1 #include <linux/atomic.h> 2 #include <linux/rwsem.h> 3 #include <linux/percpu.h> 4 #include <linux/wait.h> 5 #include <linux/lockdep.h> 6 #include <linux/percpu-rwsem.h> 7 #include <linux/rcupdate.h> 8 #include <linux/sched.h> 9 #include <linux/errno.h> 10 11 int __percpu_init_rwsem(struct percpu_rw_semaphore *brw, 12 const char *name, struct lock_class_key *rwsem_key) 13 { 14 brw->fast_read_ctr = alloc_percpu(int); 15 if (unlikely(!brw->fast_read_ctr)) 16 return -ENOMEM; 17 18 /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ 19 __init_rwsem(&brw->rw_sem, name, rwsem_key); 20 atomic_set(&brw->write_ctr, 0); 21 atomic_set(&brw->slow_read_ctr, 0); 22 init_waitqueue_head(&brw->write_waitq); 23 return 0; 24 } 25 26 void percpu_free_rwsem(struct percpu_rw_semaphore *brw) 27 { 28 free_percpu(brw->fast_read_ctr); 29 brw->fast_read_ctr = NULL; /* catch use after free bugs */ 30 } 31 32 /* 33 * This is the fast-path for down_read/up_read, it only needs to ensure 34 * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the 35 * fast per-cpu counter. The writer uses synchronize_sched_expedited() to 36 * serialize with the preempt-disabled section below. 37 * 38 * The nontrivial part is that we should guarantee acquire/release semantics 39 * in case when 40 * 41 * R_W: down_write() comes after up_read(), the writer should see all 42 * changes done by the reader 43 * or 44 * W_R: down_read() comes after up_write(), the reader should see all 45 * changes done by the writer 46 * 47 * If this helper fails the callers rely on the normal rw_semaphore and 48 * atomic_dec_and_test(), so in this case we have the necessary barriers. 49 * 50 * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or 51 * __this_cpu_add() below can be reordered with any LOAD/STORE done by the 52 * reader inside the critical section. See the comments in down_write and 53 * up_write below. 54 */ 55 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) 56 { 57 bool success = false; 58 59 preempt_disable(); 60 if (likely(!atomic_read(&brw->write_ctr))) { 61 __this_cpu_add(*brw->fast_read_ctr, val); 62 success = true; 63 } 64 preempt_enable(); 65 66 return success; 67 } 68 69 /* 70 * Like the normal down_read() this is not recursive, the writer can 71 * come after the first percpu_down_read() and create the deadlock. 72 * 73 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep, 74 * percpu_up_read() does rwsem_release(). This pairs with the usage 75 * of ->rw_sem in percpu_down/up_write(). 76 */ 77 void percpu_down_read(struct percpu_rw_semaphore *brw) 78 { 79 might_sleep(); 80 if (likely(update_fast_ctr(brw, +1))) { 81 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_); 82 return; 83 } 84 85 down_read(&brw->rw_sem); 86 atomic_inc(&brw->slow_read_ctr); 87 /* avoid up_read()->rwsem_release() */ 88 __up_read(&brw->rw_sem); 89 } 90 91 int percpu_down_read_trylock(struct percpu_rw_semaphore *brw) 92 { 93 if (unlikely(!update_fast_ctr(brw, +1))) { 94 if (!__down_read_trylock(&brw->rw_sem)) 95 return 0; 96 atomic_inc(&brw->slow_read_ctr); 97 __up_read(&brw->rw_sem); 98 } 99 100 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_); 101 return 1; 102 } 103 104 void percpu_up_read(struct percpu_rw_semaphore *brw) 105 { 106 rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_); 107 108 if (likely(update_fast_ctr(brw, -1))) 109 return; 110 111 /* false-positive is possible but harmless */ 112 if (atomic_dec_and_test(&brw->slow_read_ctr)) 113 wake_up_all(&brw->write_waitq); 114 } 115 116 static int clear_fast_ctr(struct percpu_rw_semaphore *brw) 117 { 118 unsigned int sum = 0; 119 int cpu; 120 121 for_each_possible_cpu(cpu) { 122 sum += per_cpu(*brw->fast_read_ctr, cpu); 123 per_cpu(*brw->fast_read_ctr, cpu) = 0; 124 } 125 126 return sum; 127 } 128 129 /* 130 * A writer increments ->write_ctr to force the readers to switch to the 131 * slow mode, note the atomic_read() check in update_fast_ctr(). 132 * 133 * After that the readers can only inc/dec the slow ->slow_read_ctr counter, 134 * ->fast_read_ctr is stable. Once the writer moves its sum into the slow 135 * counter it represents the number of active readers. 136 * 137 * Finally the writer takes ->rw_sem for writing and blocks the new readers, 138 * then waits until the slow counter becomes zero. 139 */ 140 void percpu_down_write(struct percpu_rw_semaphore *brw) 141 { 142 /* tell update_fast_ctr() there is a pending writer */ 143 atomic_inc(&brw->write_ctr); 144 /* 145 * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read 146 * so that update_fast_ctr() can't succeed. 147 * 148 * 2. Ensures we see the result of every previous this_cpu_add() in 149 * update_fast_ctr(). 150 * 151 * 3. Ensures that if any reader has exited its critical section via 152 * fast-path, it executes a full memory barrier before we return. 153 * See R_W case in the comment above update_fast_ctr(). 154 */ 155 synchronize_sched_expedited(); 156 157 /* exclude other writers, and block the new readers completely */ 158 down_write(&brw->rw_sem); 159 160 /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */ 161 atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); 162 163 /* wait for all readers to complete their percpu_up_read() */ 164 wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr)); 165 } 166 167 void percpu_up_write(struct percpu_rw_semaphore *brw) 168 { 169 /* release the lock, but the readers can't use the fast-path */ 170 up_write(&brw->rw_sem); 171 /* 172 * Insert the barrier before the next fast-path in down_read, 173 * see W_R case in the comment above update_fast_ctr(). 174 */ 175 synchronize_sched_expedited(); 176 /* the last writer unblocks update_fast_ctr() */ 177 atomic_dec(&brw->write_ctr); 178 } 179