtl *async) {
if (async->deGCList != nullptr && serverTL != nullptr && !serverTL->gcEpoch.isReset()) {
auto splazy = std::make_unique<LazyFree>();
auto *de = async->deGCList;
while (de != nullptr) {
splazy->vecde.push_back(de);
de = de->next;
}
async->deGCList = nullptr;
g_pserver->garbageCollector.enqueue(serverTL->gcEpoch, std::move(splazy));
}
delete async;
}