Subject: [PATCH 4/7] powerpc/64s/radix: Allow mm_cpumask trimming from external sources

From: Nicholas Piggin <[email-masked]>
Date: Thu, 17 Dec 2020 23:47:28 +1000

mm_cpumask trimming is currently restricted to be issued by the current
thread of a single-threaded mm. This patch relaxes that and allows the
mask to be trimmed from any context.

Signed-off-by: Nicholas Piggin <[email-masked]>
---
 arch/powerpc/mm/book3s64/radix_tlb.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
index 5b62e2e7371c..7b199bee4baa 100644
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -657,20 +657,16 @@ static void do_exit_flush_lazy_tlb(void *arg)
 	}
 
 	/*
-	 * This IPI is only initiated from a CPU which is running mm which
-	 * is a single-threaded process, so there will not be another racing
-	 * IPI coming in where we would find our cpumask already clear.
-	 *
-	 * Nothing else clears our bit in the cpumask except CPU offlining,
-	 * in which case we should not be taking IPIs here. However check
-	 * this just in case the logic is wrong somewhere, and don't underflow
-	 * the active_cpus count.
+	 * This IPI may be initiated from any source including those not
+	 * running the mm, so there may be a racing IPI that comes after
+	 * this one which finds the cpumask already clear. Check and avoid
+	 * underflowing the active_cpus count in that case. The race should
+	 * not otherwise be a problem, but the TLB must be flushed because
+	 * that's what the caller expects.
 	 */
 	if (cpumask_test_cpu(cpu, mm_cpumask(mm))) {
 		atomic_dec(&mm->context.active_cpus);
 		cpumask_clear_cpu(cpu, mm_cpumask(mm));
-	} else {
-		WARN_ON_ONCE(1);
 	}
 
 out_flush:
-- 
2.23.0