[Backport v3.7-branch] kernel: Fix priority boost when locking a mutex #87841
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k_thread_priority_set() on a pended thread wasn't re-inserting into the waitq, causing the incorrect thread to run based on priority. When using the scalable waitq config, this can also break assumptions of the tree and leave the owner of a waitq still being in the waitq tree, cycles in the tree, or a crash. Remove and re-add a thread to a waitq to ensure the waitq remains in order and the tree's assumptions are not violated. To illustrate the issue, consider 4 threads in decreasing priority order: A, B, C, and D along with two mutexes, m0 and m1. This is implemented in the new complex_inversion mutex_api test. 1. D locks m1 2. C locks m0 3. C pends on m1 4. B pends on m1 5. A pends on m0, boosts C's priority, now tree on m1 is not sorted 6. D unlocks m1, left-most thread on tree is B. When removing B from tree it cannot be found because it searches to the right of C due to C's boosted priority when the node is actually on the left. rb_remove silently fails. 7. B unlocks m1, left-most thread on tree is still B and it tries to unpend itself, resulting in a NULL pointer dereference on B->base.pended_on. Signed-off-by: Josh DeWitt <[email protected]> (cherry picked from commit c05cfbf)
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peter-mitsis
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May 28, 2025
dcpleung
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Backport c05cfbf from #87235.
Fixes #89331