WritePriorityMutex: Fix rare case of dropped read waiter wakes #5161
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Sonicadvance1
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Reasonable enough to me
The Race: 1. A Reader sets `READ_WAITER_BIT` (Bit 15) and sleeps on the High 16 bits (`Futex+2`). 2. Writer A unlocks. It clears `READ_WAITER_BIT` (in Low 16 bits) and `WRITE_OWNED` (in High 16 bits). 3. Writer B immediately steals the lock. It sets `WRITE_OWNED` but preserves the now-cleared `READ_WAITER_BIT`. 4. The Reader, checking `Futex+2`, sees `WRITE_OWNED` is set. Since it cannot see that Bit 15 was unset (as it is watching High 16 bits), it assumes its wait signal is still valid and sleeps. 5. Writer B unlocks. It sees no `READ_WAITER_BIT` and wakes nobody. Deadlock. The Fix: Move `READ_WAITER_BIT` to Bit 30 (High 16 bits). Now, when Writer A clears the flag, the High 16 bits change value which will prevent the wait from occurring within WaitForAddress
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Sonicadvance1
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Dec 27, 2025
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The Race:
READ_WAITER_BIT(Bit 15) and sleeps on the High 16 bits (Futex+2).READ_WAITER_BIT(in Low 16 bits) andWRITE_OWNED(in High 16 bits).WRITE_OWNEDbut preserves the now-clearedREAD_WAITER_BIT.Futex+2, seesWRITE_OWNEDis set. Since it cannot see that Bit 15 was unset (as it is watching High 16 bits), it assumes its wait signal is still valid and sleeps.READ_WAITER_BITand wakes nobody. Deadlock.The Fix:
Move
READ_WAITER_BITto Bit 30 (High 16 bits).Now, when Writer A clears the flag, the High 16 bits change value which will prevent the wait from occurring within WaitForAddress