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219 | static int |
219 | static int |
220 | linuxaio_get_events_from_ring (EV_P) |
220 | linuxaio_get_events_from_ring (EV_P) |
221 | { |
221 | { |
222 | struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx; |
222 | struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx; |
223 | |
223 | |
224 | unsigned head = ring->head; |
224 | /* the kernel reads and writes both of these variables, */ |
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225 | /* as a C extension, we assume that volatile use here */ |
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226 | /* both makes reads atomic and once-only */ |
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227 | unsigned head = *(volatile unsigned *)&ring->head; |
225 | unsigned tail = *(volatile unsigned *)&ring->tail; |
228 | unsigned tail = *(volatile unsigned *)&ring->tail; |
226 | |
229 | |
227 | if (head == tail) |
230 | if (head == tail) |
228 | return 0; |
231 | return 0; |
229 | |
232 | |
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231 | if (ecb_expect_false (ring->magic != AIO_RING_MAGIC) |
234 | if (ecb_expect_false (ring->magic != AIO_RING_MAGIC) |
232 | || ring->incompat_features != AIO_RING_INCOMPAT_FEATURES |
235 | || ring->incompat_features != AIO_RING_INCOMPAT_FEATURES |
233 | || ring->header_length != sizeof (struct aio_ring)) /* TODO: or use it to find io_event[0]? */ |
236 | || ring->header_length != sizeof (struct aio_ring)) /* TODO: or use it to find io_event[0]? */ |
234 | return 0; |
237 | return 0; |
235 | |
238 | |
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239 | /* make sure the events up to tail are visible */ |
236 | ECB_MEMORY_FENCE_ACQUIRE; |
240 | ECB_MEMORY_FENCE_ACQUIRE; |
237 | |
241 | |
238 | /* parse all available events, but only once, to avoid starvation */ |
242 | /* parse all available events, but only once, to avoid starvation */ |
239 | if (tail > head) /* normal case around */ |
243 | if (tail > head) /* normal case around */ |
240 | linuxaio_parse_events (EV_A_ ring->io_events + head, tail - head); |
244 | linuxaio_parse_events (EV_A_ ring->io_events + head, tail - head); |
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242 | { |
246 | { |
243 | linuxaio_parse_events (EV_A_ ring->io_events + head, ring->nr - head); |
247 | linuxaio_parse_events (EV_A_ ring->io_events + head, ring->nr - head); |
244 | linuxaio_parse_events (EV_A_ ring->io_events, tail); |
248 | linuxaio_parse_events (EV_A_ ring->io_events, tail); |
245 | } |
249 | } |
246 | |
250 | |
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251 | /* as an extension to C, we hope that the volatile will makethis atomic and once-only */ |
247 | *(volatile unsigned *)&ring->head = tail; |
252 | *(volatile unsigned *)&ring->head = tail; |
248 | |
253 | /* make sure kernel can see our new head value - probably not required */ |
249 | /* again, other implementations don't do this, but I think it's required for the kernel to see free slots */ |
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250 | ECB_MEMORY_FENCE_RELEASE; |
254 | ECB_MEMORY_FENCE_RELEASE; |
251 | |
255 | |
252 | return 1; |
256 | return 1; |
253 | } |
257 | } |
254 | |
258 | |