- if (!ctx_valid || (old_mm != mm)) {
- load_secondary_context(mm);
- tsb_context_switch(mm);
- }
+ /* We have to be extremely careful here or else we will miss
+ * a TSB grow if we switch back and forth between a kernel
+ * thread and an address space which has it's TSB size increased
+ * on another processor.
+ *
+ * It is possible to play some games in order to optimize the
+ * switch, but the safest thing to do is to unconditionally
+ * perform the secondary context load and the TSB context switch.
+ *
+ * For reference the bad case is, for address space "A":
+ *
+ * CPU 0 CPU 1
+ * run address space A
+ * set cpu0's bits in cpu_vm_mask
+ * switch to kernel thread, borrow
+ * address space A via entry_lazy_tlb
+ * run address space A
+ * set cpu1's bit in cpu_vm_mask
+ * flush_tlb_pending()
+ * reset cpu_vm_mask to just cpu1
+ * TSB grow
+ * run address space A
+ * context was valid, so skip
+ * TSB context switch
+ *
+ * At that point cpu0 continues to use a stale TSB, the one from
+ * before the TSB grow performed on cpu1. cpu1 did not cross-call
+ * cpu0 to update it's TSB because at that point the cpu_vm_mask
+ * only had cpu1 set in it.
+ */
+ load_secondary_context(mm);
+ tsb_context_switch(mm);