Keywords: perfbook_html

## 11.1.2 Implementation

Figure: RCU and Per-Thread Statistical Counters

Lines 1-4 of Figure show the countarray structure, which contains a ->total field for the count from previously exited threads, and a counterp[] array of pointers to the per-thread counter for each currently running thread. This structure allows a given execution of read_count() to see a total that is consistent with the indicated set of running threads.

Lines 6-8 contain the definition of the per-thread counter variable, the global pointer countarrayp referencing the current countarray structure, and the final_mutex spinlock.

Lines 10-13 show inc_count(), which is unchanged from Figure .

Lines 15-29 show read_count(), which has changed significantly. Lines 21 and 27 substitute rcu_read_lock() and rcu_read_unlock() for acquisition and release of final_mutex. Line 22 uses rcu_dereference() to snapshot the current countarray structure into local variable cap. Proper use of RCU will guarantee that this countarray structure will remain with us through at least the end of the current RCU read-side critical section at line 27. Line 23 initializes sum to cap->total, which is the sum of the counts of threads that have previously exited. Lines 24-26 add up the per-thread counters corresponding to currently running threads, and, finally, line 28 returns the sum.

The initial value for countarrayp is provided by count_init() on lines 31-39. This function runs before the first thread is created, and its job is to allocate and zero the initial structure, and then assign it to countarrayp.

Lines 41-48 show the count_register_thread() function, which is invoked by each newly created thread. Line 43 picks up the current thread's index, line 45 acquires final_mutex, line 46 installs a pointer to this thread's counter, and line 47 releases final_mutex.

Quick Quiz 11.3: Hey!!! Line 45 of Figure modifies a value in a pre-existing countarray structure! Didn't you say that this structure, once made available to read_count(), remained constant??? End Quick Quiz

Lines 50-70 shows count_unregister_thread(), which is invoked by each thread just before it exits. Lines 56-60 allocate a new countarray structure, line 61 acquires final_mutex and line 67 releases it. Line 62 copies the contents of the current countarray into the newly allocated version, line 63 adds the exiting thread's counter to new structure's total, and line 64 NULLs the exiting thread's counterp[] array element. Line 65 then retains a pointer to the current (soon to be old) countarray structure, and line 66 uses rcu_assign_pointer() to install the new version of the countarray structure. Line 68 waits for a grace period to elapse, so that any threads that might be concurrently executing in read_count, and thus might have references to the old countarray structure, will be allowed to exit their RCU read-side critical sections, thus dropping any such references. Line 69 can then safely free the old countarray structure.