stm_fraser.c

/******************************************************************************
 * stm_fraser.c
 *
 * Lock-free software transactional memory (STM).
 *
 * Copyright (c) 2002-2003, K A Fraser

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

    * Redistributions of source code must retain the above copyright
    * notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above
    * copyright notice, this list of conditions and the following
    * disclaimer in the documentation and/or other materials provided
    * with the distribution.  Neither the name of the Keir Fraser
    * nor the names of its contributors may be used to endorse or
    * promote products derived from this software without specific
    * prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "portable_defns.h"
#include "ptst.h"
#include "gc.h"
#include <assert.h>
#include <stdarg.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <setjmp.h>
#include <signal.h>

typedef struct stm_blk_st stm_blk;
typedef struct stm_tx_entry_st stm_tx_entry;
typedef struct stm_tx_st stm_tx;
typedef struct stm_st stm;

struct stm_blk_st {
    void *data;
};

struct stm_tx_entry_st {
    stm_blk *b;
    void    *old;
    void    *new;
    stm_tx_entry *next;
};

struct stm_tx_st {
    int status;
    int rc;
    stm_tx *next_free;
    stm_tx_entry *reads;
    stm_tx_entry *writes;
    stm_tx_entry *alloc_ptr, *check;
    int gc_data_id, blk_size; /* copied from 'stm' structure */
    sigjmp_buf *penv;
};

struct stm_st {
    int gc_data_id;
    int blk_size;
};

/* Private per-thread state. The array is indexed off ptst->id. */
typedef struct {
    void *arena, *arena_lim;
    stm_tx *next_descriptor;
    stm_tx *cur_tx;
    CACHE_PAD(0);
} priv_t;

static priv_t priv_ptst[MAX_THREADS];
static int gc_blk_id;  /* Allocation id for block descriptors. */
static int do_padding; /* Should all allocations be padded to a cache line? */

#define ALLOCATOR_SIZE(_s) (do_padding ? CACHE_LINE_SIZE : (_s))

#define ARENA_SIZE      40960
#define DESCRIPTOR_SIZE  4096

#define TXS_IN_PROGRESS 0
#define TXS_READ_PHASE  1
#define TXS_FAILED      2
#define TXS_SUCCESSFUL  3

#define is_descriptor(_p)     ((unsigned long)(_p) & 1)
#define ptr_to_descriptor(_p) ((stm_tx *)((unsigned long)(_p) & ~1))
#define make_marked_ptr(_p)   ((void *)((unsigned long)(_p) | 1))

/* Is transaction read-only? */
#define read_only(_t)         ((_t)->writes == NULL)

bool_t commit_stm_tx(ptst_t *ptst, stm_tx *t);

static void new_arena (priv_t *priv, int size)
{
    priv->arena = malloc(size);
    if ( priv->arena == NULL ) abort();
    priv->arena_lim = (((char *) priv->arena) + size);
}

static void release_descriptor(ptst_t *ptst, stm_tx *t)
{
    stm_tx_entry *ent;
    priv_t *priv = &priv_ptst[ptst->id];
    void *data;

    assert(t->status >= TXS_FAILED);

    t->next_free = priv->next_descriptor;
    priv->next_descriptor = t;

    if ( t->status == TXS_SUCCESSFUL )
    {
        for ( ent = t->writes; ent != NULL; ent = ent->next )
        {
            gc_free(ptst, ent->old, t->gc_data_id);
        }
    }
    else
    {
        for ( ent = t->writes; ent != NULL; ent = ent->next )
        {
            gc_unsafe_free(ptst, ent->new, t->gc_data_id);
        }
    }
}

static int rc_delta_descriptor(stm_tx *t, int delta)
{
    int rc, new_rc = t->rc;

    do { rc = new_rc; }
    while ( (new_rc = CASIO (&t->rc, rc, rc + delta)) != rc );

    return rc;
}

static void rc_up_descriptor(stm_tx *t)
{
    rc_delta_descriptor(t, 2);
    MB();
}

static void rc_down_descriptor(ptst_t *ptst, stm_tx *t)
{
    int old_rc, new_rc, cur_rc = t->rc;

    WMB();

    do {
        old_rc = cur_rc;
        new_rc = old_rc - 2;
        if ( new_rc == 0 ) new_rc = 1;
    }
    while ( (cur_rc = CASIO (&t->rc, old_rc, new_rc)) != old_rc );

    if ( old_rc == 2 ) release_descriptor(ptst, t);
}

static stm_tx *new_descriptor(priv_t *priv)
{
    stm_tx *t;

    t = priv->next_descriptor;

    if ( t != NULL )
    {
        priv->next_descriptor = t->next_free;
        /* 'Unfree' descriptor, if it was previously freed. */
        if ( (t->rc & 1) == 1 ) rc_delta_descriptor(t, 1);
    }
    else
    {
        t = (stm_tx *) priv->arena;
        priv->arena = ((char *) (priv->arena)) + DESCRIPTOR_SIZE;

        if ( priv->arena >= priv->arena_lim )
        {
            new_arena(priv, ARENA_SIZE);
            t = (stm_tx *) priv->arena;
            priv->arena = ((char *) (priv->arena)) + DESCRIPTOR_SIZE;
        }

        t->next_free = NULL;
        t->rc = 2;
    }

    return t;
}


static stm_tx_entry *alloc_stm_tx_entry(stm_tx *t)
{
    stm_tx_entry *ent = t->alloc_ptr++;
    assert(((unsigned long)t->alloc_ptr - (unsigned long)t) <=
           DESCRIPTOR_SIZE);
    return ent;
}


static stm_tx_entry **search_stm_tx_entry(stm_tx_entry **pnext, stm_blk *b)
{
    stm_tx_entry *next = *pnext;

    while ( (next != NULL) && ((unsigned long)next->b < (unsigned long)b) )
    {
        pnext = &next->next;
        next  = *pnext;
    }

    return pnext;
}


static void *read_blk_data(ptst_t *ptst, stm_blk *b)
{
    void *data;
    stm_tx *t;
    int status;
    stm_tx_entry **pent;

    for ( ; ; )
    {
        data = b->data;
        if ( !is_descriptor(data) ) return data;

        t = ptr_to_descriptor(data);
        rc_up_descriptor(t);
        if ( b->data != data )
        {
            rc_down_descriptor(ptst, t);
            continue;
        }

        /*
         * Commit even when we could just read from descriptor, as it gets
         * the descriptor out of the way in future.
         */
        commit_stm_tx(ptst, t);
    }
}


stm *new_stm(ptst_t *ptst, int blk_size)
{
    stm *mem = malloc(CACHE_LINE_SIZE);
    mem->blk_size = blk_size;
    mem->gc_data_id = gc_add_allocator(ALLOCATOR_SIZE(blk_size));
    return mem;
}


void free_stm(ptst_t *ptst, stm *mem)
{
    gc_remove_allocator(mem->gc_data_id);
    free(mem);
}


stm_blk *new_stm_blk(ptst_t *ptst, stm *mem)
{
    stm_blk *b;
    b       = gc_alloc(ptst, gc_blk_id);
    b->data = gc_alloc(ptst, mem->gc_data_id);
    return b;
}


void free_stm_blk(ptst_t *ptst, stm *mem, stm_blk *b)
{
    /*
     * We have to use read_stm_blk(), as some doomed transaction may still
     * install a marked pointer here while in its write phase.
     */
    void *data = read_blk_data(ptst, b);
    assert(!is_descriptor(data));
    gc_free(ptst, data, mem->gc_data_id);
    gc_free(ptst, b,    gc_blk_id);
}


void *init_stm_blk(ptst_t *ptst, stm *mem, stm_blk *b)
{
    return b->data;
}


int sizeof_stm_blk(ptst_t *ptst, stm *mem, stm_blk *b)
{
    return mem->blk_size;
}


stm_tx *new_stm_tx(ptst_t *ptst, stm *mem, sigjmp_buf *penv)
{
    priv_t *priv = &priv_ptst[ptst->id];
    stm_tx *t;

    if ( priv->cur_tx != NULL ) goto nesting;
    t = new_descriptor(priv);
    t->status = TXS_IN_PROGRESS;
    t->reads = t->writes = NULL;
    t->alloc_ptr = t->check = (stm_tx_entry *)(t + 1);
    t->gc_data_id = mem->gc_data_id;
    t->blk_size = mem->blk_size;
    t->penv = penv;
    priv->cur_tx = t;
    return t;

 nesting:
    fprintf(stderr, "No nesting of transactions is allowed\n");
    return NULL;
}


bool_t commit_stm_tx(ptst_t *ptst, stm_tx *t)
{
    int desired_status, other_status, old_status, new_status, final_status;
    void *marked_tx, *data;
    stm_tx *other;
    stm_tx_entry **other_pent, *ent;
    priv_t *priv = &priv_ptst[ptst->id];

    if ( priv->cur_tx == t ) priv->cur_tx = NULL;

    marked_tx = make_marked_ptr(t);
    desired_status = TXS_FAILED;

    /*
     * PHASE 1: WRITE-CHECKING PHASE.
     */
    if ( (t->status == TXS_IN_PROGRESS) && ((ent = t->writes) != NULL) )
    {
        /* Others should see up-to-date contents of descriptor. */
        WMB();

        do {
            for ( ; ; )
            {
                data = CASPO(&ent->b->data, ent->old, marked_tx);
                if ( (data == ent->old) || (data == marked_tx) ) break;

                if ( !is_descriptor(data) ) goto fail;

                other = ptr_to_descriptor(data);
                rc_up_descriptor(other);
                if ( ent->b->data != data )
                {
                    rc_down_descriptor(ptst, other);
                    continue;
                }

                commit_stm_tx(ptst, other);
            }
        }
        while ( (ent = ent->next) != NULL );
    }

    /* On success we linearise at this point. */
    WEAK_DEP_ORDER_WMB();

    /*
     * PHASE 2: READ-CHECKING PHASE.
     */
    if ( (t->status <= TXS_READ_PHASE) && (t->reads != NULL) )
    {
        if ( !read_only(t) )
        {
            CASIO(&t->status, TXS_IN_PROGRESS, TXS_READ_PHASE);
            MB_NEAR_CAS();
        }
        else MB();

        for ( ent = t->reads; ent != NULL; ent = ent->next )
        {
            for ( ; ; )
            {
                data = ent->b->data;
                if ( data == ent->old ) break;

                /* Someone else made progress at our expense. */
                if ( !is_descriptor(data) ) goto fail;
                other = ptr_to_descriptor(data);

                /*
                 * Descriptor always belongs to a contending operation.
                 * Before continuing, we must increment the reference count.
                 */
                assert(other != t);
                rc_up_descriptor(other);
                if ( ent->b->data != data )
                {
                    rc_down_descriptor(ptst, other);
                    continue;
                }

                /*
                 * What we do now depends on the status of the contending
                 * operation. This is easy for any status other than
                 * TXS_READ_PHASE -- usually we just check against the
                 * appropriate 'old' or 'new' data pointer. Transactions
                 * in their read-checking phase must be aborted, or helped
                 * to completion, depending on relative ordering of the
                 * transaction descriptors.
                 */
                while ( (other_status = other->status) == TXS_READ_PHASE )
                {
                    if ( t < other )
                    {
                        CASIO(&other->status, TXS_READ_PHASE, TXS_FAILED);
                    }
                    else
                    {
                        rc_up_descriptor(other);
                        commit_stm_tx(ptst, other);
                    }
                }

                other_pent = search_stm_tx_entry(&other->writes, ent->b);
                assert(*other_pent != NULL);
                data = (other_status == TXS_SUCCESSFUL) ?
                    (*other_pent)->new : (*other_pent)->old;
                rc_down_descriptor(ptst, other);
                if ( data != ent->old ) goto fail;

                break;
            }
        }
    }

    desired_status = TXS_SUCCESSFUL;

 fail:
    if ( read_only(t) )
    {
        /* A very fast path: we can immediately reuse the descriptor. */
        t->next_free = priv->next_descriptor;
        priv->next_descriptor = t;
        return desired_status == TXS_SUCCESSFUL;
    }

    /* Loop until we push the status to a "final decision" value. */
    old_status = t->status;
    while ( old_status <= TXS_READ_PHASE )
    {
        new_status = CASIO(&t->status, old_status, desired_status);
        if ( old_status == new_status ) break;
        old_status = new_status;
    }
    WMB_NEAR_CAS();

    /*
     * PHASE 3: CLEAN-UP.
     */
    final_status = t->status;
    for ( ent = t->writes; ent != NULL; ent = ent->next )
    {
        /* If CAS fails, someone did it for us already. */
        (void)CASPO(&ent->b->data, marked_tx,
                    (final_status == TXS_SUCCESSFUL) ? ent->new: ent->old);
    }

    rc_down_descriptor(ptst, t);
    return final_status == TXS_SUCCESSFUL;
}


bool_t validate_stm_tx(ptst_t *ptst, stm_tx *t)
{
    stm_tx_entry *ent;

    RMB();

    for ( ent = t->reads; ent != NULL; ent = ent->next )
    {
        if ( read_blk_data(ptst, ent->b) != ent->old ) goto fail;
    }

    for ( ent = t->writes; ent != NULL; ent = ent->next )
    {
        if ( read_blk_data(ptst, ent->b) != ent->old ) goto fail;
    }

    return TRUE;

 fail:
    t->status = TXS_FAILED;
    return FALSE;
}


void abort_stm_tx(ptst_t *ptst, stm_tx *t)
{
    t->status = TXS_FAILED;
}


void *read_stm_blk(ptst_t *ptst, stm_tx *t, stm_blk *b)
{
    stm_tx_entry **pent, *ent;
    sigjmp_buf *penv;
    void *result;

    pent = search_stm_tx_entry(&t->writes, b);
    ent  = *pent;
    if ( (ent != NULL) && (ent->b == b) ) goto found;

    pent = search_stm_tx_entry(&t->reads, b);
    ent  = *pent;
    if ( (ent != NULL) && (ent->b == b) ) goto found;

    ent = alloc_stm_tx_entry(t);
    ent->b    = b;
    ent->old  = read_blk_data(ptst, b);
    ent->new  = ent->old;
    ent->next = *pent;
    *pent = ent;

    assert(!is_descriptor(ent->new));
    return ent->new;

 found:
    result = ent->new;
    ent = t->check;
    if ( read_blk_data(ptst, ent->b) != ent->old ) goto fail;
    if ( ++t->check == t->alloc_ptr ) t->check = (stm_tx_entry *)(t + 1);
    return result;

 fail:
    penv = t->penv;
    abort_stm_tx(ptst, t);
    commit_stm_tx(ptst, t);
    siglongjmp(*penv, 0);
    assert(0);
    return NULL;
}


void *write_stm_blk(ptst_t *ptst, stm_tx *t, stm_blk *b)
{
    stm_tx_entry **r_pent, **w_pent, *ent;
    sigjmp_buf *penv;
    void *result;

    w_pent = search_stm_tx_entry(&t->writes, b);
    ent = *w_pent;
    if ( (ent != NULL) && (ent->b == b) ) goto found;

    r_pent = search_stm_tx_entry(&t->reads, b);
    ent = *r_pent;
    if ( (ent != NULL) && (ent->b == b) )
    {
        *r_pent = ent->next;
    }
    else
    {
        ent = alloc_stm_tx_entry(t);
        ent->b   = b;
        ent->old = read_blk_data(ptst, b);
    }

    ent->new  = gc_alloc(ptst, t->gc_data_id);
    ent->next = *w_pent;
    *w_pent = ent;
    memcpy(ent->new, ent->old, t->blk_size);

    assert(!is_descriptor(ent->old));
    assert(!is_descriptor(ent->new));
    return ent->new;

 found:
    result = ent->new;
    ent = t->check;
    if ( read_blk_data(ptst, ent->b) != ent->old ) goto fail;
    if ( ++t->check == t->alloc_ptr ) t->check = (stm_tx_entry *)(t + 1);
    return result;

 fail:
    penv = t->penv;
    abort_stm_tx(ptst, t);
    commit_stm_tx(ptst, t);
    siglongjmp(*penv, 0);
    assert(0);
    return NULL;
}


void remove_from_tx(ptst_t *ptst, stm_tx *t, stm_blk *b)
{
    stm_tx_entry **pent, *ent;
    void *data;

    pent = search_stm_tx_entry(&t->writes, b);
    ent  = *pent;
    if ( (ent != NULL) && (ent->b == b) )
    {
        *pent = ent->next;
        data = ent->new;
        assert(!is_descriptor(data));
        gc_free(ptst, data, t->gc_data_id);
        return;
    }

    pent = search_stm_tx_entry(&t->reads, b);
    ent  = *pent;
    if ( (ent != NULL) && (ent->b == b) )
    {
        *pent = ent->next;
    }
}


static void handle_fault(int sig)
{
    ptst_t *ptst;
    stm_tx *t;

    ptst = critical_enter();
    t = priv_ptst[ptst->id].cur_tx;
    if ( (t != NULL) && !validate_stm_tx(ptst, t) )
    {
        sigjmp_buf *penv = t->penv;
        commit_stm_tx(ptst, t);
        critical_exit(ptst);
        siglongjmp(*penv, 0);
    }

 fail:
    fprintf(stderr, "Error: unhandleable SIGSEGV!\n");
    abort();
}


void _init_stm_subsystem(int pad_data)
{
    struct sigaction act;

    do_padding = pad_data;
    gc_blk_id = gc_add_allocator(ALLOCATOR_SIZE(sizeof(stm_blk)));
    memset(priv_ptst, 0, sizeof(priv_ptst));

    act.sa_handler = handle_fault;
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0;
    sigaction(SIGSEGV, &act, NULL);
}