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Diffstat (limited to 'Documentation/RCU')
| -rw-r--r-- | Documentation/RCU/Design/Requirements/Requirements.html | 71 |
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diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 467251f7fef6..bdbc84f1b949 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2129,8 +2129,6 @@ 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>. @@ -2523,75 +2521,6 @@ 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->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->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 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->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> |