Re: [PATCH v2] smp: Use release stores for csd_lock_record() state

[Dmitry Ilvokhin <d@ilvokhin.com>]

On Fri, Jun 26, 2026 at 05:30:21PM +0100, Usama Arif wrote:
> 
> 
> On 25/06/2026 15:42, Dmitry Ilvokhin wrote:
> > On Mon, Jun 22, 2026 at 09:38:07AM -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 changelog only calls out the clear path here, but the publish path
> > also drops its trailing smp_mb() (plus the smp_wmb()), so on x86 both
> > paths lose a locked full barrier. Worth describing symmetrically.
> 
> Do you mean whats written in the next paragraph? The paragraph above
> is for clear, the paragraph just below is for publish. The reason for removing
> smp_wmb() is in 2nd paragraph. I think all the information regarding the changes
> is in the commit message.> 

Indeed. I clearly can not read properly.

> >>
> >> 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>
> >> ---
> >> v1 -> v2: https://lore.kernel.org/all/01437928-ff79-4d8e-823b-7f20146946f6@linux.dev/
> >> - Document where the smp_store_release() synchronizes with (Alan Stern,
> >>   Randy Dunlap and Paul McKenney).
> >> ---
> >>  kernel/smp.c | 18 ++++++++++++------
> >>  1 file changed, 12 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/kernel/smp.c b/kernel/smp.c
> >> index a0bb56bd8dda..685829875a3e 100644
> >> --- a/kernel/smp.c
> >> +++ b/kernel/smp.c
> >> @@ -182,16 +182,22 @@ 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);
> >> +		/*
> >> +		 * 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);
> >> -	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. */
> >> +	/*
> >> +	 * 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);
> >>  }
> > 
> > Since v2 is specifically about documenting the pairing, it would be good
> > to make it symmetric and add the comment on the acquire side in
> > csd_lock_wait_toolong().
> > 
> 
> Its already documented [1]
> 
> [1] https://elixir.bootlin.com/linux/v7.1.1/source/kernel/smp.c#L275

It is documented to some extent, but it doesn't explicitly state which
smp_store_release() the smp_load_acquire() pairs with. I think that's
the main benefit of these comments: making the synchronization pair
explicit so readers don't have to infer it.