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-rw-r--r--Documentation/RCU/Design/Requirements/Requirements.html73
-rw-r--r--Documentation/RCU/stallwarn.txt6
2 files changed, 78 insertions, 1 deletions
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 5a9238a2883c..467251f7fef6 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2129,6 +2129,8 @@ Some of the relevant points of interest are as follows:
<li> <a href="#Hotplug CPU">Hotplug CPU</a>.
<li> <a href="#Scheduler and RCU">Scheduler and RCU</a>.
<li> <a href="#Tracing and RCU">Tracing and RCU</a>.
+<li> <a href="#Accesses to User Memory and RCU">
+Accesses to User Memory and RCU</a>.
<li> <a href="#Energy Efficiency">Energy Efficiency</a>.
<li> <a href="#Scheduling-Clock Interrupts and RCU">
Scheduling-Clock Interrupts and RCU</a>.
@@ -2512,7 +2514,7 @@ disabled across the entire RCU read-side critical section.
<p>
It is possible to use tracing on RCU code, but tracing itself
uses RCU.
-For this reason, <tt>rcu_dereference_raw_notrace()</tt>
+For this reason, <tt>rcu_dereference_raw_check()</tt>
is provided for use by tracing, which avoids the destructive
recursion that could otherwise ensue.
This API is also used by virtualization in some architectures,
@@ -2521,6 +2523,75 @@ cannot be used.
The tracing folks both located the requirement and provided the
needed fix, so this surprise requirement was relatively painless.
+<h3><a name="Accesses to User Memory and RCU">
+Accesses to User Memory and RCU</a></h3>
+
+<p>
+The kernel needs to access user-space memory, for example, to access
+data referenced by system-call parameters.
+The <tt>get_user()</tt> macro does this job.
+
+<p>
+However, user-space memory might well be paged out, which means
+that <tt>get_user()</tt> might well page-fault and thus block while
+waiting for the resulting I/O to complete.
+It would be a very bad thing for the compiler to reorder
+a <tt>get_user()</tt> invocation into an RCU read-side critical
+section.
+For example, suppose that the source code looked like this:
+
+<blockquote>
+<pre>
+ 1 rcu_read_lock();
+ 2 p = rcu_dereference(gp);
+ 3 v = p-&gt;value;
+ 4 rcu_read_unlock();
+ 5 get_user(user_v, user_p);
+ 6 do_something_with(v, user_v);
+</pre>
+</blockquote>
+
+<p>
+The compiler must not be permitted to transform this source code into
+the following:
+
+<blockquote>
+<pre>
+ 1 rcu_read_lock();
+ 2 p = rcu_dereference(gp);
+ 3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!!
+ 4 v = p-&gt;value;
+ 5 rcu_read_unlock();
+ 6 do_something_with(v, user_v);
+</pre>
+</blockquote>
+
+<p>
+If the compiler did make this transformation in a
+<tt>CONFIG_PREEMPT=n</tt> kernel build, and if <tt>get_user()</tt> did
+page fault, the result would be a quiescent state in the middle
+of an RCU read-side critical section.
+This misplaced quiescent state could result in line&nbsp;4 being
+a use-after-free access, which could be bad for your kernel's
+actuarial statistics.
+Similar examples can be constructed with the call to <tt>get_user()</tt>
+preceding the <tt>rcu_read_lock()</tt>.
+
+<p>
+Unfortunately, <tt>get_user()</tt> doesn't have any particular
+ordering properties, and in some architectures the underlying <tt>asm</tt>
+isn't even marked <tt>volatile</tt>.
+And even if it was marked <tt>volatile</tt>, the above access to
+<tt>p-&gt;value</tt> is not volatile, so the compiler would not have any
+reason to keep those two accesses in order.
+
+<p>
+Therefore, the Linux-kernel definitions of <tt>rcu_read_lock()</tt>
+and <tt>rcu_read_unlock()</tt> must act as compiler barriers,
+at least for outermost instances of <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> within a nested set of RCU read-side critical
+sections.
+
<h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
<p>
diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 13e88fc00f01..f48f4621ccbc 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -57,6 +57,12 @@ o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
CONFIG_PREEMPT_RCU case, you might see stall-warning
messages.
+ You can use the rcutree.kthread_prio kernel boot parameter to
+ increase the scheduling priority of RCU's kthreads, which can
+ help avoid this problem. However, please note that doing this
+ can increase your system's context-switch rate and thus degrade
+ performance.
+
o A periodic interrupt whose handler takes longer than the time
interval between successive pairs of interrupts. This can
prevent RCU's kthreads and softirq handlers from running.