Does acquiring a spinlock require compare-and-swap or is swap enough?
Suppose we have a spinlock implementation:
struct Lock {
locked : Atomic(bool),
}
Then an unlock function could be:
fun unlock(lock : &Lock) {
atomic_store(&lock.locked, false, release);
}
But what about lock? Commonly, it uses a compare-and-swap like this:
fun lock(lock : &Lock) {
while atomic_compare_and_swap(&lock.locked, false, true, acquire) {}
}
But wouldn't a swap be enough for this? Something like this:
fun lock(lock : &Lock) {
while atomic_swap(&lock.locked, true, acquire) {}
}
Is there any problem with this?
concurrency atomic spinlock
asked Nov 13 '18 at 12:09
bzimbzim
753514
add a comment |
Suppose we have a spinlock implementation:
struct Lock {
locked : Atomic(bool),
}
Then an unlock function could be:
fun unlock(lock : &Lock) {
atomic_store(&lock.locked, false, release);
}
But what about lock? Commonly, it uses a compare-and-swap like this:
fun lock(lock : &Lock) {
while atomic_compare_and_swap(&lock.locked, false, true, acquire) {}
}
But wouldn't a swap be enough for this? Something like this:
fun lock(lock : &Lock) {
while atomic_swap(&lock.locked, true, acquire) {}
}
Is there any problem with this?
concurrency atomic spinlock
asked Nov 13 '18 at 12:09
bzimbzim
753514
add a comment |
Suppose we have a spinlock implementation:
struct Lock {
locked : Atomic(bool),
}
Then an unlock function could be:
fun unlock(lock : &Lock) {
atomic_store(&lock.locked, false, release);
}
But what about lock? Commonly, it uses a compare-and-swap like this:
fun lock(lock : &Lock) {
while atomic_compare_and_swap(&lock.locked, false, true, acquire) {}
}
But wouldn't a swap be enough for this? Something like this:
fun lock(lock : &Lock) {
while atomic_swap(&lock.locked, true, acquire) {}
}
Is there any problem with this?
concurrency atomic spinlock
asked Nov 13 '18 at 12:09
bzimbzim
753514
Suppose we have a spinlock implementation:
struct Lock {
locked : Atomic(bool),
}
Then an unlock function could be:
fun unlock(lock : &Lock) {
atomic_store(&lock.locked, false, release);
}
But what about lock? Commonly, it uses a compare-and-swap like this:
fun lock(lock : &Lock) {
while atomic_compare_and_swap(&lock.locked, false, true, acquire) {}
}
But wouldn't a swap be enough for this? Something like this:
fun lock(lock : &Lock) {
while atomic_swap(&lock.locked, true, acquire) {}
}
Is there any problem with this?
concurrency atomic spinlock
concurrency atomic spinlock
asked Nov 13 '18 at 12:09
bzimbzim
753514
asked Nov 13 '18 at 12:09
bzimbzim
753514
edited Nov 13 '18 at 12:15
bzim
asked Nov 13 '18 at 12:09
bzimbzim
753514
asked Nov 13 '18 at 12:09
bzimbzim
753514
asked Nov 13 '18 at 12:09
bzimbzim
753514
753514
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
A compare-and-swap isn't really necessary. Atomically setting a flag true if it is false is logically equivalent to unconditionally setting it true.
An unconditional swap might be slightly faster since it does not have to compare anything, although the real cost of an atomic read-modify-write operation is in obtaining and locking the cache line.
Here is an example of a C++ spinlock that uses exchange()
#include <atomic>
class mutex {
std::atomic<bool> flag{false};
public:
void lock()
{
while (flag.exchange(true, std::memory_order_acquire));
}
void unlock()
{
flag.store(false, std::memory_order_release);
}
};
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
add a comment |
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1 Answer
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active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
A compare-and-swap isn't really necessary. Atomically setting a flag true if it is false is logically equivalent to unconditionally setting it true.
An unconditional swap might be slightly faster since it does not have to compare anything, although the real cost of an atomic read-modify-write operation is in obtaining and locking the cache line.
Here is an example of a C++ spinlock that uses exchange()
#include <atomic>
class mutex {
std::atomic<bool> flag{false};
public:
void lock()
{
while (flag.exchange(true, std::memory_order_acquire));
}
void unlock()
{
flag.store(false, std::memory_order_release);
}
};
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
add a comment |
A compare-and-swap isn't really necessary. Atomically setting a flag true if it is false is logically equivalent to unconditionally setting it true.
An unconditional swap might be slightly faster since it does not have to compare anything, although the real cost of an atomic read-modify-write operation is in obtaining and locking the cache line.
Here is an example of a C++ spinlock that uses exchange()
#include <atomic>
class mutex {
std::atomic<bool> flag{false};
public:
void lock()
{
while (flag.exchange(true, std::memory_order_acquire));
}
void unlock()
{
flag.store(false, std::memory_order_release);
}
};
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
add a comment |
A compare-and-swap isn't really necessary. Atomically setting a flag true if it is false is logically equivalent to unconditionally setting it true.
An unconditional swap might be slightly faster since it does not have to compare anything, although the real cost of an atomic read-modify-write operation is in obtaining and locking the cache line.
Here is an example of a C++ spinlock that uses exchange()
#include <atomic>
class mutex {
std::atomic<bool> flag{false};
public:
void lock()
{
while (flag.exchange(true, std::memory_order_acquire));
}
void unlock()
{
flag.store(false, std::memory_order_release);
}
};
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
A compare-and-swap isn't really necessary. Atomically setting a flag true if it is false is logically equivalent to unconditionally setting it true.
An unconditional swap might be slightly faster since it does not have to compare anything, although the real cost of an atomic read-modify-write operation is in obtaining and locking the cache line.
Here is an example of a C++ spinlock that uses exchange()
#include <atomic>
class mutex {
std::atomic<bool> flag{false};
public:
void lock()
{
while (flag.exchange(true, std::memory_order_acquire));
}
void unlock()
{
flag.store(false, std::memory_order_release);
}
};
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
edited Nov 14 '18 at 3:32
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
answered Nov 14 '18 at 3:01
LWimseyLWimsey
3,0501727
3,0501727
add a comment |
add a comment |
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