Documentation/staging/static-keys.rst

Source file repositories/reference/linux-study-clean/Documentation/staging/static-keys.rst

File Facts

System: Linux kernel
Corpus path: Documentation/staging/static-keys.rst
Extension: .rst
Size: 13120 bytes
Lines: 329
Domain: Support Tooling And Documentation
Bucket: Documentation
Inferred role: Support Tooling And Documentation: syscall or user/kernel boundary
Status: core implementation candidate

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
Defines or participates in a user/kernel boundary; inspect argument validation, copy_from_user/copy_to_user, credentials, and dispatch target.
Uses kernel synchronization; read lock ordering, sleepability, and interrupt context assumptions before translating.
Defines or uses C structs; map object ownership, embedded links, reference counts, and lock ownership.

Dependency Surface

No C-style include directives detected by the generator.

Detected Declarations

syscall getppid
function SYSCALL_DEFINE0

Annotated Snippet

SYSCALL_DEFINE0(getppid)
  {
        int pid;

  +     if (static_branch_unlikely(&key))
  +             printk("I am the true branch\n");

        rcu_read_lock();
        pid = task_tgid_vnr(rcu_dereference(current->real_parent));
        rcu_read_unlock();

        return pid;
  }

The resulting instructions with jump labels generated by GCC is::

  ffffffff81044290 <sys_getppid>:
  ffffffff81044290:       55                      push   %rbp
  ffffffff81044291:       48 89 e5                mov    %rsp,%rbp
  ffffffff81044294:       e9 00 00 00 00          jmpq   ffffffff81044299 <sys_getppid+0x9>
  ffffffff81044299:       65 48 8b 04 25 c0 b6    mov    %gs:0xb6c0,%rax
  ffffffff810442a0:       00 00
  ffffffff810442a2:       48 8b 80 80 02 00 00    mov    0x280(%rax),%rax
  ffffffff810442a9:       48 8b 80 b0 02 00 00    mov    0x2b0(%rax),%rax
  ffffffff810442b0:       48 8b b8 e8 02 00 00    mov    0x2e8(%rax),%rdi
  ffffffff810442b7:       e8 f4 d9 00 00          callq  ffffffff81051cb0 <pid_vnr>
  ffffffff810442bc:       5d                      pop    %rbp
  ffffffff810442bd:       48 98                   cltq
  ffffffff810442bf:       c3                      retq
  ffffffff810442c0:       48 c7 c7 e3 54 98 81    mov    $0xffffffff819854e3,%rdi
  ffffffff810442c7:       31 c0                   xor    %eax,%eax
  ffffffff810442c9:       e8 71 13 6d 00          callq  ffffffff8171563f <printk>
  ffffffff810442ce:       eb c9                   jmp    ffffffff81044299 <sys_getppid+0x9>

Without the jump label optimization it looks like::

  ffffffff810441f0 <sys_getppid>:
  ffffffff810441f0:       8b 05 8a 52 d8 00       mov    0xd8528a(%rip),%eax        # ffffffff81dc9480 <key>
  ffffffff810441f6:       55                      push   %rbp
  ffffffff810441f7:       48 89 e5                mov    %rsp,%rbp
  ffffffff810441fa:       85 c0                   test   %eax,%eax
  ffffffff810441fc:       75 27                   jne    ffffffff81044225 <sys_getppid+0x35>
  ffffffff810441fe:       65 48 8b 04 25 c0 b6    mov    %gs:0xb6c0,%rax
  ffffffff81044205:       00 00
  ffffffff81044207:       48 8b 80 80 02 00 00    mov    0x280(%rax),%rax
  ffffffff8104420e:       48 8b 80 b0 02 00 00    mov    0x2b0(%rax),%rax
  ffffffff81044215:       48 8b b8 e8 02 00 00    mov    0x2e8(%rax),%rdi
  ffffffff8104421c:       e8 2f da 00 00          callq  ffffffff81051c50 <pid_vnr>
  ffffffff81044221:       5d                      pop    %rbp
  ffffffff81044222:       48 98                   cltq
  ffffffff81044224:       c3                      retq
  ffffffff81044225:       48 c7 c7 13 53 98 81    mov    $0xffffffff81985313,%rdi
  ffffffff8104422c:       31 c0                   xor    %eax,%eax
  ffffffff8104422e:       e8 60 0f 6d 00          callq  ffffffff81715193 <printk>
  ffffffff81044233:       eb c9                   jmp    ffffffff810441fe <sys_getppid+0xe>
  ffffffff81044235:       66 66 2e 0f 1f 84 00    data32 nopw %cs:0x0(%rax,%rax,1)
  ffffffff8104423c:       00 00 00 00

Thus, the disable jump label case adds a 'mov', 'test' and 'jne' instruction
vs. the jump label case just has a 'no-op' or 'jmp 0'. (The jmp 0, is patched
to a 5 byte atomic no-op instruction at boot-time.) Thus, the disabled jump
label case adds::

  6 (mov) + 2 (test) + 2 (jne) = 10 - 5 (5 byte jump 0) = 5 addition bytes.

If we then include the padding bytes, the jump label code saves, 16 total bytes
of instruction memory for this small function. In this case the non-jump label
function is 80 bytes long. Thus, we have saved 20% of the instruction
footprint. We can in fact improve this even further, since the 5-byte no-op
really can be a 2-byte no-op since we can reach the branch with a 2-byte jmp.

Annotation

Detected declarations: `syscall getppid`, `function SYSCALL_DEFINE0`.
Atlas domain: Support Tooling And Documentation / Documentation.
Implementation status: core implementation candidate.
Synchronization appears in or near this file; preserve lock ordering, sleepability, and interrupt-context constraints.

Implementation Notes

This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
Driver-family pages are intentionally pattern-oriented unless they are part of the selected PCIe/NVMe representative device path.