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

[Usama Arif <usama.arif@linux.dev>]

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 <usama.arif@linux.dev>
>>> ---
>>>  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);
 }