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.
+ */
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);
+ /*
+ * 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);
}
