Re: [boost] [thread] shared_spinlock_t idea or faster shared_lock

Subject: Re: [boost] [thread] shared_spinlock_t idea or faster shared_lock
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-05-02 05:04:58

On Friday 02 May 2014 09:37:46 Antony Polukhin wrote:
> Hi,
> 
> This is just a refining of idea that was once implemented and worked well.
> The idea was to make shared spin lock. Here is some pseudo-code:
> 
> class shared_spin_lock {
>     atomic_max_uint_t state_;
> 
> lock_shared() {
>     do { // CAS loop
>         auto snapshot = state_.load();
>         if (snapshot.has_writer_bit_set()) {
>              reader_wait(); // waiting for writer to finish
>              continue;
>         }
> 
>         auto new_state = snapshot + 1;
>      } while(!CAS(state, new_state));
> }
> 
> unlock_shared() {
>         --state;
> }
> 
> lock() {
>     do { // CAS loop
>         auto snapshot = state_.load();
>         if (snapshot.has_writer_bit_set()) {
>              writer_wait(); // waiting for other writer to finish
>              continue;
>         }
> 
>         auto new_state = snapshot.set_writer_bit();
>      } while(!CAS(state, new_state));
> 
>     // we have set the writer bit, now waiting for readers to finish
>     while (state_.readers_count) ; // busy loop, can we do better?
> 
>      writer_started();
> }
> 
> unlock() {
>         auto snapshot = state_.reset_writer_bit();
>         writer_finished();
>         ASSERT (snapshot.has_writer_bit_set());
> }
> 
> };
> 
> When all the *_wait() and writer_*() functions do nothing we get a shared
> spinlock.
> 
> Now if we add helper mutex and make all the *_wait() functions to lock and
> unlock mutex; writer_start() - lock the mutex; writer finished() - unlock
> the mutex... we get a simplified shared_mutex that is faster (in theory)
> than current implementation.
> 
> Is there an interest in such shared_spinlock_t? Any comments, suggestions?

There is shared_spin_mutex in Boost.Sync:

boost/sync/mutexes/shared_spin_mutex.hpp