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

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

[Usama Arif]

__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);
 }
 
 static __always_inline void csd_lock_record(call_single_data_t *csd)
-- 
2.53.0-Meta

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

[Alan Stern]

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.

Alan Stern

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

[Randy Dunlap]

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.


-- 
~Randy

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

[Paul E. McKenney]

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 <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.

Is smp_store_release() a memory barrier?  Sorry, couldn't resist... ;-)

							Thanx, Paul

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

[Usama Arif]

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);
 }

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

[Paul E. McKenney]

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 <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.
> +                */

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

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

[Usama Arif]

On 18/06/2026 15:57, Paul E. McKenney wrote:
> 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 <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.
>> +                */
> 
> Please replace the spaces with tabs.  (Probably a copy-pasta issue.)

Yes, its tabs in my commit, looks like the email client messed it up.
> 
>>                 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.

Will do, Thanks!

> 
>>                 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

Thanks!
> 
> 						Thanx, Paul