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 <alastair@d-silva.org>
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

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 */

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;