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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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687 lines
42 KiB
HTML
687 lines
42 KiB
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<meta name="description" content="The perfect emulation setup to study and develop the <<linux-kernel>> v5.9.2, kernel modules, <<qemu-buildroot-setup,QEMU>>, <<gem5-buildroot-setup,gem5>> and x86_64, ARMv7 and ARMv8 <<userland-assembly,userland>> and <<baremetal-setup,baremetal>> assembly, <<c,ANSI C>>, <<cpp,C++>> and <<posix,POSIX>>. <<gdb>> and <<kgdb>> just work. Powered by <<about-the-qemu-buildroot-setup,Buildroot>> and <<about-the-baremetal-setup,crosstool-NG>>. Highly automated. Thoroughly documented. Automated <<test-this-repo,tests>>. "Tested" in an Ubuntu 20.04 host.">
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<title>Linux Kernel Module Cheat</title>
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</head>
|
||
<body id="atomic-cpp" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect4">
|
||
<h5 id="atomic-cpp"><a class="anchor" href="#atomic-cpp"></a><a class="link" href="#atomic-cpp">27.3.4.1. atomic.cpp</a></h5>
|
||
<div class="paragraph">
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/">userland/cpp/atomic/</a></p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>C version at: <a href="atomic-c.html">atomic.c</a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>In this set of examples, we exemplify various synchronization mechanisms, including assembly specific ones, by using the convenience of C++ multithreading:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/main.hpp">userland/cpp/atomic/main.hpp</a>: contains all the code which is then specialized in separated <code>.cpp</code> files with macros</p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/aarch64_add.cpp">userland/cpp/atomic/aarch64_add.cpp</a>: non synchronized aarch64 inline assembly</p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/aarch64_ldaxr_stlxr.cpp">userland/cpp/atomic/aarch64_ldaxr_stlxr.cpp</a>: see: <a href="arm-ldxr-and-stxr-instructions.html">ARM LDXR and STXR instructions</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/aarch64_ldadd.cpp">userland/cpp/atomic/aarch64_ldadd.cpp</a>: synchronized aarch64 inline assembly with the <a href="arm-lse.html">ARM Large System Extensions (LSE)</a> LDADD instruction</p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/fail.cpp">userland/cpp/atomic/fail.cpp</a>: non synchronized C operator ``</p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/mutex.cpp">userland/cpp/atomic/mutex.cpp</a>: synchronized <code>std::mutex</code>. <code>std;</code></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/std_atomic.cpp">userland/cpp/atomic/std_atomic.cpp</a>: synchronized <code>std::atomic_ulong</code></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/x86_64_inc.cpp">userland/cpp/atomic/x86_64_inc.cpp</a>: non synchronized x86_64 inline assembly</p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/x86_64_lock_inc.cpp">userland/cpp/atomic/x86_64_lock_inc.cpp</a>: synchronized x86_64 inline assembly with the <a href="x86-lock-prefix.html">x86 LOCK prefix</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>All examples do exactly the same thing: span N threads and loop M times in each thread incrementing a global integer.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>For inputs large enough, the non-synchronized examples are extremely likely to produce "wrong" results, for example on <a href="p51.html">2017 Lenovo ThinkPad P51</a> Ubuntu 19.10 <a href="userland-setup-getting-started-natively.html">native</a> with 2 threads and 10000 loops:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./fail.out 2 10000</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>we could get an output such as:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>expect 20000
|
||
global 12676</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The actual value is much smaller, because the threads have often overwritten one another with older values.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>With <a href="optimization-level-of-a-build.html"><code>--optimization-level 3</code></a>, the result almost always equals that of a single thread, e.g.:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./build --optimization-level 3 --force-rebuild fail.cpp
|
||
./fail.out 4 1000000</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>usually gives:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>expect 40000
|
||
global 10000</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This is because now, instead of the horribly inefficient <code>-O0</code> assembly that reads <code>global</code> from memory every time, the code:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>reads <code>global</code> to a register</p>
|
||
</li>
|
||
<li>
|
||
<p>increments the register</p>
|
||
</li>
|
||
<li>
|
||
<p>at end the end, the resulting value of each thread gets written back, overwriting each other with the increment of each thread</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The <code>-O0</code> code therefore mixes things up much more because it reads and write back to memory many many times.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This can be easily seen from the disassembly with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>gdb -batch -ex "disassemble threadMain" fail.out</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which gives for <code>-O0</code>:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre> 0x0000000000402656 <+0>: endbr64
|
||
0x000000000040265a <+4>: push %rbp
|
||
0x000000000040265b <+5>: mov %rsp,%rbp
|
||
0x000000000040265e <+8>: movq $0x0,-0x8(%rbp)
|
||
0x0000000000402666 <+16>: mov 0x5c2b(%rip),%rax # 0x408298 <niters>
|
||
0x000000000040266d <+23>: cmp %rax,-0x8(%rbp)
|
||
0x0000000000402671 <+27>: jae 0x40269b <threadMain()+69>
|
||
0x0000000000402673 <+29>: mov 0x5c26(%rip),%rdx # 0x4082a0 <global>
|
||
0x000000000040267a <+36>: mov -0x8(%rbp),%rax
|
||
0x000000000040267e <+40>: mov %rax,-0x8(%rbp)
|
||
0x0000000000402682 <+44>: mov 0x5c17(%rip),%rax # 0x4082a0 <global>
|
||
0x0000000000402689 <+51>: add $0x1,%rax
|
||
0x000000000040268d <+55>: mov %rax,0x5c0c(%rip) # 0x4082a0 <global>
|
||
0x0000000000402694 <+62>: addq $0x1,-0x8(%rbp)
|
||
0x0000000000402699 <+67>: jmp 0x402666 <threadMain()+16>
|
||
0x000000000040269b <+69>: nop
|
||
0x000000000040269c <+70>: pop %rbp
|
||
0x000000000040269d <+71>: retq</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and for <code>-O3</code>:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre> 0x00000000004017f0 <+0>: endbr64
|
||
0x00000000004017f4 <+4>: mov 0x2a25(%rip),%rcx # 0x404220 <niters>
|
||
0x00000000004017fb <+11>: test %rcx,%rcx
|
||
0x00000000004017fe <+14>: je 0x401824 <threadMain()+52>
|
||
0x0000000000401800 <+16>: mov 0x2a11(%rip),%rdx # 0x404218 <global>
|
||
0x0000000000401807 <+23>: xor %eax,%eax
|
||
0x0000000000401809 <+25>: nopl 0x0(%rax)
|
||
0x0000000000401810 <+32>: add $0x1,%rax
|
||
0x0000000000401814 <+36>: add $0x1,%rdx
|
||
0x0000000000401818 <+40>: cmp %rcx,%rax
|
||
0x000000000040181b <+43>: jb 0x401810 <threadMain()+32>
|
||
0x000000000040181d <+45>: mov %rdx,0x29f4(%rip) # 0x404218 <global>
|
||
0x0000000000401824 <+52>: retq</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>We can now look into how <code>std::atomic</code> is implemented. In <code>-O3</code> the disassembly is:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre> 0x0000000000401770 <+0>: endbr64
|
||
0x0000000000401774 <+4>: cmpq $0x0,0x297c(%rip) # 0x4040f8 <niters>
|
||
0x000000000040177c <+12>: je 0x401796 <threadMain()+38>
|
||
0x000000000040177e <+14>: xor %eax,%eax
|
||
0x0000000000401780 <+16>: lock addq $0x1,0x2967(%rip) # 0x4040f0 <global>
|
||
0x0000000000401789 <+25>: add $0x1,%rax
|
||
0x000000000040178d <+29>: cmp %rax,0x2964(%rip) # 0x4040f8 <niters>
|
||
0x0000000000401794 <+36>: ja 0x401780 <threadMain()+16>
|
||
0x0000000000401796 <+38>: retq</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>so we clearly see that basically a <code>lock addq</code> is used to do an atomic read and write to memory every single time, just like in our other example <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/x86_64_lock_inc.cpp">userland/cpp/atomic/x86_64_lock_inc.cpp</a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This setup can also be used to benchmark different synchronization mechanisms. For example, <code>std::mutex</code> was about 1.5x slower with two cores than <code>std::atomic</code>, presumably because it relies on the <a href="futex-system-call.html"><code>futex</code> system call</a> as can be seen from <code>strace -f -s999 -v</code> logs, while <code>std::atomic</code> uses just userland instructions: <a href="https://www.quora.com/How-does-std-atomic-work-in-C++11/answer/Ciro-Santilli" class="bare">https://www.quora.com/How-does-std-atomic-work-in-C++11/answer/Ciro-Santilli</a> Tested in <code>-O3</code> with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>time ./std_atomic.out 4 100000000
|
||
time ./mutex.out 4 100000000</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Related examples:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>POSIX <a href="pthread-mutex.html">pthread_mutex</a></p>
|
||
</li>
|
||
<li>
|
||
<p>C11 <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/c/atomic.c">userland/c/atomic.c</a> documented at <a href="c-multithreading.html">C multithreading</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Bibliography:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://stackoverflow.com/questions/31978324/what-exactly-is-stdatomic/58904448#58904448" class="bare">https://stackoverflow.com/questions/31978324/what-exactly-is-stdatomic/58904448#58904448</a> "What exactly is std::atomic?"</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="detailed-gem5-analysis-of-how-data-races-happen.html">Detailed gem5 analysis of how data races happen</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>↑ Up: <a href="cpp-multithreading.html">C++ multithreading</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="detailed-gem5-analysis-of-how-data-races-happen.html">Detailed gem5 analysis of how data races happen</a> →</p>
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