[tip: x86/urgent] x86/mm: Fix SMP ordering in switch_mm_irqs_off()

[tip-bot2 for Ingo Molnar]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     83b0177a6c4889b3a6e865da5e21b2c9d97d0551
Gitweb:        https://git.kernel.org/tip/83b0177a6c4889b3a6e865da5e21b2c9d97d0551
Author:        Ingo Molnar <mingo@xxxxxxxxxx>
AuthorDate:    Fri, 16 May 2025 15:43:04 +02:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Mon, 13 Oct 2025 13:55:53 -07:00

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 <sdolan@xxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/CAHDw0oGd0B4=uuv8NGqbUQ_ZVmSheU2bN70e4QhFXWvuAZdt2w@xxxxxxxxxxxxxx/
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---
 arch/x86/mm/tlb.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 39f8011..5d22170 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);