The Gcs_tagged_lock class Implements a tagged lock for optimistic read-side sections. More...

#include <gcs_tagged_lock.h>

Public Types
using	Tag = std::uint64_t

Public Member Functions
	Gcs_tagged_lock () noexcept

	~Gcs_tagged_lock ()=default

Tag	optimistic_read () const
	Starts an optimistic read-side section. More...

bool	validate_optimistic_read (Tag const &tag) const
	Validates an optimistic read-side section. More...

bool	try_lock ()
	Attempts to start a write-side section, i.e. More...

void	unlock ()
	Finishes the write-side section, i.e. More...

bool	is_locked () const
	Checks whether the lock is currently acquired. More...

Private Member Functions
std::uint64_t	get_lock_word (std::memory_order order=std::memory_order_acquire) const

Private Attributes
std::atomic< std::uint64_t >	m_lock_word

Detailed Description

The Gcs_tagged_lock class Implements a tagged lock for optimistic read-side sections.

In a nutshell, the tagged lock is a read-write spin lock which offers the following API:

try_lock() -> bool
unlock()
optimistic_read() -> tag
validate_optimistic_read(tag) -> bool

For the write-side section, one uses it as a typical spin lock, e.g.:

do:
  lock_acquired := try_lock()
while (not lock_acquired)
write-side section
unlock()

For the read-side section, one can use it as follows:

done := false
while (not done):
  tag := optimistic_read()
  unsynchronised read-side section
  done := validate_optimistic_read(tag)
  if (not done):
    rollback unsynchronized read-side section

The idea is to allow an optimistic read-side section that does not perform any memory stores. This is in contrast with a typical read-write lock, where the read side performs some memory stores to account for the reader, e.g. keeping a reader counter. The trade off is that:

a. the execution of the read-side of a tagged lock may be concurrent with the write-side section if meanwhile the tagged lock is acquired b. the read-side of a tagged lock may fail if meanwhile the tagged lock is acquired, in which case one may want to rollback the effects of the failed read-side section

The tagged lock is implemented over a single atomic 64-bit word with the following bit layout:

bit #    64  63  62        3   2   1
       +---+---+---+-...-+---+---+---+
       |   |   |   |     |   |   |   |
       +---+---+---+-...-+---+---+---+
        \__________  ___________/ \ /
                   \/              v
                   tag            locked?

Member Typedef Documentation

◆ Tag

using Gcs_tagged_lock::Tag = std::uint64_t

Constructor & Destructor Documentation

◆ Gcs_tagged_lock()

Gcs_tagged_lock::Gcs_tagged_lock ( )

noexcept

◆ ~Gcs_tagged_lock()

Gcs_tagged_lock::~Gcs_tagged_lock ( )

default

Member Function Documentation

◆ get_lock_word()

std::uint64_t Gcs_tagged_lock::get_lock_word ( std::memory_order order = std::memory_order_acquire ) const

private

◆ is_locked()

bool Gcs_tagged_lock::is_locked ( ) const

Checks whether the lock is currently acquired.

Returns: true if the lock is currently acquired, false otherwise

◆ optimistic_read()

Gcs_tagged_lock::Tag Gcs_tagged_lock::optimistic_read ( ) const

Starts an optimistic read-side section.

Returns: the tag associated with the optimistic execution.

◆ try_lock()

bool Gcs_tagged_lock::try_lock ( )

Attempts to start a write-side section, i.e.

acquire the lock.

Returns: true if the write-side section was successfully started, i.e. we acquired the lock, false otherwise

◆ unlock()

void Gcs_tagged_lock::unlock ( )

Finishes the write-side section, i.e.

releases the lock.

◆ validate_optimistic_read()

bool Gcs_tagged_lock::validate_optimistic_read ( Gcs_tagged_lock::Tag const & tag ) const

Validates an optimistic read-side section.

Parameters

tag	The tag returned by the corresponding `optimistic_read`

Returns: true if the optimistic read-side was atomically executed while the lock was free, false otherwise

Member Data Documentation

◆ m_lock_word

std::atomic<std::uint64_t> Gcs_tagged_lock::m_lock_word