Commit 3bafb6cd authored by Xavier Thompson's avatar Xavier Thompson

Enable lock-free contention detection in trylock mode

parent 0474ac90
......@@ -19,9 +19,18 @@
#define CyObject_ATOMIC_REFCOUNT_TYPE atomic_int
#define CyObject_NO_OWNER -1
#define CyObject_MANY_OWNERS -2
#define CyObject_MAX_READERS (1 << 30)
#define CyObject_LOCK_ERROR_OTHER_WRITER (1 << 0)
#define CyObject_LOCK_ERROR_OTHER_READER (1 << 1)
#define CyObject_WRITER_OFFSET (16)
#define CyObject_FETCH_CONTENDERS_ADD_WRITER(n) n.fetch_add((1 << CyObject_WRITER_OFFSET))
#define CyObject_FETCH_CONTENDERS_SUB_WRITER(n) n.fetch_sub((1 << CyObject_WRITER_OFFSET))
#define CyObject_FETCH_CONTENDERS_ADD_READER(n) n.fetch_add(1)
#define CyObject_FETCH_CONTENDERS_SUB_READER(n) n.fetch_sub(1)
#define CyObject_WRITERS_FROM_CONTENDERS(n) (n >> CyObject_WRITER_OFFSET)
#define CyObject_READERS_FROM_CONTENDERS(n) (n & ((1 << CyObject_WRITER_OFFSET) -1))
#define CyObject_HAS_WRITER_CONTENDERS(n) (n > (1 << CyObject_WRITER_OFFSET) - 1)
#define CyObject_CONTENDING_WRITER_FLAG (1 << 0)
#define CyObject_CONTENDING_READER_FLAG (1 << 1)
#include <pthread.h>
......@@ -40,6 +49,7 @@
pthread_cond_t wait_writer_depart;
atomic<pid_t> owner_id;
atomic_int32_t readers_nb;
atomic_uint32_t contenders;
uint32_t write_count;
public:
RecursiveUpgradeableRWLock() {
......@@ -49,6 +59,7 @@
this->owner_id = CyObject_NO_OWNER;
this->readers_nb = 0;
this->write_count = 0;
this->contenders = 0;
}
void wlock();
void rlock();
......@@ -310,6 +321,8 @@
void RecursiveUpgradeableRWLock::rlock() {
pid_t caller_id = syscall(SYS_gettid);
CyObject_FETCH_CONTENDERS_ADD_READER(this->contenders);
if (this->owner_id == caller_id) {
++this->readers_nb;
return;
......@@ -329,18 +342,26 @@ void RecursiveUpgradeableRWLock::rlock() {
int RecursiveUpgradeableRWLock::tryrlock() {
pid_t caller_id = syscall(SYS_gettid);
int contenders = CyObject_FETCH_CONTENDERS_ADD_READER(this->contenders);
if (this->owner_id == caller_id) {
++this->readers_nb;
return 0;
}
if (CyObject_HAS_WRITER_CONTENDERS(contenders)) {
CyObject_FETCH_CONTENDERS_SUB_READER(this->contenders);
return CyObject_CONTENDING_WRITER_FLAG;
}
// we must lock here, because a trylock could fail also when another thread is currently read-locking or read-unlocking
// but this means we might miss a writer arriving and leaving
pthread_mutex_lock(&this->guard);
if (this->write_count > 0) {
return CyObject_LOCK_ERROR_OTHER_WRITER;
pthread_mutex_unlock(&this->guard);
CyObject_FETCH_CONTENDERS_SUB_READER(this->contenders);
return CyObject_CONTENDING_WRITER_FLAG;
}
this->owner_id = this->readers_nb++ ? CyObject_MANY_OWNERS : caller_id;
......@@ -364,11 +385,15 @@ void RecursiveUpgradeableRWLock::unrlock() {
}
pthread_mutex_unlock(&this->guard);
CyObject_FETCH_CONTENDERS_SUB_READER(this->contenders);
}
void RecursiveUpgradeableRWLock::wlock() {
pid_t caller_id = syscall(SYS_gettid);
CyObject_FETCH_CONTENDERS_ADD_WRITER(this->contenders);
if (this->owner_id == caller_id) {
if (this->write_count) {
++this->write_count;
......@@ -411,6 +436,8 @@ void RecursiveUpgradeableRWLock::wlock() {
int RecursiveUpgradeableRWLock::trywlock() {
pid_t caller_id = syscall(SYS_gettid);
uint32_t contenders = CyObject_FETCH_CONTENDERS_ADD_WRITER(this->contenders);
if (this->owner_id == caller_id) {
if (this->write_count) {
++this->write_count;
......@@ -418,25 +445,36 @@ int RecursiveUpgradeableRWLock::trywlock() {
}
}
if (CyObject_HAS_WRITER_CONTENDERS(contenders) > 0) {
CyObject_FETCH_CONTENDERS_SUB_WRITER(this->contenders);
return CyObject_CONTENDING_WRITER_FLAG;
}
if (contenders > 0) {
CyObject_FETCH_CONTENDERS_SUB_WRITER(this->contenders);
return CyObject_CONTENDING_READER_FLAG;
}
if (pthread_mutex_trylock(&this->guard) != 0) {
// another thread is currently doing a lock operation on this lock, but we don't know what it is.
// we could choose to do a blocking lock instead of a trylock here.
// that way we would not detect when an unlock overlaps with this trylock,
// but also all the contending readers and / or writers could leave while we block,
// so we wouldn't detect those contentions either.
return CyObject_LOCK_ERROR_OTHER_WRITER | CyObject_LOCK_ERROR_OTHER_READER;
return CyObject_CONTENDING_WRITER_FLAG | CyObject_CONTENDING_READER_FLAG;
}
if (this->owner_id != caller_id) {
if (this->readers_nb > 0) {
pthread_mutex_unlock(&this->guard);
return CyObject_LOCK_ERROR_OTHER_READER;
CyObject_FETCH_CONTENDERS_SUB_WRITER(this->contenders);
return CyObject_CONTENDING_READER_FLAG;
}
if (this->write_count > 0) {
pthread_mutex_unlock(&this->guard);
return CyObject_LOCK_ERROR_OTHER_WRITER;
CyObject_FETCH_CONTENDERS_SUB_WRITER(this->contenders);
return CyObject_CONTENDING_WRITER_FLAG;
}
this->owner_id = caller_id;
......
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