From: Ingo Molnar Date: Fri, 16 May 2025 13:43:04 +0000 (+0200) Subject: x86/mm: Fix SMP ordering in switch_mm_irqs_off() X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=83b0177a6c4889b3a6e865da5e21b2c9d97d0551;p=users%2Fhch%2Fmisc.git 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 Closes: https://lore.kernel.org/all/CAHDw0oGd0B4=uuv8NGqbUQ_ZVmSheU2bN70e4QhFXWvuAZdt2w@mail.gmail.com/ Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Signed-off-by: Dave Hansen --- diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 39f80111e6f1..5d221709353e 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -911,11 +911,31 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next, * 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);