2026年6月24日 23:27, "Usama Arif" <[email protected] mailto:[email protected]?to=%22Usama%20Arif%22%20%3Cusama.arif%40linux.dev%3E > 写到:
> > On 24/06/2026 15:15, Kunwu Chan wrote: > > > > > On 6/23/26 00:38, 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. > > > > > The original comments around those updates seem to suggest a stronger > > relationship: > > /* Update cur_csd before function call. */ > > /* NULL cur_csd after unlock. */ > > > > The changelog characterizes cur_csd as best-effort diagnostic context. > > > > Is there prior consensus that cur_csd carries no ordering relationship > > to callback execution / unlock state, > > > > or is this patch effectively relaxing that historical assumption? > > > > > > Thanks, Kunwu > > > Thanks for the review Kunwu! > > The patch preserves the ordering that reader actually needs: > csd_lock_wait_toolong() observes non-NULL cur_csd, the matching func/info have > been published. If it observes NULL, the prior callback/unlock work is ordered > before the clear. > > So yes, this relaxes the historical implementation ordering around the > diagnostic marker, but not the CSD execution/reuse ordering. > Thanks for the v3 update. The explanation of reader-side tolerance for snapshot-vs-completion races addresses my concern. > > > > > > > > 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]> > > > --- > > > v1 -> v2: > > > https://lore.kernel.org/all/[email protected]/ > > > - 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); > > > } > > > > > > static __always_inline void csd_lock_record(call_single_data_t *csd) > > > > > >
