On Wed, Jun 17, 2026 at 08:30:32PM -0700, 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.
Is smp_store_release() a memory barrier? Sorry, couldn't resist... ;-)
Thanx, Paul
