[PATCH] x86/tsc: Fix misplaced seqcount_latch_init() in cyc2ns_init_secondary_cpus()

["Bo Li" <libo.gcs85@bytedance.com>]

In cyc2ns_init_secondary_cpus(), seqcount_latch_init(&c2n->seq) is
called _before_ c2n is advanced to the next CPU via per_cpu_ptr().
As a result:

 1. On the first iteration, c2n still points at the BSP's struct, so
    the BSP's seqcount_latch is re-initialized. cyc2ns_init_boot_cpu()
    already did this correctly, so it happens to work by accident
    because no concurrent readers are live at __init time.

 2. On subsequent iterations, seqcount_latch_init() initializes the
    previous CPU's seqcount (because c2n was advanced by the prior
    per_cpu_ptr()), so all CPUs except the last one in the for_each
    loop happen to get initialized. The last secondary CPU's seqcount
    is left uninitialized.

Move seqcount_latch_init() _after_ c2n is pointed at the target CPU's
struct, so each secondary CPU's seqcount is correctly initialized.

Fixes: e2a9ca29b5ed ("x86/tsc: Initialize cyc2ns when tsc frequency is determined")
Cc: stable@vger.kernel.org
Signed-off-by: Bo Li <libo.gcs85@bytedance.com>
---
 arch/x86/kernel/tsc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index ce10ae4b298b..84fb80492b01 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -223,8 +223,8 @@ static void __init cyc2ns_init_secondary_cpus(void)
 
 	for_each_possible_cpu(cpu) {
 		if (cpu != this_cpu) {
-			seqcount_latch_init(&c2n->seq);
 			c2n = per_cpu_ptr(&cyc2ns, cpu);
+			seqcount_latch_init(&c2n->seq);
 			c2n->data[0] = data[0];
 			c2n->data[1] = data[1];
 		}
-- 
2.20.1