# membarrier

## NAME



           membarrier - issue memory barriers on a set of threads
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <linux/membarrier.h> **/* Definition of **MEMBARRIER_*** constants */
           **#include <sys/syscall.h>      **/* Definition of **SYS_*** constants */
           **#include <unistd.h>**
    
           **int syscall(SYS_membarrier, int** _cmd_**, unsigned int** _flags_**, int** _cpu_id_**);**
    
           _Note_ : glibc provides no wrapper for **membarrier**(), necessitating
           the use of syscall(2).
    


## DESCRIPTION



           The **membarrier**() system call helps reducing the overhead of the
           memory barrier instructions required to order memory accesses on
           multi-core systems.  However, this system call is heavier than a
           memory barrier, so using it effectively is _not_ as simple as
           replacing memory barriers with this system call, but requires
           understanding of the details below.
    
           Use of memory barriers needs to be done taking into account that a
           memory barrier always needs to be either matched with its memory
           barrier counterparts, or that the architecture's memory model
           doesn't require the matching barriers.
    
           There are cases where one side of the matching barriers (which we
           will refer to as "fast side") is executed much more often than the
           other (which we will refer to as "slow side").  This is a prime
           target for the use of **membarrier**().  The key idea is to replace,
           for these matching barriers, the fast-side memory barriers by
           simple compiler barriers, for example:
    
               asm volatile ("" : : : "memory")
    
           and replace the slow-side memory barriers by calls to
           **membarrier**().
    
           This will add overhead to the slow side, and remove overhead from
           the fast side, thus resulting in an overall performance increase
           as long as the slow side is infrequent enough that the overhead of
           the **membarrier**() calls does not outweigh the performance gain on
           the fast side.
    
           The _cmd_ argument is one of the following:
    
           **MEMBARRIER_CMD_QUERY**(since Linux 4.3)
                  Query the set of supported commands.  The return value of
                  the call is a bit mask of supported commands.
                  **MEMBARRIER_CMD_QUERY** , which has the value 0, is not itself
                  included in this bit mask.  This command is always
                  supported (on kernels where **membarrier**() is provided).
    
           **MEMBARRIER_CMD_GLOBAL**(since Linux 4.16)
                  Ensure that all threads from all processes on the system
                  pass through a state where all memory accesses to user-
                  space addresses match program order between entry to and
                  return from the **membarrier**() system call.  All threads on
                  the system are targeted by this command.
    
           **MEMBARRIER_CMD_GLOBAL_EXPEDITED**(since Linux 4.16)
                  Execute a memory barrier on all running threads of all
                  processes that previously registered with
                  **MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED**.
    
                  Upon return from the system call, the calling thread has a
                  guarantee that all running threads have passed through a
                  state where all memory accesses to user-space addresses
                  match program order between entry to and return from the
                  system call (non-running threads are de facto in such a
                  state).  This guarantee is provided only for the threads of
                  processes that previously registered with
                  **MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED**.
    
                  Given that registration is about the intent to receive the
                  barriers, it is valid to invoke
                  **MEMBARRIER_CMD_GLOBAL_EXPEDITED** from a process that has not
                  employed **MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED**.
    
                  The "expedited" commands complete faster than the non-
                  expedited ones; they never block, but have the downside of
                  causing extra overhead.
    
           **MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED**(since Linux 4.16)
                  Register the process's intent to receive
                  **MEMBARRIER_CMD_GLOBAL_EXPEDITED** memory barriers.
    
           **MEMBARRIER_CMD_PRIVATE_EXPEDITED**(since Linux 4.14)
                  Execute a memory barrier on each running thread belonging
                  to the same process as the calling thread.
    
                  Upon return from the system call, the calling thread has a
                  guarantee that all its running thread siblings have passed
                  through a state where all memory accesses to user-space
                  addresses match program order between entry to and return
                  from the system call (non-running threads are de facto in
                  such a state).  This guarantee is provided only for threads
                  in the same process as the calling thread.
    
                  The "expedited" commands complete faster than the non-
                  expedited ones; they never block, but have the downside of
                  causing extra overhead.
    
                  A process must register its intent to use the private
                  expedited command prior to using it.
    
           **MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED**(since Linux 4.14)
                  Register the process's intent to use
                  **MEMBARRIER_CMD_PRIVATE_EXPEDITED**.
    
           **MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE**(since Linux 4.16)
                  In addition to providing the memory ordering guarantees
                  described in **MEMBARRIER_CMD_PRIVATE_EXPEDITED** , upon return
                  from system call the calling thread has a guarantee that
                  all its running thread siblings have executed a core
                  serializing instruction.  This guarantee is provided only
                  for threads in the same process as the calling thread.
    
                  The "expedited" commands complete faster than the non-
                  expedited ones, they never block, but have the downside of
                  causing extra overhead.
    
                  A process must register its intent to use the private
                  expedited sync core command prior to using it.
    
           **MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE**(since Linux
           4.16)
                  Register the process's intent to use
                  **MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE**.
    
           **MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ**(since Linux 5.10)
                  Ensure the caller thread, upon return from system call,
                  that all its running thread siblings have any currently
                  running rseq critical sections restarted if _flags_ parameter
                  is 0; if _flags_ parameter is **MEMBARRIER_CMD_FLAG_CPU** , then
                  this operation is performed only on CPU indicated by
                  _cpu_id_.  This guarantee is provided only for threads in the
                  same process as the calling thread.
    
                  RSEQ membarrier is only available in the "private
                  expedited" form.
    
                  A process must register its intent to use the private
                  expedited rseq command prior to using it.
    
           **MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ**(since Linux 5.10)
                  Register the process's intent to use
                  **MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ**.
    
           **MEMBARRIER_CMD_SHARED**(since Linux 4.3)
                  This is an alias for **MEMBARRIER_CMD_GLOBAL** that exists for
                  header backward compatibility.
    
           The _flags_ argument must be specified as 0 unless the command is
           **MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ** , in which case _flags_ can be
           either 0 or **MEMBARRIER_CMD_FLAG_CPU**.
    
           The _cpu_id_ argument is ignored unless _flags_ is
           **MEMBARRIER_CMD_FLAG_CPU** , in which case it must specify the CPU
           targeted by this membarrier command.
    
           All memory accesses performed in program order from each targeted
           thread are guaranteed to be ordered with respect to **membarrier**().
    
           If we use the semantic _barrier()_ to represent a compiler barrier
           forcing memory accesses to be performed in program order across
           the barrier, and _smp_mb()_ to represent explicit memory barriers
           forcing full memory ordering across the barrier, we have the
           following ordering table for each pairing of _barrier()_ ,
           **membarrier**(), and _smp_mb()_.  The pair ordering is detailed as (O:
           ordered, X: not ordered):
    
                                 barrier()   smp_mb()   membarrier()
                  barrier()          X          X            O
                  smp_mb()           X          O            O
                  membarrier()       O          O            O
    


## RETURN VALUE



           On success, the **MEMBARRIER_CMD_QUERY** operation returns a bit mask
           of supported commands, and the **MEMBARRIER_CMD_GLOBAL** ,
           **MEMBARRIER_CMD_GLOBAL_EXPEDITED** ,
           **MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED** ,
           **MEMBARRIER_CMD_PRIVATE_EXPEDITED** ,
           **MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED** ,
           **MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE** , and
           **MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE** operations
           return zero.  On error, -1 is returned, and _errno_ is set to
           indicate the error.
    
           For a given command, with _flags_ set to 0, this system call is
           guaranteed to always return the same value until reboot.  Further
           calls with the same arguments will lead to the same result.
           Therefore, with _flags_ set to 0, error handling is required only
           for the first call to **membarrier**().
    


## ERRORS



           **EINVAL** _cmd_ is invalid, or _flags_ is nonzero, or the
                  **MEMBARRIER_CMD_GLOBAL** command is disabled because the
                  _nohz_full_ CPU parameter has been set, or the
                  **MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE** and
                  **MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE**
                  commands are not implemented by the architecture.
    
           **ENOSYS** The **membarrier**() system call is not implemented by this
                  kernel.
    
           **EPERM** The current process was not registered prior to using
                  private expedited commands.
    


## STANDARDS



           Linux.
    


## NOTES



           A memory barrier instruction is part of the instruction set of
           architectures with weakly ordered memory models.  It orders memory
           accesses prior to the barrier and after the barrier with respect
           to matching barriers on other cores.  For instance, a load fence
           can order loads prior to and following that fence with respect to
           stores ordered by store fences.
    
           Program order is the order in which instructions are ordered in
           the program assembly code.
    
           Examples where **membarrier**() can be useful include implementations
           of Read-Copy-Update libraries and garbage collectors.
    


## EXAMPLES



           Assuming a multithreaded application where "fast_path()" is
           executed very frequently, and where "slow_path()" is executed
           infrequently, the following code (x86) can be transformed using
           **membarrier**():
    
               #include <stdlib.h>
    
               static volatile int a, b;
    
               static void
               fast_path(int *read_b)
               {
                   a = 1;
                   asm volatile ("mfence" : : : "memory");
                   *read_b = b;
               }
    
               static void
               slow_path(int *read_a)
               {
                   b = 1;
                   asm volatile ("mfence" : : : "memory");
                   *read_a = a;
               }
    
               int
               main(void)
               {
                   int read_a, read_b;
    
                   /*
                    * Real applications would call fast_path() and slow_path()
                    * from different threads. Call those from main() to keep
                    * this example short.
                    */
    
                   slow_path(&read_a);
                   fast_path(&read_b);
    
                   /*
                    * read_b == 0 implies read_a == 1 and
                    * read_a == 0 implies read_b == 1.
                    */
    
                   if (read_b == 0 && read_a == 0)
                       abort();
    
                   exit(EXIT_SUCCESS);
               }
    
           The code above transformed to use **membarrier**() becomes:
    
               #define _GNU_SOURCE
               #include <stdlib.h>
               #include <stdio.h>
               #include <unistd.h>
               #include <sys/syscall.h>
               #include <linux/membarrier.h>
    
               static volatile int a, b;
    
               static int
               membarrier(int cmd, unsigned int flags, int cpu_id)
               {
                   return syscall(__NR_membarrier, cmd, flags, cpu_id);
               }
    
               static int
               init_membarrier(void)
               {
                   int ret;
    
                   /* Check that membarrier() is supported. */
    
                   ret = membarrier(MEMBARRIER_CMD_QUERY, 0, 0);
                   if (ret < 0) {
                       perror("membarrier");
                       return -1;
                   }
    
                   if (!(ret & MEMBARRIER_CMD_GLOBAL)) {
                       fprintf(stderr,
                           "membarrier does not support MEMBARRIER_CMD_GLOBAL\n");
                       return -1;
                   }
    
                   return 0;
               }
    
               static void
               fast_path(int *read_b)
               {
                   a = 1;
                   asm volatile ("" : : : "memory");
                   *read_b = b;
               }
    
               static void
               slow_path(int *read_a)
               {
                   b = 1;
                   membarrier(MEMBARRIER_CMD_GLOBAL, 0, 0);
                   *read_a = a;
               }
    
               int
               main(int argc, char *argv[])
               {
                   int read_a, read_b;
    
                   if (init_membarrier())
                       exit(EXIT_FAILURE);
    
                   /*
                    * Real applications would call fast_path() and slow_path()
                    * from different threads. Call those from main() to keep
                    * this example short.
                    */
    
                   slow_path(&read_a);
                   fast_path(&read_b);
    
                   /*
                    * read_b == 0 implies read_a == 1 and
                    * read_a == 0 implies read_b == 1.
                    */
    
                   if (read_b == 0 && read_a == 0)
                       abort();
    
                   exit(EXIT_SUCCESS);
               }
    


* * *