Erlang ets safe_fixtable的源碼分析

使用範例

• 在遍歷的時候進行ets處理，如刪除，更新，插入字段內容等
• 配合ets:first和ets:next的迭代器使用

實踐例子

• mnesia的使用
• httpc的使用
• snmpm_net 使用
• dets
• ets自己的foldr使用

作用

可以將ets表鎖住，避免ets數據修改導致遍歷數據錯誤

具體的使用用法

• 使用時：ets:safe_fixtable(Tab, true)
• 使用完畢後：ets:safe_fixtable(Tab, false)

源碼設計

BIF_RETTYPE ets_safe_fixtable_2(BIF_ALIST_2)
{
    DbTable *tb;
    db_lock_kind_t kind;
#ifdef HARDDEBUG
    erts_fprintf(stderr,
		"ets:safe_fixtable(%T,%T); Process: %T, initial: %T:%T/%bpu\n",
		BIF_ARG_1, BIF_ARG_2, BIF_P->common.id,
		BIF_P->u.initial[0], BIF_P->u.initial[1], BIF_P->u.initial[2]);
#endif
    kind = (BIF_ARG_2 == am_true) ? LCK_READ : LCK_WRITE_REC; 

    DB_BIF_GET_TABLE(tb, DB_READ, kind, BIF_ets_safe_fixtable_2);

    if (BIF_ARG_2 == am_true) {
	fix_table_locked(BIF_P, tb);
    }
    else if (BIF_ARG_2 == am_false) {
	if (IS_FIXED(tb)) {
	    unfix_table_locked(BIF_P, tb, &kind);
	}
    }
    else {
	db_unlock(tb, kind);
	BIF_ERROR(BIF_P, BADARG);
    }
    db_unlock(tb, kind);
    BIF_RET(am_true);
}


/*  SMP note: table only need to be LCK_READ locked */
static void fix_table_locked(Process* p, DbTable* tb)
{
    DbFixation *fix;
    int use_locks = !DB_LOCK_FREE(tb);

    if (use_locks)
        erts_mtx_lock(&tb->common.fixlock);

    erts_refc_inc(&tb->common.fix_count,1);
    fix = tb->common.fixing_procs;
    if (fix == NULL) {
	tb->common.time.monotonic
	    = erts_get_monotonic_time(erts_proc_sched_data(p));
	tb->common.time.offset = erts_get_time_offset();
    }
    else {
	fix = fixing_procs_rbt_lookup(fix, p);
	if (fix) {
	    ASSERT(fixed_tabs_find(NULL, fix));
	    ++(fix->counter);
            if (use_locks)
                erts_mtx_unlock(&tb->common.fixlock);
	    return;
	}
    }
    fix = (DbFixation *) erts_db_alloc(ERTS_ALC_T_DB_FIXATION,
				       tb, sizeof(DbFixation));
    ERTS_ETS_MISC_MEM_ADD(sizeof(DbFixation));
    fix->tabs.btid = tb->common.btid;
    erts_refc_inc(&fix->tabs.btid->intern.refc, 2);
    fix->procs.p = p;
    fix->counter = 1;
    fixing_procs_rbt_insert(&tb->common.fixing_procs, fix);

    if (use_locks)
        erts_mtx_unlock(&tb->common.fixlock);

    p->flags |= F_USING_DB;

    fixed_tabs_insert(p, fix);
}

static void fixed_tabs_insert(Process* p, DbFixation* fix)
{
    DbFixation* first = erts_psd_get(p, ERTS_PSD_ETS_FIXED_TABLES);

    if (!first) {
        fix->tabs.next = fix->tabs.prev = fix;
        erts_psd_set(p, ERTS_PSD_ETS_FIXED_TABLES, fix);
    }
    else {
        ASSERT(!fixed_tabs_find(first, fix));
        fix->tabs.prev = first->tabs.prev;
        fix->tabs.next = first;
        fix->tabs.prev->tabs.next = fix;
        first->tabs.prev = fix;
    }
}

/* SMP note: May re-lock table 
*/
static void unfix_table_locked(Process* p,  DbTable* tb,
			       db_lock_kind_t* kind_p)
{
    DbFixation* fix;
    int use_locks = !DB_LOCK_FREE(tb);

    if (use_locks)
        erts_mtx_lock(&tb->common.fixlock);

    fix = fixing_procs_rbt_lookup(tb->common.fixing_procs, p);

    if (fix) {
	erts_refc_dec(&tb->common.fix_count,0);
	--(fix->counter);
	ASSERT(fix->counter >= 0);
	if (fix->counter == 0) {
	    fixing_procs_rbt_delete(&tb->common.fixing_procs, fix);
            if (use_locks)
                erts_mtx_unlock(&tb->common.fixlock);
	    fixed_tabs_delete(p, fix);

	    erts_refc_dec(&fix->tabs.btid->intern.refc, 1);

	    erts_db_free(ERTS_ALC_T_DB_FIXATION,
			 tb, (void *) fix, sizeof(DbFixation));
	    ERTS_ETS_MISC_MEM_ADD(-sizeof(DbFixation));
	    goto unlocked;
	}
    }
    if (use_locks)
        erts_mtx_unlock(&tb->common.fixlock);
unlocked:

    if (!IS_FIXED(tb) && IS_HASH_TABLE(tb->common.status)
	&& erts_atomic_read_nob(&tb->hash.fixdel) != (erts_aint_t)NULL) {
	if (*kind_p == LCK_READ && tb->common.is_thread_safe) {
	    /* Must have write lock while purging pseudo-deleted (OTP-8166) */
            if (use_locks) {
                erts_rwmtx_runlock(&tb->common.rwlock);
                erts_rwmtx_rwlock(&tb->common.rwlock);
            }
	    *kind_p = LCK_WRITE;
	    if (tb->common.status & (DB_DELETE|DB_BUSY))
                return;
	}
	db_unfix_table_hash(&(tb->hash));
    }
}

SWord db_unfix_table_hash(DbTableHash *tb)
{
    FixedDeletion* fixdel;
    SWord work = 0;

    ERTS_LC_ASSERT(IS_TAB_WLOCKED(tb)
                   || (erts_lc_rwmtx_is_rlocked(&tb->common.rwlock)
                       && !tb->common.is_thread_safe));
restart:
    fixdel = (FixedDeletion*) erts_atomic_xchg_mb(&tb->fixdel,
                                                  (erts_aint_t) NULL);
    while (fixdel) {
        FixedDeletion *free_me;

        do {
            HashDbTerm **bp;
            HashDbTerm *b;
            HashDbTerm *free_us = NULL;
            erts_rwmtx_t* lck;

            lck = WLOCK_HASH(tb, fixdel->slot);

            if (IS_FIXED(tb)) { /* interrupted by fixer */
                WUNLOCK_HASH(lck);
                restore_fixdel(tb,fixdel);
                if (!IS_FIXED(tb)) {
                    goto restart; /* unfixed again! */
                }
                return work;
            }
            if (fixdel->slot < NACTIVE(tb)) {
                bp = &BUCKET(tb, fixdel->slot);
                b = *bp;

                while (b != NULL) {
                    if (is_pseudo_deleted(b)) {
                        HashDbTerm* nxt = b->next;
                        b->next = free_us;
                        free_us = b;
                        work++;
                        b = *bp = nxt;
                    } else {
                        bp = &b->next;
                        b = b->next;
                    }
                }
            }
            /* else slot has been joined and purged by shrink() */
            WUNLOCK_HASH(lck);
            free_term_list(tb, free_us);

        }while (fixdel->all && fixdel->slot-- > 0);

        free_me = fixdel;
        fixdel = fixdel->next;
        free_fixdel(tb, free_me);
        work++;
    }

    /* ToDo: Maybe try grow/shrink the table as well */

    return work;
}