alpha: do not skip TLB shootdown IPIs for kthread-borrowed mms flush_tlb_mm(), flush_tlb_page() and flush_icache_user_page() skip the shootdown IPI when mm_users <= 1, on the assumption that no other CPU can be running the mm. A task that borrows an mm through kthread_use_mm() takes mmgrab() rather than mmget(), so it never appears in mm_users, and kthread_use_mm() may be handed an mm that is already the caller's active_mm and loaded on that CPU. Such a CPU was not only left without an IPI, its mm->context[cpu] was cleared from under it. mm->context[cpu] is already the record of which CPUs hold an ASN for the mm. Test it rather than clearing it: take the shortcut only when no other CPU has one, and otherwise fall through to the IPI, which invalidates those CPUs properly. A CPU that merely ran the mm in the past also holds a context and now costs an IPI, which errs in the safe direction. Change ipi_flush_tlb_mm(), ipi_flush_icache_page() and the migration callback ipi_flush_mm_and_page() to test current->mm, like ipi_flush_tlb_page(). A CPU that only retains the mm lazily then clears its own slot through flush_tlb_other(), rather than publishing a new ASN on every IPI or migration rendezvous. The migration callback still does its per-VA tbi() afterwards. Returning to the mm allocates a fresh ASN through ev5_switch_mm(), or through the direct-load path when kthread_use_mm() switches current's mm. Once these historical slots are gone, the shortcut is available again if mm_users remains at most one and no other CPU has taken or kept a context. An explicit kthread_use_mm() borrower has current->mm set and still receives the invalidate it needs. The caller-side tests in flush_tlb_mm() and flush_icache_user_page() remain current->active_mm. A kernel thread that flushes its lazily held mm can therefore publish its own context slot. Those branches also perform the shortcut check, which excludes the calling CPU's slot, so leave them unchanged. This is distinct from a remote lazy CPU keeping its slot populated through repeated callbacks. Together with the preceding patch, dropping these clearing loops leaves every runtime write to mm->context[] targeting the writing CPU's own slot, so the array becomes a lockless publication of which CPUs may be using the mm, with a single writer per slot. Mark the two stores that are now observed from other CPUs; flush_tlb_other() already uses WRITE_ONCE(). The uniprocessor flush_icache_user_page() is left alone, as nothing reads another CPU's slot there, and init_new_context() runs before the mm is shared. Order the slot reads after the page-table changes with smp_mb(). On the publishing CPU, the store in ev5_switch_mm() precedes the mb() in Alpha's arch_spin_unlock(), when finish_lock_switch() drops the rq lock. This relies on Alpha's unlock implementation, not a generic scheduler promise of a full barrier on unlock; document that reliance at the store. Also issue smp_mb() after publishing in __load_new_mm_context(), before loading and using the mm. This covers every caller, including check_mmu_context(), which can publish a replacement after finish_lock_switch() has already run. This requires the preceding removal of remote slot clearing from migrate_flush_tlb_page(). It also requires the stale-TLB series to set need_new_asn on both the reused and newly allocated ASN paths. Otherwise an IPI between finish_lock_switch() and check_mmu_context() can zero a new slot while asn_lock is set, and the task can return to user space with a live ASN that the shortcut cannot see. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: # 53ab52cfe5ce: alpha: load the MMU context when switch_mm() switches the current task Cc: # 01df8f401888: alpha: run check_mmu_context() from finish_arch_post_lock_switch() Cc: # ef43cfc28c42: alpha: only use a targeted tbi() when the target mm is really current Cc: # 738cf8f7a78f: alpha: invalidate the local context in flush_tlb_page() Cc: # 62c92086f30e: alpha: fix the local TLB invalidate in flush_tlb_page() Cc: # 3faee097b939: alpha: fix the local TLB invalidate in the UP flush_tlb_page() Cc: # b2c3b654bb64: alpha: invalidate the local context in flush_tlb_mm() Cc: # f19ae6f51a38: alpha: invalidate the local context in flush_icache_user_page() Cc: # 471e7f9c890b: alpha: do not clear remote MMU contexts in migrate_flush_tlb_page() Cc: stable@vger.kernel.org Reviewed-by: Matt Turner Signed-off-by: Magnus Lindholm Link: https://lore.kernel.org/r/20261010144621.974850-3-linmag7@gmail.com Signed-off-by: Magnus Lindholm