source: trunk/kernel/kern/thread.c @ 147

/*
 * thread.c -  implementation of thread operations (user & kernel)
 * 
 * Author  Ghassan Almaless (2008,2009,2010,2011,2012)
 *         Alain Greiner (2016,2017)
 *
 * Copyright (c) UPMC Sorbonne Universites
 *
 * This file is part of ALMOS-MKH.
 *
 * ALMOS-MKH 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 2.0 of the License.
 *
 * ALMOS-MKH 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 ALMOS-MKH; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include <kernel_config.h>
#include <hal_types.h>
#include <hal_context.h>
#include <hal_irqmask.h>
#include <hal_special.h>
#include <hal_remote.h>
#include <memcpy.h>
#include <printk.h>
#include <cluster.h>
#include <process.h>
#include <scheduler.h>
#include <dev_icu.h>
#include <core.h>
#include <list.h>
#include <xlist.h>
#include <page.h>
#include <kmem.h>
#include <ppm.h>
#include <thread.h>

//////////////////////////////////////////////////////////////////////////////////////
// Extern global variables
//////////////////////////////////////////////////////////////////////////////////////

extern process_t      process_zero;

//////////////////////////////////////////////////////////////////////////////////////
// This function returns a printable string for the thread type.
//////////////////////////////////////////////////////////////////////////////////////
char * thread_type_str( uint32_t type )
{
    if     ( type == THREAD_USER   ) return "USER";
    else if( type == THREAD_RPC    ) return "RPC";
    else if( type == THREAD_DEV    ) return "DEV";
    else if( type == THREAD_KERNEL ) return "KERNEL";
    else if( type == THREAD_IDLE   ) return "IDLE";
    else                             return "undefined";
}

/////////////////////////////////////////////////////////////////////////////////////
// This static function allocates physical memory for a thread descriptor.
// It can be called by the three functions:
// - thread_user_create()
// - thread_user_fork()
// - thread_kernel_create()
/////////////////////////////////////////////////////////////////////////////////////
// @ return pointer on thread descriptor if success / return NULL if failure.
/////////////////////////////////////////////////////////////////////////////////////
static thread_t * thread_alloc()
{
        page_t       * page;   // pointer on page descriptor containing thread descriptor
        kmem_req_t     req;    // kmem request 

        // allocates memory for thread descriptor + kernel stack
        req.type  = KMEM_PAGE;
        req.size  = CONFIG_THREAD_DESC_ORDER;
        req.flags = AF_KERNEL | AF_ZERO;
        page      = kmem_alloc( &req );

    // return pointer on new thread descriptor
        if( page == NULL ) return NULL;
    else               return (thread_t *)ppm_page2vaddr( page );
}  

/////////////////////////////////////////////////////////////////////////////////////
// This static function releases the physical memory for a thread descriptor.
// It is called by the three functions:
// - thread_user_create()
// - thread_user_fork()
// - thread_kernel_create()
/////////////////////////////////////////////////////////////////////////////////////
// @ thread  : pointer on thread descriptor.
/////////////////////////////////////////////////////////////////////////////////////
static void thread_release( thread_t * thread )
{
    kmem_req_t   req;

    req.type  = KMEM_PAGE;
    req.ptr   = ppm_vaddr2page( thread );
    kmem_free( &req );
}

/////////////////////////////////////////////////////////////////////////////////////
// This static function initializes a thread descriptor (kernel or user).
// It can be called by the four functions:
// - thread_user_create()
// - thread_user_fork()
// - thread_kernel_create()
// - thread_user_init()
/////////////////////////////////////////////////////////////////////////////////////
// @ thread       : pointer on thread descriptor
// @ process      : pointer on process descriptor.
// @ type         : thread type.
// @ func         : pointer on thread entry function.
// @ args         : pointer on thread entry function arguments.
// @ core_lid     : target core local index.
// @ u_stack_base : stack base (user thread only)
// @ u_stack_size : stack base (user thread only)
/////////////////////////////////////////////////////////////////////////////////////
static error_t thread_init( thread_t      * thread,
                            process_t     * process,
                            thread_type_t   type,
                            void          * func,
                            void          * args,
                            lid_t           core_lid,
                            intptr_t        u_stack_base,
                            uint32_t        u_stack_size )
{
    error_t        error;
    trdid_t        trdid;      // allocated thread identifier

        cluster_t    * local_cluster = LOCAL_CLUSTER;

    // register new thread in process descriptor, and get a TRDID
    spinlock_lock( &process->th_lock );
    error = process_register_thread( process, thread , &trdid );
    spinlock_unlock( &process->th_lock );

    if( error ) 
    {
        printk("\n[ERROR] in %s : cannot get TRDID\n", __FUNCTION__ );
        return EINVAL;
    }

        // Initialize new thread descriptor
    thread->trdid           = trdid;
        thread->type            = type;  
    thread->quantum         = 0;            // TODO
    thread->ticks_nr        = 0;            // TODO
    thread->time_last_check = 0;
        thread->core            = &local_cluster->core_tbl[core_lid];
        thread->process         = process;

    thread->local_locks     = 0;
    list_root_init( &thread->locks_root );

    thread->remote_locks    = 0;
    xlist_root_init( XPTR( local_cxy , &thread->xlocks_root ) );

    thread->u_stack_base    = u_stack_base;     
    thread->u_stack_size    = u_stack_size;
    thread->k_stack_base    = (intptr_t)thread;     
    thread->k_stack_size    = CONFIG_THREAD_DESC_SIZE;

    thread->entry_func      = func;         // thread entry point
    thread->entry_args      = args;         // thread function arguments
    thread->flags           = 0;            // all flags reset  
    thread->signals         = 0;            // no pending signal
    thread->errno           = 0;            // no error detected
    thread->fork_user       = 0;            // no fork required 
    thread->fork_cxy        = 0;

    // thread blocked
    thread->blocked = THREAD_BLOCKED_GLOBAL;

    // reset children list
    xlist_root_init( XPTR( local_cxy , &thread->children_root ) );
    thread->children_nr = 0;

    // reset sched list and brothers list
    list_entry_init( &thread->sched_list );
    xlist_entry_init( XPTR( local_cxy , &thread->brothers_list ) );

    // reset thread info
    memset( &thread->info , 0 , sizeof(thread_info_t) );

    // initialise signature
        thread->signature = THREAD_SIGNATURE;

    // update local DQDT
    dqdt_local_update_threads( 1 );

    // register new thread in core scheduler 
    sched_register_thread( thread->core , thread );

        return 0;

} // end thread_init()


/////////////////////////////////////////////////////////
error_t thread_user_create( pid_t             pid,
                            void            * start_func,
                            void            * start_arg,
                            pthread_attr_t  * attr,
                            thread_t       ** new_thread )
{
    error_t        error;
        thread_t     * thread;       // pointer on created thread descriptor
    process_t    * process;      // pointer to local process descriptor
    lid_t          core_lid;     // selected core local index
    vseg_t       * vseg;         // stack vseg

    thread_dmsg("\n[INFO] %s : enters for process %x\n", __FUNCTION__ , pid );

    // get process descriptor local copy
    process = process_get_local_copy( pid );

    if( process == NULL )
    {
                printk("\n[ERROR] in %s : cannot get process descriptor %x\n",
               __FUNCTION__ , pid );
        return ENOMEM;
    }

    // select a target core in local cluster 
    if( attr->attributes & PT_ATTR_CORE_DEFINED ) core_lid = attr->lid;
    else                                          core_lid = cluster_select_local_core();

    // check core local index
    if( core_lid >= LOCAL_CLUSTER->cores_nr )
    {
            printk("\n[ERROR] in %s : illegal core index attribute = %d\n",
               __FUNCTION__ , core_lid );
       
        return EINVAL;
    }

    // allocate a stack from local VMM 
    vseg = vmm_create_vseg( process, 0 , 0 , VSEG_TYPE_STACK );

    if( vseg == NULL );
    {
            printk("\n[ERROR] in %s : cannot create stack vseg\n", __FUNCTION__ );
                return ENOMEM;
    } 

    // allocates memory tor thread descriptor
    thread = thread_alloc();

    if( thread == NULL )
    {
            printk("\n[ERROR] in %s : cannot create new thread\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        return ENOMEM;
    }

    // initializes thread descriptor
    error = thread_init( thread,
                         process,
                         THREAD_USER,
                         start_func,
                         start_arg,
                         core_lid,
                         vseg->min,
                         vseg->max - vseg->min );

    if( error ) 
    {
            printk("\n[ERROR] in %s : cannot initialize new thread\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        thread_release( thread );
        return EINVAL;
    }

    // set LOADABLE flag 
    thread->flags = THREAD_FLAG_LOADABLE;

    // set DETACHED flag if required
    if( attr->attributes & PT_ATTR_DETACH ) thread->flags |= THREAD_FLAG_DETACHED;

    // allocate & initialise CPU context 
        error = hal_cpu_context_create( thread ); 

    if( error ) 
    {
            printk("\n[ERROR] in %s : cannot create CPU context\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        thread_release( thread );
        return ENOMEM;
    }

    // allocate & initialise FPU context 
    error = hal_fpu_context_create( thread ); 

    if( error )
    {
            printk("\n[ERROR] in %s : cannot create FPU context\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        thread_release( thread );
        return ENOMEM;
    }

    thread_dmsg("\n[INFO] %s : exit / trdid = %x / process %x / core = %d\n", 
                __FUNCTION__ , thread->trdid , process->pid , core_lid );

    *new_thread = thread;
        return 0;

} // end thread_user_create()


//////////////////////////////////////////////
error_t thread_user_fork( process_t * process,
                          thread_t ** new_thread )
{
    error_t        error;
        thread_t     * thread;       // pointer on new thread descriptor
    lid_t          core_lid;     // selected core local index
        vseg_t       * vseg;         // stack vseg

    thread_dmsg("\n[INFO] %s : enters\n", __FUNCTION__ );

    // allocate a stack from local VMM 
    vseg = vmm_create_vseg( process, 0 , 0 , VSEG_TYPE_STACK );

    if( vseg == NULL );
    {
            printk("\n[ERROR] in %s : cannot create stack vseg\n", __FUNCTION__ );
                return ENOMEM;
    } 

    // select a target core in local cluster
    core_lid = cluster_select_local_core();

    // get pointer on calling thread descriptor
    thread_t * this = CURRENT_THREAD;

    // allocated memory for new thread descriptor
    thread = thread_alloc();

    if( thread == NULL )
    {
        printk("\n[ERROR] in %s : cannot allocate new thread\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        return ENOMEM;
    }

    // initializes thread descriptor
    error = thread_init( thread,
                         process,
                         THREAD_USER,
                         this->entry_func,
                         this->entry_args,
                         core_lid,
                         vseg->min,
                         vseg->max - vseg->min );

    if( error )
    {
            printk("\n[ERROR] in %s : cannot initialize new thread\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        thread_release( thread );
        return EINVAL;
    }

    // set ATTACHED flag if set in this thread
    if( this->flags & THREAD_FLAG_DETACHED ) thread->flags = THREAD_FLAG_DETACHED;

    // allocate & initialise CPU context from calling thread
        error = hal_cpu_context_copy( thread , this ); 

    if( error )
    {
            printk("\n[ERROR] in %s : cannot create CPU context\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        thread_release( thread );
        return ENOMEM;
    }

    // allocate & initialise FPU context from calling thread
        error = hal_fpu_context_copy( thread , this ); 

    if( error )
    {
            printk("\n[ERROR] in %s : cannot create CPU context\n", __FUNCTION__ );
        vmm_remove_vseg( vseg );
        thread_release( thread );
        return ENOMEM;
    }

    thread_dmsg("\n[INFO] %s : exit / thread %x for process %x on core %d in cluster %x\n", 
                 __FUNCTION__, thread->trdid, process->pid, core_lid, local_cxy );

    *new_thread = thread;
        return 0;

} // end thread_user_fork()



/////////////////////////////////////////////////////////
error_t thread_kernel_create( thread_t     ** new_thread,
                              thread_type_t   type,
                              void          * func, 
                              void          * args, 
                                              lid_t           core_lid )
{
    error_t        error;
        thread_t     * thread;       // pointer on new thread descriptor
        kmem_req_t     req;          // kmem request (for release) 

    thread_dmsg("\n[INFO] %s : enters for type %s in cluster %x\n",
                __FUNCTION__ , thread_type_str( type ) , local_cxy );

    assert( ( (type == THREAD_KERNEL) || (type == THREAD_RPC) || 
              (type == THREAD_IDLE)   || (type == THREAD_DEV) ) ,
              __FUNCTION__ , "illegal thread type" );

    assert( (core_lid < LOCAL_CLUSTER->cores_nr) , 
            __FUNCTION__ , "illegal core_lid" );

    // allocated memory for new thread descriptor
    thread = thread_alloc();

    if( thread == NULL ) return ENOMEM;

    // initializes thread descriptor
    error = thread_init( thread,
                         &process_zero,
                         type,
                         func,
                         args,
                         core_lid,
                         0 , 0 );  // no user stack for a kernel thread

    if( error ) // release allocated memory for thread descriptor 
    {
            req.type  = KMEM_PAGE;
        req.ptr   = ppm_vaddr2page( thread );
        kmem_free( &req );
        return EINVAL;
    }


    // allocate & initialise CPU context
        hal_cpu_context_create( thread ); 

    thread_dmsg("\n[INFO] %s : exit in cluster %x / trdid = %x / core_lid = %d\n", 
                 __FUNCTION__ , local_cxy , thread->trdid , core_lid );

    *new_thread = thread; 
        return 0;

} // end thread_kernel_create()

///////////////////////////////////////////////////
error_t thread_kernel_init( thread_t      * thread,
                            thread_type_t   type,
                            void          * func, 
                            void          * args, 
                                            lid_t           core_lid )
{
    assert( ( (type == THREAD_KERNEL) || (type == THREAD_RPC) || 
              (type == THREAD_IDLE)   || (type == THREAD_DEV) ) ,
              __FUNCTION__ , "illegal thread type" );

    if( core_lid >= LOCAL_CLUSTER->cores_nr ) 
    {
        printk("\n[PANIC] in %s : illegal core_lid / cores = %d / lid = %d / cxy = %x\n", 
               __FUNCTION__ , LOCAL_CLUSTER->cores_nr , core_lid , local_cxy );
        hal_core_sleep();
    }

    error_t  error = thread_init( thread,
                                  &process_zero,
                                  type,
                                  func,
                                  args,
                                  core_lid,
                                  0 , 0 );   // no user stack for a kernel thread

    // allocate & initialize CPU context if success
    if( error == 0 ) hal_cpu_context_create( thread );
     
    return error;

}  // end thread_kernel_init()

///////////////////////////////////////////////////////////////////////////////////////
// TODO: check that all memory dynamically allocated during thread execution
// has been released, using a cache of mmap and malloc requests. [AG]
///////////////////////////////////////////////////////////////////////////////////////
void thread_destroy( thread_t * thread )
{
        uint32_t     tm_start;
        uint32_t     tm_end;
    reg_t        state;

    process_t  * process    = thread->process;
    core_t     * core       = thread->core;

    thread_dmsg("\n[INFO] %s : enters for thread %x in process %x / type = %s\n",
                __FUNCTION__ , thread->trdid , process->pid , thread_type_str( thread->type ) );

    assert( (thread->children_nr == 0) , __FUNCTION__ , "still attached children" );

    assert( (thread->local_locks == 0) , __FUNCTION__ , "all local locks not released" );
    
    assert( (thread->remote_locks == 0) , __FUNCTION__ , "all remote locks not released" );

        tm_start = hal_get_cycles();

    // update intrumentation values
    uint32_t pgfaults = thread->info.pgfault_nr;
    uint32_t u_errors = thread->info.u_err_nr;
    uint32_t m_errors = thread->info.m_err_nr;

        process->vmm.pgfault_nr += pgfaults;
        process->vmm.u_err_nr   += u_errors;
        process->vmm.m_err_nr   += m_errors;

    // release memory allocated for CPU context and FPU context
        hal_cpu_context_destroy( thread );
        hal_fpu_context_destroy( thread );
        
    // release FPU if required
    // TODO This should be done before calling thread_destroy()
        hal_disable_irq( &state );
        if( core->fpu_owner == thread )
        {
                core->fpu_owner = NULL;
                hal_fpu_disable();
        }
        hal_restore_irq( state );

    // remove thread from process th_tbl[] 
    // TODO This should be done before calling thread_destroy()
    ltid_t ltid = LTID_FROM_TRDID( thread->trdid );

        spinlock_lock( &process->th_lock );
        process->th_tbl[ltid] = XPTR_NULL;
        process->th_nr--;
        spinlock_unlock( &process->th_lock );
        
    // update local DQDT
    dqdt_local_update_threads( -1 );

    // invalidate thread descriptor
        thread->signature = 0;

    // release memory for thread descriptor
    thread_release( thread );

        tm_end = hal_get_cycles();

        thread_dmsg("\n[INFO] %s : exit for thread %x in process %x / duration = %d\n",
                       __FUNCTION__, thread->trdid , process->pid , tm_end - tm_start );

}  // end thread_destroy()


/////////////////////////////////////////////////
void thread_child_parent_link( xptr_t  xp_parent,
                               xptr_t  xp_child )
{
    // get extended pointers on children list root 
    cxy_t      parent_cxy = GET_CXY( xp_parent );    
    thread_t * parent_ptr = (thread_t *)GET_PTR( xp_parent );
    xptr_t     root       = XPTR( parent_cxy , &parent_ptr->children_root );

    // get extended pointer on children list entry 
    cxy_t      child_cxy  = GET_CXY( xp_child );    
    thread_t * child_ptr  = (thread_t *)GET_PTR( xp_child );
    xptr_t     entry      = XPTR( child_cxy , &child_ptr->brothers_list );

    // set the link
    xlist_add_first( root , entry );
    hal_remote_atomic_add( XPTR( parent_cxy , &parent_ptr->children_nr ) , 1 );
} 

///////////////////////////////////////////////////
void thread_child_parent_unlink( xptr_t  xp_parent,
                                 xptr_t  xp_child )
{
    // get extended pointer on children list lock
    cxy_t      parent_cxy = GET_CXY( xp_parent );    
    thread_t * parent_ptr = (thread_t *)GET_PTR( xp_parent );
    xptr_t     lock       = XPTR( parent_cxy , &parent_ptr->children_lock );

    // get extended pointer on children list entry 
    cxy_t      child_cxy  = GET_CXY( xp_child );    
    thread_t * child_ptr  = (thread_t *)GET_PTR( xp_child );
    xptr_t     entry      = XPTR( child_cxy , &child_ptr->brothers_list );

    // get the lock
    remote_spinlock_lock( lock );

    // remove the link
    xlist_unlink( entry );
    hal_remote_atomic_add( XPTR( parent_cxy , &parent_ptr->children_nr ) , -1 );
    
    // release the lock
    remote_spinlock_unlock( lock );
}

/////////////////////////////////////////////////
inline void thread_set_signal( thread_t * thread,
                               uint32_t   mask )
{
    hal_atomic_or( &thread->signals , mask );
}
 
///////////////////////////////////////////////////
inline void thread_reset_signal( thread_t * thread,
                                 uint32_t   mask )
{
    hal_atomic_and( &thread->signals , ~mask );
}
 
//////////////////////////////////
inline bool_t thread_is_joinable()
{
    thread_t * this = CURRENT_THREAD;
    return( (this->brothers_list.next != XPTR_NULL) &&
            (this->brothers_list.pred != XPTR_NULL) );
}

//////////////////////////////////
inline bool_t thread_is_runnable()
{
    thread_t * this = CURRENT_THREAD;
    return( this->blocked == 0 );
}

////////////////////////////////
inline bool_t thread_can_yield()
{
    thread_t * this = CURRENT_THREAD;
    return ( (this->local_locks == 0) && (this->remote_locks == 0) );
}

///////////////////////////
bool_t thread_check_sched()
{
        thread_t * this = CURRENT_THREAD;

    // check locks count
        if( (this->local_locks != 0) || (this->remote_locks != 0) ) return false;

    // compute elapsed time, taking into account 32 bits register wrap
    uint32_t elapsed;
    uint32_t time_now   = hal_get_cycles();
    uint32_t time_last  = this->time_last_check;
    if( time_now < time_last ) elapsed = (0xFFFFFFFF - time_last) + time_now;
        else                       elapsed = time_now - time_last;

    // update thread time
    this->time_last_check = time_now;

        // check elapsed time
        if( elapsed < CONFIG_CORE_CHECK_EVERY ) return false;
    else                                    return true;
}

/////////////////////
error_t thread_exit()
{
    reg_t      sr_save;

        thread_t * this = CURRENT_THREAD;

    // test if this thread can be descheduled
        if( !thread_can_yield() )
        {
        printk("ERROR in %s : thread %x in process %x on core %d in cluster %x\n"
               " did not released all locks\n",
               __FUNCTION__ , this->trdid , this->process->pid ,
               CURRENT_CORE->lid , local_cxy );
        return EINVAL;
    }

    if( this->flags & THREAD_FLAG_DETACHED )
    {
        // if detached set signal and set blocking cause atomically
        hal_disable_irq( &sr_save );
        thread_set_signal( this , THREAD_SIG_KILL );
        thread_block( this , THREAD_BLOCKED_EXIT );
        hal_restore_irq( sr_save );
    }
    else 
    {
        // if attached, set blocking cause 
        thread_block( this , THREAD_BLOCKED_EXIT );
    }

    // deschedule
    sched_yield();
    return 0;

} // end thread_exit()

/////////////////////////////////////
void thread_block( thread_t * thread,
                   uint32_t   cause )
{
    // set blocking cause 
    hal_atomic_or( &thread->blocked , cause );

}  // end thread_block()

////////////////////////////////////
void thread_unblock( xptr_t   thread,
                    uint32_t cause )
{
    // get thread cluster and local pointer
    cxy_t      cxy = GET_CXY( thread ); 
    thread_t * ptr = (thread_t *)GET_PTR( thread );

    // reset blocking cause
    hal_remote_atomic_and( XPTR( cxy , &ptr->blocked ) , ~cause );

}  // end thread_unblock()

/////////////////////////////////////
void thread_kill( thread_t * target )
{
    // set SIG_KILL signal in target thread descriptor
    thread_set_signal( target , THREAD_SIG_KILL );

    // set the global blocked bit in target thread descriptor.
    thread_block( target , THREAD_BLOCKED_GLOBAL );

    // send an IPI to reschedule the target thread core.
    dev_icu_send_ipi( local_cxy , target->core->lid );

}  // end thread_kill()


///////////////////////
void thread_idle_func()
{

#if CONFIG_IDLE_DEBUG
    lid_t  lid = CURRENT_CORE->lid;
#endif

    while( 1 )
    {
        idle_dmsg("\n[INFO] %s : core[%x][%d] goes to sleep at cycle %d\n",
                    __FUNCTION__ , local_cxy , lid , hal_get_cycles() );

        // force core to sleeping state
        hal_core_sleep();

        idle_dmsg("\n[INFO] %s : core[%x][%d] wake up at cycle %d\n",
                    __FUNCTION__ , local_cxy , lid , hal_get_cycles() );

                // acknowledge IRQ
        dev_icu_irq_handler();

        // force scheduling
        sched_yield();
   }
}  // end thread_idle()

/////////////////////////////////////////////////
void thread_user_time_update( thread_t * thread )
{
    // TODO
    printk("\n[WARNING] function %s not implemented\n", __FUNCTION__ );
}

///////////////////////////////////////////////////
void thread_kernel_time_update( thread_t * thread )
{
    // TODO
    printk("\n[WARNING] function %s not implemented\n", __FUNCTION__ );
}

////////////////////////////////////////////////
void thread_signals_handle( thread_t * thread )
{
    // TODO
    printk("\n[WARNING] function %s not implemented\n", __FUNCTION__ );
}

/////////////////////////////////////
xptr_t thread_get_xptr( pid_t    pid,
                        trdid_t  trdid )
{
    cxy_t         target_cxy;          // target thread cluster identifier
    ltid_t        target_thread_ltid;  // target thread local index
    thread_t    * target_thread_ptr;   // target thread local pointer           
    xptr_t        target_process_xp;   // extended pointer on target process descriptor
    process_t   * target_process_ptr;  // local pointer on target process descriptor 
    pid_t         target_process_pid;  // target process identifier
    xlist_entry_t root;                // root of list of process in target cluster
    xptr_t        lock_xp;             // extended pointer on lock protecting  this list

    // get target cluster identifier and local thread identifier
    target_cxy         = CXY_FROM_TRDID( trdid );
    target_thread_ltid = LTID_FROM_TRDID( trdid );

    // get root of list of process descriptors in target cluster
    hal_remote_memcpy( XPTR( local_cxy  , &root ),
                       XPTR( target_cxy , &LOCAL_CLUSTER->pmgr.local_root ),
                       sizeof(xlist_entry_t) );

    // get extended pointer on lock protecting the list of processes 
    lock_xp = XPTR( target_cxy , &LOCAL_CLUSTER->pmgr.local_lock );

    // take the lock protecting the list of processes in target cluster
    remote_spinlock_lock( lock_xp );

    // loop on list of process in target cluster to find the PID process
    xptr_t  iter;
    bool_t  found = false;
    XLIST_FOREACH( XPTR( target_cxy , &LOCAL_CLUSTER->pmgr.local_root ) , iter )
    {
        target_process_xp  = XLIST_ELEMENT( iter , process_t , local_list );
        target_process_ptr = (process_t *)GET_PTR( target_process_xp );
        target_process_pid = hal_remote_lw( XPTR( target_cxy , &target_process_ptr->pid ) );
        if( target_process_pid == pid )
        {
            found = true;
            break;
        }
    }

    // release the lock protecting the list of processes in target cluster
    remote_spinlock_unlock( lock_xp );

    // check target thread found
    if( found == false )
    {
        return XPTR_NULL;
    }

    // get target thread local pointer
    xptr_t xp = XPTR( target_cxy , &target_process_ptr->th_tbl[target_thread_ltid] );
    target_thread_ptr = (thread_t *)hal_remote_lpt( xp );    

    if( target_thread_ptr == NULL )
    {
        return XPTR_NULL;
    }

    return XPTR( target_cxy , target_thread_ptr );

}  // end thread_get_xptr()