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

On Thu, Jun 18, 2026 at 03:44:36PM +0100, Usama Arif wrote:
> On 18/06/2026 04:30, Randy Dunlap wrote:
> > On 6/17/26 6:44 PM, Alan Stern wrote:
> >> 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 <[email protected]>
> >>> ---
> >>>  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.
> > 
> > in Documentation/process/submit-checklist.rst:
> > 
> > 3) All memory barriers {e.g., ``barrier()``, ``rmb()``, ``wmb()``} need a
> >    comment in the source code that explains the logic of what they are doing
> >    and why.
> > 
> > in Documentation/process/4.Coding.rst:
> > 
> > Certain things should always be commented.  Uses of memory barriers should
> > be accompanied by a line explaining why the barrier is necessary.
> > 
> > but looking in the 3000+ lines of Documentation/memory-barriers.txt won't tell
> > anyone about that.
> > 
> > 
> 
> Thanks!
> I will send a v2 with the below diff if there are no objections?
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 5ba4a20ba77d..685829875a3e 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -182,11 +182,21 @@ static atomic_t csd_bug_count = ATOMIC_INIT(0);
>  static void __csd_lock_record(call_single_data_t *csd)
>  {
>         if (!csd) {
> +               /*
> +                * Pairs with smp_load_acquire() of cur_csd in
> +                * csd_lock_wait_toolong(): orders any preceding CSD
> +                * callback/unlock before a remote reader observes NULL.
> +                */

Please replace the spaces with tabs.  (Probably a copy-pasta issue.)

>                 smp_store_release(this_cpu_ptr(&cur_csd), NULL);

		smp_store_release(this_cpu_ptr(&cur_csd), NULL); /* ^^^ */

Adding the comment as show above will satisfy tools such as checkpatch.

>                 return;
>         }
>         __this_cpu_write(cur_csd_func, csd->func);
>         __this_cpu_write(cur_csd_info, csd->info);
> +       /*
> +        * Pairs with smp_load_acquire() of cur_csd in
> +        * csd_lock_wait_toolong(): publishes cur_csd_func and
> +        * cur_csd_info before the non-NULL pointer becomes visible.
> +        */
>         smp_store_release(this_cpu_ptr(&cur_csd), csd);
>  }

The comments look good to me, but I must defer to Alan and Randy.

						Thanx, Paul