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<p class="MsoNormal">Hi all,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’ve been trying to understand the usage of RCU and am struggling a bit with one issue.<o:p></o:p></p>
<p class="MsoNormal">Let’s say one intends to use “classic RCU” as described in whatisRCU.txt where readers use<o:p></o:p></p>
<p class="MsoNormal">sequences of rcu_read_lock(), rcu_dereference(), rcu_read_unlock) and updaters use<o:p></o:p></p>
<p class="MsoNormal">rcu_assign_pointer()/synchronize_rcu().<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If one uses rcu_dereference() to access an RCU protect pointer twice within the same rcu<o:p></o:p></p>
<p class="MsoNormal">read locked critical section, is it guaranteed that the pointers returned by rcu_dereference()<o:p></o:p></p>
<p class="MsoNormal">will point to the same block of memory?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">For instance, in the following code (trying make this as simple as possible even if it does nothing useful):<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">char *global;<o:p></o:p></p>
<p class="MsoNormal">char x[1];<o:p></o:p></p>
<p class="MsoNormal">char y[1];<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">void reader(void)<o:p></o:p></p>
<p class="MsoNormal">{<o:p></o:p></p>
<p class="MsoNormal"> char *a;<o:p></o:p></p>
<p class="MsoNormal"> char *b;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> rcu_read_lock();<o:p></o:p></p>
<p class="MsoNormal"> a = rcu_dereference(global);<o:p></o:p></p>
<p class="MsoNormal"> b = rcu_dereference(global);<o:p></o:p></p>
<p class="MsoNormal"> rcu_read_unlock();<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">void updater(void)<o:p></o:p></p>
<p class="MsoNormal">{<o:p></o:p></p>
<p class="MsoNormal"> rcu_assign_pointer(global, x);<o:p></o:p></p>
<p class="MsoNormal"> rcu_assign_pointer(global, y);<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Following the second call to rcu_deference() in reader(), is it guaranteed that a and b will be<o:p></o:p></p>
<p class="MsoNormal">equal (either both equal to x or both equal y)? Or is it possible what a will equal x and b will equal y?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">whatisRCU.txt seems to imply that though this is bad practice because its wasteful, both calls<o:p></o:p></p>
<p class="MsoNormal">to rcu_dereference() will return the same pointer:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">“228 rcu_dereference()<o:p></o:p></p>
<p class="MsoNormal">…<o:p></o:p></p>
<p class="MsoNormal">244 Common coding practice uses rcu_dereference() to copy an<o:p></o:p></p>
<p class="MsoNormal">245 RCU-protected pointer to a local variable, then dereferences<o:p></o:p></p>
<p class="MsoNormal">246 this local variable, for example as follows:<o:p></o:p></p>
<p class="MsoNormal">247 <o:p></o:p></p>
<p class="MsoNormal">248 p = rcu_dereference(head.next);<o:p></o:p></p>
<p class="MsoNormal">249 return p->data;<o:p></o:p></p>
<p class="MsoNormal">250 <o:p></o:p></p>
<p class="MsoNormal">251 However, in this case, one could just as easily combine these<o:p></o:p></p>
<p class="MsoNormal">252 into one statement:<o:p></o:p></p>
<p class="MsoNormal">253 <o:p></o:p></p>
<p class="MsoNormal">254 return rcu_dereference(head.next)->data;<o:p></o:p></p>
<p class="MsoNormal">255 <o:p></o:p></p>
<p class="MsoNormal">256 If you are going to be fetching multiple fields from the<o:p></o:p></p>
<p class="MsoNormal">257 RCU-protected structure, using the local variable is of<o:p></o:p></p>
<p class="MsoNormal">258 course preferred. Repeated rcu_dereference() calls look<o:p></o:p></p>
<p class="MsoNormal">259 ugly and incur unnecessary overhead on Alpha CPUs.”<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">From lines 256 to 259 I conclude that reader()’s code is considered ugly and wasteful,<o:p></o:p></p>
<p class="MsoNormal">but a will always equal b.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But looking at how rcu_dereference() and rcu_assign_pointer() are implemented, I’m having a<o:p></o:p></p>
<p class="MsoNormal">hard time seeing how reader() would always see a and b equal.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks in advance,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Jeff Haran<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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