Re: [PATCH] smp: Use release stores for csd_lock_record() state

[Alan Stern]

On Wed, Jun 17, 2026 at 02:20:01PM -0700, Usama Arif wrote:
> __csd_lock_record() publishes per-CPU CSD debug state that is read by
> csd_lock_wait_toolong() on another CPU.  The remote side first reads
> cur_csd with smp_load_acquire() and, when non-NULL, may then read the
> matching cur_csd_func and cur_csd_info fields.
> 
> Use smp_store_release() when publishing cur_csd so that the preceding
> cur_csd_func and cur_csd_info stores are ordered before the pointer
> that csd_lock_wait_toolong() acquires.  This replaces the open-coded
> smp_wmb() plus plain cur_csd store with the release operation that
> matches the smp_load_acquire() in csd_lock_wait_toolong().
> 
> For the clear path, use smp_store_release(&cur_csd, NULL) so that
> clearing the diagnostic state remains ordered after the preceding
> callback/unlock work, without requiring a full barrier before the
> store.  On x86 this removes the locked full barrier from the clear
> path; on weaker memory models it uses the release operation needed by
> the smp_load_acquire() in csd_lock_wait_toolong().
> 
> The old code also had smp_mb() calls around cur_csd updates. Those would
> only be needed if cur_csd were treated as an exact live-state marker whose
> publication had to be observed before callback execution or CSD unlock.
> CSD stall warnings do not currently have RCU-style stall-ended checks, so
> they already allow the stall to end while diagnostics are being assembled.
> The cur_csd record is therefore best-effort diagnostic context, not a
> precise completion/stall boundary.
> 
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---
>  kernel/smp.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index a0bb56bd8dda..5ba4a20ba77d 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -182,16 +182,12 @@ static atomic_t csd_bug_count = ATOMIC_INIT(0);
>  static void __csd_lock_record(call_single_data_t *csd)
>  {
>  	if (!csd) {
> -		smp_mb(); /* NULL cur_csd after unlock. */
> -		__this_cpu_write(cur_csd, NULL);
> +		smp_store_release(this_cpu_ptr(&cur_csd), NULL);
>  		return;
>  	}
>  	__this_cpu_write(cur_csd_func, csd->func);
>  	__this_cpu_write(cur_csd_info, csd->info);
> -	smp_wmb(); /* func and info before csd. */
> -	__this_cpu_write(cur_csd, csd);
> -	smp_mb(); /* Update cur_csd before function call. */
> -		  /* Or before unlock, as the case may be. */
> +	smp_store_release(this_cpu_ptr(&cur_csd), csd);

Isn't there a general policy in the kernel that memory barriers should 
be accompanied by a comment explaining what other memory barriers they 
synchronize with?  Including such comments is a good idea in any case.

Alan Stern