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

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.


-- 
~Randy