sync: add mem::forget to RwLockWriteGuard::downgrade. #2957
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Currently when `RwLockWriteGuard::downgrade` the `MAX_READS - 1` permits are added to the semaphore. When `RwLockWriteGuard::drop` gets invoked however another `MAX_READS` permits are added. This results in releasing more permits that were actually aquired when downgrading a write to a read lock. This is why we need to call `mem::forget` on the `RwLockWriteGuard` in order to avoid invoking the destructor. Fixes: tokio-rs#2941
Darksonn
added
A-tokio
carllerche
requested changes
Oct 14, 2020
| @@ -76,3 +76,24 @@ fn concurrent_read_write() { | |||
| assert_eq!(10, *guard); | |||
| }); | |||
| } | |||
| #[test] | |||
| fn downgrade() { | |||
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It does not look like loom_rwlock.rs is being included here.
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It is not. I was not sure whether this is intentional or not. Will include
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Also, maybe this PR should be submitted against 0.2 and then forward ported. |
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Sorry for the delay, I will come back to this soon. Working on v0.3.1 atm. |
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Btw, should we call out the deadlock in the docs and explain that it would deadlock because it is fair? |
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@carllerche yes we should. Just need to think of a succinct way to put it. :) |
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Motivation
Currently when
RwLockWriteGuard::downgradetheMAX_READS - 1permits are added to the semaphore. When
RwLockWriteGuard::dropgets invoked however another
MAX_READSpermits are added. Thisresults in releasing more permits that were actually aquired when
downgrading a write to a read lock.
Solution
This is why we need to call
mem::forgeton theRwLockWriteGuardin order to avoidinvoking the destructor.
Additionally doc test for
downgradewas not correct as it can hang in the situation where after downgrading all available permits are assigned to the waiting writer before the additional reader gets the change to obtain a permit.Fixes: #2941