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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 04, 2026 at 03:11:28PM +0800, Bo Li wrote:
> 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];
>  		}

No, this looks wrong. Note that per the 'cpu != this_cpu' there are only
N-1 invocations. One CPU will not be initialized. Further note the name
of this function, and the name of the function above it.