membarrier(2) - Linux manual page (original) (raw)

membarrier(2) System Calls Manual membarrier(2)

NAME top

   membarrier - issue memory barriers on a set of threads

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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** _cpuid_**);**

   _Note_: glibc provides no wrapper for **membarrier**(), necessitating
   the use of [syscall(2)](../man2/syscall.2.html).

DESCRIPTION top

   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
          _cpuid_.  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 _cpuid_ 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 _smpmb()_ 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 _smpmb()_.  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 top

   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](../man3/errno.3.html)_ 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 top

   **EINVAL** _cmd_ is invalid, or _flags_ is nonzero, or the
          **MEMBARRIER_CMD_GLOBAL** command is disabled because the
          _nohzfull_ 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 top

   Linux.

HISTORY top

   Linux 4.3.

   Before Linux 5.10, the prototype was:

       **int membarrier(int** _cmd_**, int** _flags_**);**

NOTES top

   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 top

   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);
       }

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Linux man-pages 6.10 2024-07-23 membarrier(2)

Pages that refer to this page:syscalls(2)