[PATCH RESEND v2 0/4] Add new system call for non-destructive peek and inspection to posix ipc mqueue

[Mathura_Kumar]

Patch series overview:

  1. Add New system call do_mq_timedreceive2() and handler implementation
  2. Add system call number in all  most common arch.
  3. Prepared Documentation and test
  4. Add entry in performance tools all most common file

This is a fresh patch series starting from v1. It completely replaces 
the previous series (v1–v5) sent about a month ago, so please do not 
confuse it with the older thread.I preferred to start a new series instead of 
continuing the old one, as it had become messy and almost everything has been rewritten.
All feedback from the earlier series has been taken into account and incorporated here.



Short Description:

    POSIX message queues currently lack a mechanism to read
    a message without removing it from the queue. This is a
    long-standing limitation,when we require inspection of queue state
    without altering it.


    Design considerations:
    Two approaches for copying message data to userspace
    were evaluated:

    1) Refcount-based message lifecycle handling
       - This can help us  Avoids intermediate temp kernel copy
       - Extends message lifetime
       -But this may increase writer starvation under heavy load and
        add unnecessary complication on priority  management and
        delay more time to free space in inode due refcount may prevent

    2) Temporary kernel buffer copy
       - Copies message into a bounded kernel buffer
       - Reduces time message remains locked
       - Improves fairness under write-heavy workloads
       - Simpler lifetime management

    My implementation adopts the temporary buffer approach
    to minimize starvation and reduce locking complexity.
    The design allows future transition if refcounting is
    deemed preferable.

    Testing:
      - 17 functional test cases
      - Multi-threaded producer/consumer scenarios
      - concurrent pop and peek
      - Edge cases: empty queue, FIFO
        invalid flags, signal interruption etc.

Use Case:

1) Observability in distributed systems
 e.g Monitoring tools can inspect queue contents without interfering with
    normal processing

2) Check pointing system like CRIU can have a look into queue without consuming and can store messages

3) Resource-aware processing

4) Selective Consumption for Specialized Workers

change since v1:
 - minor update, header guard was removed from include/uapi/linux/mqueue.h 
 - v1 Link: https://lore.kernel.org/all/20260315040827.156558-1-academic1mathura@xxxxxxxxx/T/#t

note for 

  Thanks for reviewing.


 Documentation/userspace-api/index.rst         |   1 +
 Documentation/userspace-api/ipc.rst           | 222 +++++
 arch/alpha/kernel/syscalls/syscall.tbl        |   1 +
 arch/arm/tools/syscall.tbl                    |   1 +
 arch/arm64/tools/syscall_32.tbl               |   1 +
 arch/m68k/kernel/syscalls/syscall.tbl         |   1 +
 arch/microblaze/kernel/syscalls/syscall.tbl   |   1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl     |   1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl     |   1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl     |   1 +
 arch/parisc/kernel/syscalls/syscall.tbl       |   1 +
 arch/powerpc/kernel/syscalls/syscall.tbl      |   1 +
 arch/s390/kernel/syscalls/syscall.tbl         |   1 +
 arch/sh/kernel/syscalls/syscall.tbl           |   1 +
 arch/sparc/kernel/syscalls/syscall.tbl        |   1 +
 arch/x86/entry/syscalls/syscall_32.tbl        |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl        |   1 +
 arch/xtensa/kernel/syscalls/syscall.tbl       |   1 +
 include/linux/compat.h                        |   6 +-
 include/linux/syscalls.h                      |   6 +
 include/uapi/asm-generic/unistd.h             |   7 +-
 include/uapi/linux/mqueue.h                   |  14 +-
 ipc/mqueue.c                                  | 186 ++++-
 ipc/msg.c                                     |   2 +-
 ipc/msgutil.c                                 |  48 +-
 ipc/util.h                                    |   3 +-
 kernel/sys_ni.c                               |   1 +
 scripts/syscall.tbl                           |   1 +
 .../arch/alpha/entry/syscalls/syscall.tbl     |   1 +
 .../perf/arch/arm/entry/syscalls/syscall.tbl  |   1 +
 .../arch/arm64/entry/syscalls/syscall_32.tbl  |   1 +
 .../arch/mips/entry/syscalls/syscall_n64.tbl  |   1 +
 .../arch/parisc/entry/syscalls/syscall.tbl    |   1 +
 .../arch/powerpc/entry/syscalls/syscall.tbl   |   1 +
 .../perf/arch/s390/entry/syscalls/syscall.tbl |   1 +
 tools/perf/arch/sh/entry/syscalls/syscall.tbl |   1 +
 .../arch/sparc/entry/syscalls/syscall.tbl     |   1 +
 .../arch/x86/entry/syscalls/syscall_32.tbl    |   1 +
 .../arch/x86/entry/syscalls/syscall_64.tbl    |   1 +
 .../arch/xtensa/entry/syscalls/syscall.tbl    |   1 +
 tools/scripts/syscall.tbl                     |   1 +
 tools/testing/selftests/ipc/.gitignore        |   1 +
 tools/testing/selftests/ipc/Makefile          |   9 +-
 tools/testing/selftests/ipc/mq_peek.c         | 785 ++++++++++++++++++
 44 files changed, 1278 insertions(+), 43 deletions(-)
 create mode 100644 Documentation/userspace-api/ipc.rst
 create mode 100644 tools/testing/selftests/ipc/mq_peek.c

-- 
2.43.0