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The entropy queue stores entropy gathered from environmental sources. Periodically (every 100ms currently), the random kthread will drain this queue and mix it into the CSPRNG's entropy pool(s). The old scheme uses a ring buffer with a mutex to serialize producers, while the sole consumer, the random kthread, avoids using a mutex on the basis that no serialization is needed since nothing else is updating the consumer index. On platforms without total store ordering, however, this isn't sufficient: when a producer inserts a queue entry and updates `ring.in`, there is no guarantee that the consumer will see the updated queue entry upon observing the updated producer index. That is, the update to `ring.in` may be visible before the updated queue entry is visible. As a result, we could end up mixing in zero'ed queue entries, though this race is fairly unlikely in practice given how infrequently the kthread runs. The easiest way to fix this is to make the kthread acquire the mutex as well, and hold it while processing queue entries. However, this might result in a long hold time if there are many queue entries, and we really want the hold times to be short, e.g., to avoid delaying interrupt processing. We could introduce a proper MPSC queue, but this is probably overcomplicated for a consumer which runs at 10Hz. Instead, define two buffers, always with one designated as the "active" buffer. Producers queue entries in the active buffer, and the kthread uses the mutex to atomically flip the two buffers, so it can process entries from the inactive buffer without holding the mutex. This requires more memory, but keeps mutex hold times short and lets us keep the queue implementation very simple. Reviewed by: cem MFC after: 1 month Sponsored by: Stormshield Sponsored by: Klara, Inc. Differential Revision: https://reviews.freebsd.org/D51112 |
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| .. | ||
| fenestrasX | ||
| armv8rng.c | ||
| build.sh | ||
| darn.c | ||
| fortuna.c | ||
| fortuna.h | ||
| hash.c | ||
| hash.h | ||
| ivy.c | ||
| nehemiah.c | ||
| other_algorithm.c | ||
| other_algorithm.h | ||
| random_harvestq.c | ||
| random_harvestq.h | ||
| random_infra.c | ||
| randomdev.c | ||
| randomdev.h | ||
| uint128.h | ||
| unit_test.c | ||
| unit_test.h | ||