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<section id="membarrier-system-call">
<h1>membarrier() System Call<a class="headerlink" href="#membarrier-system-call" title="Link to this heading">¶</a></h1>
<section id="membarrier-cmd-private-global-expedited-architecture-requirements">
<h2>MEMBARRIER_CMD_{PRIVATE,GLOBAL}_EXPEDITED - Architecture requirements<a class="headerlink" href="#membarrier-cmd-private-global-expedited-architecture-requirements" title="Link to this heading">¶</a></h2>
<section id="memory-barriers-before-updating-rq-curr">
<h3>Memory barriers before updating rq->curr<a class="headerlink" href="#memory-barriers-before-updating-rq-curr" title="Link to this heading">¶</a></h3>
<p>The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED
require each architecture to have a full memory barrier after coming from
user-space, before updating rq->curr. This barrier is implied by the sequence
<code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">rq_lock()</span></code>; <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">smp_mb__after_spinlock()</span></code> in <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">__schedule()</span></code>. The barrier matches a full
barrier in the proximity of the membarrier system call exit, cf.
membarrier_{private,global}<code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">_expedited()</span></code>.</p>
</section>
<section id="memory-barriers-after-updating-rq-curr">
<h3>Memory barriers after updating rq->curr<a class="headerlink" href="#memory-barriers-after-updating-rq-curr" title="Link to this heading">¶</a></h3>
<p>The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED
require each architecture to have a full memory barrier after updating rq->curr,
before returning to user-space. The schemes providing this barrier on the various
architectures are as follows.</p>
<blockquote>
<div><ul class="simple">
<li><p>alpha, arc, arm, hexagon, mips rely on the full barrier implied by
<code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">spin_unlock()</span></code> in <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">finish_lock_switch()</span></code>.</p></li>
<li><p>arm64 relies on the full barrier implied by <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">switch_to()</span></code>.</p></li>
<li><p>powerpc, riscv, s390, sparc, x86 rely on the full barrier implied by
<code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">switch_mm()</span></code>, if mm is not NULL; they rely on the full barrier implied
by <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">mmdrop()</span></code>, otherwise. On powerpc and riscv, <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">switch_mm()</span></code> relies on
<code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">membarrier_arch_switch_mm()</span></code>.</p></li>
</ul>
</div></blockquote>
<p>The barrier matches a full barrier in the proximity of the membarrier system call
entry, cf. membarrier_{private,global}<code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">_expedited()</span></code>.</p>
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