x86/mm: Fix SMP ordering in switch_mm_irqs_off()

Stephen noted that it is possible to not have an smp_mb() between
the loaded_mm store and the tlb_gen load in switch_mm(), meaning the
ordering against flush_tlb_mm_range() goes out the window, and it
becomes possible for switch_mm() to not observe a recent tlb_gen
update and fail to flush the TLBs.

[ dhansen: merge conflict fixed by Ingo ]

Fixes: 209954cbc7d0 ("x86/mm/tlb: Update mm_cpumask lazily")
Reported-by: Stephen Dolan <[e-mail]>
Closes: https://lore.kernel.org/all/CAHDw0oGd0B4=[e-mail]/
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>

arch/x86/mm/tlb.c

···
 		 * CR3 and cpu_tlbstate.loaded_mm are not all in sync.
 		 */
 		this_cpu_write(cpu_tlbstate.loaded_mm, LOADED_MM_SWITCHING);
-		barrier();
 
-		/* Start receiving IPIs and then read tlb_gen (and LAM below) */
+		/*
+		 * Make sure this CPU is set in mm_cpumask() such that we'll
+		 * receive invalidation IPIs.
+		 *
+		 * Rely on the smp_mb() implied by cpumask_set_cpu()'s atomic
+		 * operation, or explicitly provide one. Such that:
+		 *
+		 * switch_mm_irqs_off()				flush_tlb_mm_range()
+		 *   smp_store_release(loaded_mm, SWITCHING);     atomic64_inc_return(tlb_gen)
+		 *   smp_mb(); // here                            // smp_mb() implied
+		 *   atomic64_read(tlb_gen);                      this_cpu_read(loaded_mm);
+		 *
+		 * we properly order against flush_tlb_mm_range(), where the
+		 * loaded_mm load can happen in mative_flush_tlb_multi() ->
+		 * should_flush_tlb().
+		 *
+		 * This way switch_mm() must see the new tlb_gen or
+		 * flush_tlb_mm_range() must see the new loaded_mm, or both.
+		 */
 		if (next != &init_mm && !cpumask_test_cpu(cpu, mm_cpumask(next)))
 			cpumask_set_cpu(cpu, mm_cpumask(next));
+		else
+			smp_mb();
+
 		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
 
 		ns = choose_new_asid(next, next_tlb_gen);