membarrier

MEMBARRIER(2)              Linux Programmer's Manual             MEMBARRIER(2)

NAME
       membarrier - issue memory barriers on a set of threads

SYNOPSIS
       #include <linux/membarrier.h>

       int membarrier(int cmd, int flags);

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 mem-
       ory 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 match-
       ing barriers, the fast-side memory barriers by simple  compiler  barri-
       ers, 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 membar-
       rier() 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 ad-
              dresses match program order between entry to and return from the
              membarrier()  system  call.   All threads on the system are tar-
              geted 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_REGIS-
              TER_GLOBAL_EXPEDITED.

              Upon return from the system call, the calling thread has a guar-
              antee 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  pro-
              vided  only  for the threads of processes that previously regis-
              tered with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED.

              Given that registration is about the intent to receive the  bar-
              riers,  it  is  valid  to invoke MEMBARRIER_CMD_GLOBAL_EXPEDITED
              from a  process  that  has  not  employed  MEMBARRIER_CMD_REGIS-
              TER_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   MEMBAR-
              RIER_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 guar-
              antee 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 guar-
              antee 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_EX-
              PEDITED.

       MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE (since Linux 4.16)
              In  addition  to  providing  the  memory ordering guarantees de-
              scribed in MEMBARRIER_CMD_PRIVATE_EXPEDITED,  upon  return  from
              system call the calling thread has a guarantee that all its run-
              ning thread siblings have executed a core  serializing  instruc-
              tion.   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_EX-
              PEDITED_SYNC_CORE.

       MEMBARRIER_CMD_SHARED (since Linux 4.3)
              This is an  alias  for  MEMBARRIER_CMD_GLOBAL  that  exists  for
              header backward compatibility.

       The flags argument is currently unused and must be specified as 0.

       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  forc-
       ing  memory  accesses  to be performed in program order across the bar-
       rier, and smp_mb() to represent explicit memory barriers  forcing  full
       memory  ordering across the barrier, we have the following ordering ta-
       ble 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,    MEMBAR-
       RIER_CMD_GLOBAL_EXPEDITED,    MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED,
       MEMBARRIER_CMD_PRIVATE_EXPEDITED, MEMBARRIER_CMD_REGISTER_PRIVATE_EXPE-
       DITED,    MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE,    and    MEMBAR-
       RIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE operations  return  zero.
       On error, -1 is returned, and errno is set appropriately.

       For  a  given command, with flags set to 0, this system call is guaran-
       teed 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 membar-
       rier().

ERRORS
       EINVAL cmd   is   invalid,   or   flags  is  nonzero,  or  the  MEMBAR-
              RIER_CMD_GLOBAL command is disabled because  the  nohz_full  CPU
              parameter  has  been  set,  or  the MEMBARRIER_CMD_PRIVATE_EXPE-
              DITED_SYNC_CORE    and     MEMBARRIER_CMD_REGISTER_PRIVATE_EXPE-
              DITED_SYNC_CORE  commands  are  not implemented by the architec-
              ture.

       ENOSYS The membarrier() system call is not implemented by this kernel.

       EPERM  The current process was not registered prior  to  using  private
              expedited commands.

VERSIONS
       The membarrier() system call was added in Linux 4.3.

CONFORMING TO
       membarrier() is Linux-specific.

NOTES
       A  memory  barrier instruction is part of the instruction set of archi-
       tectures 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.

EXAMPLE
       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(int argc, char **argv)
           {
               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, int flags)
           {
               return syscall(__NR_membarrier, cmd, flags);
           }

           static int
           init_membarrier(void)
           {
               int ret;

               /* Check that membarrier() is supported. */

               ret = membarrier(MEMBARRIER_CMD_QUERY, 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);
               *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);
           }

COLOPHON
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Linux                             2018-04-30                     MEMBARRIER(2)
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