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