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 > 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)
