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/* Copyright (c) 2009, 2010, Oracle and/or its affiliates. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA */
#ifndef PFS_LOCK_H
#define PFS_LOCK_H
/**
@file storage/perfschema/pfs_lock.h
Performance schema internal locks (declarations).
*/
#include "pfs_atomic.h"
/**
@addtogroup Performance_schema_buffers
@{
*/
/**
State of a free record.
Values of a free record should not be read by a reader.
Writers can concurrently attempt to allocate a free record.
*/
#define PFS_LOCK_FREE 0
/**
State of a dirty record.
Values of a dirty record should not be read by a reader,
as the record is currently being modified.
Only one writer, the writer which owns the record, should
modify the record content.
*/
#define PFS_LOCK_DIRTY 1
/**
State of an allocated record.
Values of an allocated record are safe to read by a reader.
A writer may modify some but not all properties of the record:
only modifying values that can never cause the reader to crash is allowed.
*/
#define PFS_LOCK_ALLOCATED 2
/**
A 'lock' protecting performance schema internal buffers.
This lock is used to mark the state of a record.
Access to the record is not enforced here,
it's up to the readers and writers to look at the record state
before making an actual read or write operation.
*/
struct pfs_lock
{
/**
The record internal state.
@sa PFS_LOCK_FREE
@sa PFS_LOCK_DIRTY
@sa PFS_LOCK_ALLOCATED
*/
volatile int32 m_state;
/**
The record internal version number.
This version number is to transform the 'ABA' problem
(see http://en.wikipedia.org/wiki/ABA_problem)
into an 'A(n)BA(n + 1)' problem, where 'n' is the m_version number.
When the performance schema instrumentation deletes a record,
then create a different record reusing the same memory allocation,
the version number is incremented, so that a reader can detect that
the record was changed. Note that the version number is never
reset to zero when a new record is created.
*/
volatile uint32 m_version;
/** Returns true if the record is free. */
bool is_free(void)
{
/* This is a dirty read */
return (m_state == PFS_LOCK_FREE);
}
/** Returns true if the record contains values that can be read. */
bool is_populated(void)
{
int32 copy= m_state; /* non volatile copy, and dirty read */
return (copy == PFS_LOCK_ALLOCATED);
}
/**
Execute a free to dirty transition.
This transition is safe to execute concurrently by multiple writers.
Only one writer will succeed to acquire the record.
@return true if the operation succeed
*/
bool free_to_dirty(void)
{
int32 old_state= PFS_LOCK_FREE;
int32 new_state= PFS_LOCK_DIRTY;
return (PFS_atomic::cas_32(&m_state, &old_state, new_state));
}
/**
Execute a dirty to allocated transition.
This transition should be executed by the writer that owns the record,
after the record is in a state ready to be read.
*/
void dirty_to_allocated(void)
{
DBUG_ASSERT(m_state == PFS_LOCK_DIRTY);
PFS_atomic::add_u32(&m_version, 1);
PFS_atomic::store_32(&m_state, PFS_LOCK_ALLOCATED);
}
/**
Execute a dirty to free transition.
This transition should be executed by the writer that owns the record.
*/
void dirty_to_free(void)
{
DBUG_ASSERT(m_state == PFS_LOCK_DIRTY);
PFS_atomic::store_32(&m_state, PFS_LOCK_FREE);
}
/**
Execute an allocated to free transition.
This transition should be executed by the writer that owns the record.
*/
void allocated_to_free(void)
{
/*
If this record is not in the ALLOCATED state and the caller is trying
to free it, this is a bug: the caller is confused,
and potentially damaging data owned by another thread or object.
The correct assert to use here to guarantee data integrity is simply:
DBUG_ASSERT(m_state == PFS_LOCK_ALLOCATED);
*/
DBUG_ASSERT(m_state == PFS_LOCK_ALLOCATED);
PFS_atomic::store_32(&m_state, PFS_LOCK_FREE);
}
/**
Start an optimistic read operation.
@sa end_optimist_lock.
*/
void begin_optimistic_lock(struct pfs_lock *copy)
{
copy->m_version= PFS_atomic::load_u32(&m_version);
copy->m_state= PFS_atomic::load_32(&m_state);
}
/**
End an optimistic read operation.
@sa begin_optimist_lock.
@return true if the data read is safe to use.
*/
bool end_optimistic_lock(struct pfs_lock *copy)
{
/*
return true if:
- the version + state has not changed
- and there was valid data to look at
*/
return ((copy->m_version == PFS_atomic::load_u32(&m_version)) &&
(copy->m_state == PFS_atomic::load_32(&m_state)) &&
(copy->m_state == PFS_LOCK_ALLOCATED));
}
};
/** @} */
#endif
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