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Use Mutexes not Atomics #4

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Use Mutexes not Atomics #4

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3f66973
Replace the atomic by a mutexes
Kerollmops Dec 9, 2025
f1d66a7
Make it compile on Windows
Kerollmops Dec 10, 2025
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38 changes: 32 additions & 6 deletions libraries/liblmdb/mdb.c
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Original file line number Diff line number Diff line change
Expand Up @@ -137,7 +137,6 @@ static NtCloseFunc *NtClose;
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <stdatomic.h>

#ifdef _MSC_VER
#include <io.h>
Expand Down Expand Up @@ -1304,7 +1303,9 @@ struct MDB_txn {
/** Nested txn under this txn, set together with flag #MDB_TXN_HAS_CHILD */
MDB_txn *mt_child;
/** The count of nested RDONLY txns under this txn also named child txns */
atomic_uint mt_rdonly_child_count;
unsigned int mt_rdonly_child_count;
/** Mutex protecting mt_rdonly_child_count */
pthread_mutex_t mt_child_mutex;
pgno_t mt_next_pgno; /**< next unallocated page */
#ifdef MDB_VL32
pgno_t mt_last_pgno; /**< last written page */
Expand Down Expand Up @@ -3148,6 +3149,11 @@ mdb_txn_renew0(MDB_txn *txn)
#endif
txn->mt_child = NULL;
txn->mt_rdonly_child_count = 0;
#ifdef _WIN32
txn->mt_child_mutex = CreateMutex(NULL, FALSE, NULL);
#else
pthread_mutex_init(&txn->mt_child_mutex, NULL);
#endif
txn->mt_loose_pgs = NULL;
txn->mt_loose_count = 0;
txn->mt_dirty_room = MDB_IDL_UM_MAX;
Expand Down Expand Up @@ -3290,9 +3296,13 @@ mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **ret)
parent->mt_flags |= MDB_TXN_HAS_CHILD;
parent->mt_child = txn;
if (F_ISSET(flags, MDB_RDONLY)) {
atomic_fetch_add(&parent->mt_rdonly_child_count, 1);
pthread_mutex_lock(&parent->mt_child_mutex);
parent->mt_rdonly_child_count++;
pthread_mutex_unlock(&parent->mt_child_mutex);
} else {
pthread_mutex_lock(&parent->mt_child_mutex);
parent->mt_rdonly_child_count = 0;
pthread_mutex_unlock(&parent->mt_child_mutex);
}
txn->mt_parent = parent;
txn->mt_numdbs = parent->mt_numdbs;
Expand Down Expand Up @@ -3448,7 +3458,14 @@ mdb_txn_end(MDB_txn *txn, unsigned mode)
UNLOCK_MUTEX(env->me_wmutex);
} else {
/* mark parent txn has no longer having children if this is the last nested txn */
if (!F_ISSET(flags, MDB_RDONLY) || atomic_fetch_sub(&txn->mt_parent->mt_rdonly_child_count, 1) == 1) {
int last_child = 0;
if (F_ISSET(flags, MDB_RDONLY)) {
pthread_mutex_lock(&txn->mt_parent->mt_child_mutex);
txn->mt_parent->mt_rdonly_child_count--;
last_child = (txn->mt_parent->mt_rdonly_child_count == 0);
pthread_mutex_unlock(&txn->mt_parent->mt_child_mutex);
}
if (!F_ISSET(flags, MDB_RDONLY) || last_child) {
txn->mt_parent->mt_child = NULL;
txn->mt_parent->mt_flags &= ~MDB_TXN_HAS_CHILD;
env->me_pgstate = ((MDB_ntxn *)txn)->mnt_pgstate;
Expand Down Expand Up @@ -3487,8 +3504,14 @@ mdb_txn_end(MDB_txn *txn, unsigned mode)
free(tl);
}
#endif
if (mode & MDB_END_FREE)
if (mode & MDB_END_FREE) {
#ifdef _WIN32
CloseHandle(txn->mt_child_mutex);
#else
pthread_mutex_destroy(&txn->mt_child_mutex);
#endif
free(txn);
}
}

void
Expand All @@ -3512,7 +3535,10 @@ _mdb_txn_abort(MDB_txn *txn)

if (txn->mt_parent && txn->mt_flags & MDB_RDONLY) {
// You must first abort the child before the parent
mdb_tassert(txn, txn->mt_parent && atomic_load(&txn->mt_rdonly_child_count) == 0);
pthread_mutex_lock(&txn->mt_child_mutex);
int count = txn->mt_rdonly_child_count;
pthread_mutex_unlock(&txn->mt_child_mutex);
mdb_tassert(txn, txn->mt_parent && count == 0);
}

if (txn->mt_child)
Expand Down