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
