Re: [PATCH v3] smp: Use release stores for csd_lock_record() state

[Dmitry Ilvokhin <d@ilvokhin.com>]

On Mon, Jun 29, 2026 at 03:57:45AM -0700, Usama Arif wrote:
> __csd_lock_record() publishes per-CPU diagnostic state (cur_csd,
> cur_csd_func, cur_csd_info) that is consumed from a remote CPU by
> csd_lock_wait_toolong() via smp_load_acquire(&cur_csd). To order the
> matching cur_csd_func/cur_csd_info stores before the cur_csd
> publication, the producer issues smp_wmb() before writing cur_csd;
> to order the publication before the subsequent callback execution or
> CSD unlock, it issues smp_mb() after the write. The clear path
> mirrors this with smp_mb() before storing NULL into cur_csd so the
> preceding callback/unlock is observed first.
> 
> The smp_mb() pair is heavier than what the consumer actually
> requires (on x86 each emits a locked full barrier). The consumer
> only needs to observe the matching cur_csd_func/cur_csd_info when it
> sees a non-NULL cur_csd, and to observe the preceding callback/unlock
> when it sees NULL -- both of which a release/acquire pair provides.
> The extra two-way ordering enforced by smp_mb() -- that cur_csd
> publication be observed before callback execution or unlock becomes
> visible -- would only matter if cur_csd were an exact live-state
> marker. csd_lock_wait_toolong() does not treat it that way: it
> snapshots cur_csd via smp_load_acquire() and then prints / dumps /
> re-IPIs without an RCU-style stall-ended recheck, so the diagnostic
> already tolerates the remote CPU completing its work between snapshot
> and report. cur_csd is best-effort context, not a precise stall
> boundary.
> 
> Replace the smp_wmb() + plain store + smp_mb() in the publish path,
> and the smp_mb() + plain store in the clear path, with
> smp_store_release(). This pairs with the smp_load_acquire() in
> csd_lock_wait_toolong(): preceding cur_csd_func/cur_csd_info stores
> become visible before a remote reader observes the non-NULL
> publication, and any preceding callback/unlock becomes visible before
> a reader observes the NULL clear.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>