1 /*
2 *
3 * Copyright 2017 gRPC authors.
4 *
5 * Licensed under the Apache License, Version 2.0 (the "License");
6 * you may not use this file except in compliance with the License.
7 * You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 *
17 */
18
19 #include <grpc/support/port_platform.h>
20
21 #include "src/core/lib/iomgr/lockfree_event.h"
22
23 #include <grpc/support/log.h>
24
25 #include "src/core/lib/debug/trace.h"
26 #include "src/core/lib/iomgr/exec_ctx.h"
27
28 extern grpc_core::DebugOnlyTraceFlag grpc_polling_trace;
29
30 /* 'state' holds the to call when the fd is readable or writable respectively.
31 It can contain one of the following values:
32 kClosureReady : The fd has an I/O event of interest but there is no
33 closure yet to execute
34
35 kClosureNotReady : The fd has no I/O event of interest
36
37 closure ptr : The closure to be executed when the fd has an I/O
38 event of interest
39
40 shutdown_error | kShutdownBit :
41 'shutdown_error' field ORed with kShutdownBit.
42 This indicates that the fd is shutdown. Since all
43 memory allocations are word-aligned, the lower two
44 bits of the shutdown_error pointer are always 0. So
45 it is safe to OR these with kShutdownBit
46
47 Valid state transitions:
48
49 <closure ptr> <-----3------ kClosureNotReady -----1-------> kClosureReady
50 | | ^ | ^ | |
51 | | | | | | |
52 | +--------------4----------+ 6 +---------2---------------+ |
53 | | |
54 | v |
55 +-----5-------> [shutdown_error | kShutdownBit] <-------7---------+
56
57 For 1, 4 : See SetReady() function
58 For 2, 3 : See NotifyOn() function
59 For 5,6,7: See SetShutdown() function */
60
61 namespace grpc_core {
62
LockfreeEvent()63 LockfreeEvent::LockfreeEvent() { InitEvent(); }
64
InitEvent()65 void LockfreeEvent::InitEvent() {
66 /* Perform an atomic store to start the state machine.
67
68 Note carefully that LockfreeEvent *MAY* be used whilst in a destroyed
69 state, while a file descriptor is on a freelist. In such a state it may
70 be SetReady'd, and so we need to perform an atomic operation here to
71 ensure no races */
72 gpr_atm_no_barrier_store(&state_, kClosureNotReady);
73 }
74
DestroyEvent()75 void LockfreeEvent::DestroyEvent() {
76 gpr_atm curr;
77 do {
78 curr = gpr_atm_no_barrier_load(&state_);
79 if (curr & kShutdownBit) {
80 GRPC_ERROR_UNREF((grpc_error*)(curr & ~kShutdownBit));
81 } else {
82 GPR_ASSERT(curr == kClosureNotReady || curr == kClosureReady);
83 }
84 /* we CAS in a shutdown, no error value here. If this event is interacted
85 with post-deletion (see the note in the constructor) we want the bit
86 pattern to prevent error retention in a deleted object */
87 } while (!gpr_atm_no_barrier_cas(&state_, curr,
88 kShutdownBit /* shutdown, no error */));
89 }
90
NotifyOn(grpc_closure * closure)91 void LockfreeEvent::NotifyOn(grpc_closure* closure) {
92 while (true) {
93 /* This load needs to be an acquire load because this can be a shutdown
94 * error that we might need to reference. Adding acquire semantics makes
95 * sure that the shutdown error has been initialized properly before us
96 * referencing it. */
97 gpr_atm curr = gpr_atm_acq_load(&state_);
98 if (GRPC_TRACE_FLAG_ENABLED(grpc_polling_trace)) {
99 gpr_log(GPR_DEBUG,
100 "LockfreeEvent::NotifyOn: %p curr=%" PRIxPTR " closure=%p", this,
101 curr, closure);
102 }
103 switch (curr) {
104 case kClosureNotReady: {
105 /* kClosureNotReady -> <closure>.
106
107 We're guaranteed by API that there's an acquire barrier before here,
108 so there's no need to double-dip and this can be a release-only.
109
110 The release itself pairs with the acquire half of a set_ready full
111 barrier. */
112 if (gpr_atm_rel_cas(&state_, kClosureNotReady,
113 reinterpret_cast<gpr_atm>(closure))) {
114 return; /* Successful. Return */
115 }
116
117 break; /* retry */
118 }
119
120 case kClosureReady: {
121 /* Change the state to kClosureNotReady. Schedule the closure if
122 successful. If not, the state most likely transitioned to shutdown.
123 We should retry.
124
125 This can be a no-barrier cas since the state is being transitioned to
126 kClosureNotReady; set_ready and set_shutdown do not schedule any
127 closure when transitioning out of CLOSURE_NO_READY state (i.e there
128 is no other code that needs to 'happen-after' this) */
129 if (gpr_atm_no_barrier_cas(&state_, kClosureReady, kClosureNotReady)) {
130 ExecCtx::Run(DEBUG_LOCATION, closure, GRPC_ERROR_NONE);
131 return; /* Successful. Return */
132 }
133
134 break; /* retry */
135 }
136
137 default: {
138 /* 'curr' is either a closure or the fd is shutdown(in which case 'curr'
139 contains a pointer to the shutdown-error). If the fd is shutdown,
140 schedule the closure with the shutdown error */
141 if ((curr & kShutdownBit) > 0) {
142 grpc_error* shutdown_err =
143 reinterpret_cast<grpc_error*>(curr & ~kShutdownBit);
144 ExecCtx::Run(DEBUG_LOCATION, closure,
145 GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
146 "FD Shutdown", &shutdown_err, 1));
147 return;
148 }
149
150 /* There is already a closure!. This indicates a bug in the code */
151 gpr_log(GPR_ERROR,
152 "LockfreeEvent::NotifyOn: notify_on called with a previous "
153 "callback still pending");
154 abort();
155 }
156 }
157 }
158
159 GPR_UNREACHABLE_CODE(return );
160 }
161
SetShutdown(grpc_error * shutdown_error)162 bool LockfreeEvent::SetShutdown(grpc_error* shutdown_error) {
163 gpr_atm new_state = reinterpret_cast<gpr_atm>(shutdown_error) | kShutdownBit;
164
165 while (true) {
166 gpr_atm curr = gpr_atm_no_barrier_load(&state_);
167 if (GRPC_TRACE_FLAG_ENABLED(grpc_polling_trace)) {
168 gpr_log(GPR_DEBUG,
169 "LockfreeEvent::SetShutdown: %p curr=%" PRIxPTR " err=%s",
170 &state_, curr, grpc_error_string(shutdown_error));
171 }
172 switch (curr) {
173 case kClosureReady:
174 case kClosureNotReady:
175 /* Need a full barrier here so that the initial load in notify_on
176 doesn't need a barrier */
177 if (gpr_atm_full_cas(&state_, curr, new_state)) {
178 return true; /* early out */
179 }
180 break; /* retry */
181
182 default: {
183 /* 'curr' is either a closure or the fd is already shutdown */
184
185 /* If fd is already shutdown, we are done */
186 if ((curr & kShutdownBit) > 0) {
187 GRPC_ERROR_UNREF(shutdown_error);
188 return false;
189 }
190
191 /* Fd is not shutdown. Schedule the closure and move the state to
192 shutdown state.
193 Needs an acquire to pair with setting the closure (and get a
194 happens-after on that edge), and a release to pair with anything
195 loading the shutdown state. */
196 if (gpr_atm_full_cas(&state_, curr, new_state)) {
197 ExecCtx::Run(DEBUG_LOCATION, reinterpret_cast<grpc_closure*>(curr),
198 GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
199 "FD Shutdown", &shutdown_error, 1));
200 return true;
201 }
202
203 /* 'curr' was a closure but now changed to a different state. We will
204 have to retry */
205 break;
206 }
207 }
208 }
209
210 GPR_UNREACHABLE_CODE(return false);
211 }
212
SetReady()213 void LockfreeEvent::SetReady() {
214 while (true) {
215 gpr_atm curr = gpr_atm_no_barrier_load(&state_);
216
217 if (GRPC_TRACE_FLAG_ENABLED(grpc_polling_trace)) {
218 gpr_log(GPR_DEBUG, "LockfreeEvent::SetReady: %p curr=%" PRIxPTR, &state_,
219 curr);
220 }
221
222 switch (curr) {
223 case kClosureReady: {
224 /* Already ready. We are done here */
225 return;
226 }
227
228 case kClosureNotReady: {
229 /* No barrier required as we're transitioning to a state that does not
230 involve a closure */
231 if (gpr_atm_no_barrier_cas(&state_, kClosureNotReady, kClosureReady)) {
232 return; /* early out */
233 }
234 break; /* retry */
235 }
236
237 default: {
238 /* 'curr' is either a closure or the fd is shutdown */
239 if ((curr & kShutdownBit) > 0) {
240 /* The fd is shutdown. Do nothing */
241 return;
242 }
243 /* Full cas: acquire pairs with this cas' release in the event of a
244 spurious set_ready; release pairs with this or the acquire in
245 notify_on (or set_shutdown) */
246 else if (gpr_atm_full_cas(&state_, curr, kClosureNotReady)) {
247 ExecCtx::Run(DEBUG_LOCATION, reinterpret_cast<grpc_closure*>(curr),
248 GRPC_ERROR_NONE);
249 return;
250 }
251 /* else the state changed again (only possible by either a racing
252 set_ready or set_shutdown functions. In both these cases, the closure
253 would have been scheduled for execution. So we are done here */
254 return;
255 }
256 }
257 }
258 }
259
260 } // namespace grpc_core
261