From 71cc64a85d8d99936f6851709a07f18c87a0adab Mon Sep 17 00:00:00 2001 From: Alastair D'Silva Date: Fri, 11 May 2018 16:12:59 +1000 Subject: [PATCH] powerpc: use task_pid_nr() for TID allocation The current implementation of TID allocation, using a global IDR, may result in an errant process starving the system of available TIDs. Instead, use task_pid_nr(), as mentioned by the original author. The scenario described which prevented it's use is not applicable, as set_thread_tidr can only be called after the task struct has been populated. In the unlikely event that 2 threads share the TID and are waiting, all potential outcomes have been determined safe. Signed-off-by: Alastair D'Silva Reviewed-by: Frederic Barrat Reviewed-by: Andrew Donnellan Signed-off-by: Michael Ellerman --- arch/powerpc/include/asm/switch_to.h | 1 - arch/powerpc/kernel/process.c | 122 ++++++--------------------- 2 files changed, 28 insertions(+), 95 deletions(-) diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h index be8c9fa23983..5b03d8a82409 100644 --- a/arch/powerpc/include/asm/switch_to.h +++ b/arch/powerpc/include/asm/switch_to.h @@ -94,6 +94,5 @@ static inline void clear_task_ebb(struct task_struct *t) extern int set_thread_uses_vas(void); extern int set_thread_tidr(struct task_struct *t); -extern void clear_thread_tidr(struct task_struct *t); #endif /* _ASM_POWERPC_SWITCH_TO_H */ diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c index e8b1d3c30669..ebcd3956f2be 100644 --- a/arch/powerpc/kernel/process.c +++ b/arch/powerpc/kernel/process.c @@ -1479,103 +1479,41 @@ int set_thread_uses_vas(void) } #ifdef CONFIG_PPC64 -static DEFINE_SPINLOCK(vas_thread_id_lock); -static DEFINE_IDA(vas_thread_ida); - -/* - * We need to assign a unique thread id to each thread in a process. +/** + * Assign a TIDR (thread ID) for task @t and set it in the thread + * structure. For now, we only support setting TIDR for 'current' task. * - * This thread id, referred to as TIDR, and separate from the Linux's tgid, - * is intended to be used to direct an ASB_Notify from the hardware to the - * thread, when a suitable event occurs in the system. + * Since the TID value is a truncated form of it PID, it is possible + * (but unlikely) for 2 threads to have the same TID. In the unlikely event + * that 2 threads share the same TID and are waiting, one of the following + * cases will happen: * - * One such event is a "paste" instruction in the context of Fast Thread - * Wakeup (aka Core-to-core wake up in the Virtual Accelerator Switchboard - * (VAS) in POWER9. + * 1. The correct thread is running, the wrong thread is not + * In this situation, the correct thread is woken and proceeds to pass it's + * condition check. * - * To get a unique TIDR per process we could simply reuse task_pid_nr() but - * the problem is that task_pid_nr() is not yet available copy_thread() is - * called. Fixing that would require changing more intrusive arch-neutral - * code in code path in copy_process()?. + * 2. Neither threads are running + * In this situation, neither thread will be woken. When scheduled, the waiting + * threads will execute either a wait, which will return immediately, followed + * by a condition check, which will pass for the correct thread and fail + * for the wrong thread, or they will execute the condition check immediately. * - * Further, to assign unique TIDRs within each process, we need an atomic - * field (or an IDR) in task_struct, which again intrudes into the arch- - * neutral code. So try to assign globally unique TIDRs for now. + * 3. The wrong thread is running, the correct thread is not + * The wrong thread will be woken, but will fail it's condition check and + * re-execute wait. The correct thread, when scheduled, will execute either + * it's condition check (which will pass), or wait, which returns immediately + * when called the first time after the thread is scheduled, followed by it's + * condition check (which will pass). * - * NOTE: TIDR 0 indicates that the thread does not need a TIDR value. - * For now, only threads that expect to be notified by the VAS - * hardware need a TIDR value and we assign values > 0 for those. - */ -#define MAX_THREAD_CONTEXT ((1 << 16) - 1) -static int assign_thread_tidr(void) -{ - int index; - int err; - unsigned long flags; - -again: - if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL)) - return -ENOMEM; - - spin_lock_irqsave(&vas_thread_id_lock, flags); - err = ida_get_new_above(&vas_thread_ida, 1, &index); - spin_unlock_irqrestore(&vas_thread_id_lock, flags); - - if (err == -EAGAIN) - goto again; - else if (err) - return err; - - if (index > MAX_THREAD_CONTEXT) { - spin_lock_irqsave(&vas_thread_id_lock, flags); - ida_remove(&vas_thread_ida, index); - spin_unlock_irqrestore(&vas_thread_id_lock, flags); - return -ENOMEM; - } - - return index; -} - -static void free_thread_tidr(int id) -{ - unsigned long flags; - - spin_lock_irqsave(&vas_thread_id_lock, flags); - ida_remove(&vas_thread_ida, id); - spin_unlock_irqrestore(&vas_thread_id_lock, flags); -} - -/* - * Clear any TIDR value assigned to this thread. - */ -void clear_thread_tidr(struct task_struct *t) -{ - if (!t->thread.tidr) - return; - - if (!cpu_has_feature(CPU_FTR_P9_TIDR)) { - WARN_ON_ONCE(1); - return; - } - - mtspr(SPRN_TIDR, 0); - free_thread_tidr(t->thread.tidr); - t->thread.tidr = 0; -} - -void arch_release_task_struct(struct task_struct *t) -{ - clear_thread_tidr(t); -} - -/* - * Assign a unique TIDR (thread id) for task @t and set it in the thread - * structure. For now, we only support setting TIDR for 'current' task. + * 4. Both threads are running + * Both threads will be woken. The wrong thread will fail it's condition check + * and execute another wait, while the correct thread will pass it's condition + * check. + * + * @t: the task to set the thread ID for */ int set_thread_tidr(struct task_struct *t) { - int rc; - if (!cpu_has_feature(CPU_FTR_P9_TIDR)) return -EINVAL; @@ -1585,11 +1523,7 @@ int set_thread_tidr(struct task_struct *t) if (t->thread.tidr) return 0; - rc = assign_thread_tidr(); - if (rc < 0) - return rc; - - t->thread.tidr = rc; + t->thread.tidr = (u16)task_pid_nr(t); mtspr(SPRN_TIDR, t->thread.tidr); return 0; -- 2.45.2