CS510 Concurrent Systems “What is RCU, Fundamentally?” By Paul McKenney and Jonathan Walpole Daniel Mansour (Slides adapted from Professor Walpole’s)

2 Review of Concurrency  The Story So Far o Critical Sections Shared global objects Blocking/Non-Blocking o Reader/Writers Reads are “safer” Writes are Fewer o Hardware/Compiler Optimizations Operations are not always in order o Memory Reclamation Deleted Objects can still be read

3 Read-Copy Update (RCU)  Read/Writes o Do not allow reads while writes are occurring o RCU solves this by allowing multiple versions  Insertion o Publish-Subscribe Mechanism  Deletion o Wait for Readers to complete o Non-Blocking

4 Publish-Subscribe Mechanism 1 struct foo { 2 int a; 3 int b; 4 int c; 5 }; 6 struct foo *gp = NULL; 7 8 /*... */ 9 10 p = kmalloc(sizeof(*p), GFP_KERNEL); 11 p->a = 1; 12 p->b = 2; 13 p->c = 3; 14 gp = p;

5 Publish-Subscribe Mechanism 1 struct foo { 2 int a; 3 int b; 4 int c; 5 }; 6 struct foo *gp = NULL; 7 8 /*... */ 9 10 p = kmalloc(sizeof(*p), GFP_KERNEL); 11 gp = p; 12 p->a = 1; 13 p->b = 2; 14 p->c = 3; Reordered, concurrent readers would get uninitialized values

6 Publish-Subscribe Mechanism  Publish the Object o rcu_assign_pointer 1 p->a = 1; 2 p->b = 2; 3 p->c = 3; 4 rcu_assign_pointer(gp, p);

7 Publish-Subscribe Mechanism  Readers o Ordering can still be an issue o p->a can be read before p in some compiler optimizations 1 p = gp; 2 if (p != NULL) { 3 do_something_with(p->a, p->b, p-c); 4 }

8 Publish-Subscribe Mechanism  Subscribe to the object o rcu_dereference o Implements any necessary memory barriers 1 rcu_read_lock() 2 p = rcu_dereference(gp) 3 if (p != NULL { 4 do_something_with(p->a, p->b, p->c); 5 } 6 rcu_read_unlock();

9 Implementation  Doubly Linked Lists o list_head o Head links to tail with prev and vice versa

10 Implementation - list_head  Publishing 1 struct foo { 2 struct list_head list; 3 int a; 4 int b; 5 int c; 6 }; 7 LIST_HEAD(head); 8 9 /*... */ p = kmalloc(sizeof(*p), GFP_KERNEL); 12 p->a = 1; 13 p->b = 2; 14 p->c = 3; 15 list_add_rcu(&p->list, &head); Still needs to be protected

11 Implementation - list_head  Subscribing 1 rcu_read_lock(); 2 list_for_each_entry_rcu(p, head, list) { 3 do_something_with(p->a, p->b, p->c); 4 } 5 rcu_read_unlock();

12 Implementation - hlist_head/hlist_node  Doubly Linked Lists o hlist_head/hlist_node o Linear list

13 Implementation - hlist_head/hlist_node  Publishing 1 struct foo { 2 struct hlist_node *list; 3 int a; 4 int b; 5 int c; 6 }; 7 HLIST_HEAD(head); 8 9 /*... */ p = kmalloc(sizeof(*p), GFP_KERNEL); 12 p->a = 1; 13 p->b = 2; 14 p->c = 3; 15 hlist_add_head_rcu(&p->list, &head);

14 Implementation - list_head  Subscribing 1 rcu_read_lock(); 2 hlist_for_each_entry_rcu(p, q, head, list) { 3 do_something_with(p->a, p->b, p->c); 4 } 5 rcu_read_unlock(); Iteration requires checking for null Instead of p = head

15 Reclaiming Memory  Waiting o rcu_read_lock and rcu_read_unlock Must not block or sleep

Reclaiming Memory  Basic Process 1. Alter the data (replace/delete an element) 2. Wait for all pre-existing RCU reads to completely finish synchronize_rcu New RCU reads have no way to access removed element 3. Reclaim/Recycle memory 16

Reclaiming Memory  Example 17 1 struct foo { 2 struct list_head list; 3 int a; 4 int b; 5 int c; 6 }; 7 LIST_HEAD(head); 8 9 /*... */ p = search(head, key); 12 if (p == NULL) { 13 /* Take appropriate action, unlock, and return. */ 14 } 15 q = kmalloc(sizeof(*p), GFP_KERNEL); 16 *q = *p; 17 q->b = 2; 18 q->c = 3; 19 list_replace_rcu(&p->list, &q->list); 20 synchronize_rcu(); 21 kfree(p);

Reclaiming Memory  synchronize_rcu o rcu_read_lock and rcu_read_unlock do nothing on non- CONFIG-PREEMPT kernels o Recall that code in rcu_read blocks can not sleep or block, preventing context switches o When a context switch occurs, we can deduce that the read has completed o synchronize_rcu waits for each cpu to context switch conceptually: 18 1 for_each_online_cpu(cpu) 2 run_on(cpu);

Maintaining Multiple Versions - Deletion  During deletion  Initial State 19 1 p = search(head, key); 2 if (p != NULL) { 3 list_del_rcu(&p->list); 4 synchronize_rcu(); 5 kfree(p); 6 }

Maintaining Multiple Versions - Deletion  list_del_rcu(&p->list)  p has been deleted, but other readers can still be accessing it 20

Maintaining Multiple Versions - Deletion  synchronize_rcu()  kfree(p) 21

Maintaining Multiple Versions - Replacement  During replacement  Initial State 22 1 q = kmalloc(sizeof(*p), GFP_KERNEL); 2 *q = *p; 3 q->b = 2; 4 q->c = 3; 5 list_replace_rcu(&p->list, &q->list); 6 synchronize_rcu(); 7 kfree(p);

Maintaining Multiple Versions - Replacement  kmalloc() 23

Maintaining Multiple Versions - Replacement  *q = *p 24

Maintaining Multiple Versions - Replacement  q->b = 2; q->c = 3 25

Maintaining Multiple Versions - Replacement  list_replace_rcu  There are now two versions of the list, but both are well formed. 26

Maintaining Multiple Versions - Replacement  synchronize_rcu  kfree() 27

Issues  Very much a read/write implementation, not ideally suited for rapidly updated objects  Responsibility on the programmer to make sure that no sleeping/blocking occurs in rcu_read_locks  RCU updaters can still slow readers via cache invalidation 28