[PATCH 3/7] powerpc: use task_pid_nr() for TID allocation

[Alastair D'Silva]

From: Alastair D'Silva <alastair at d-silva.org>

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.

Signed-off-by: Alastair D'Silva <alastair at d-silva.org>
---
 arch/powerpc/kernel/process.c | 97 +------------------------------------------
 1 file changed, 1 insertion(+), 96 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index a3e0a3e06d5a..56ff7eb5ff79 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1496,103 +1496,12 @@ 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.
- *
- * 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.
- *
- * 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.
- *
- * 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()?.
- *
- * 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.
- *
- * 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.
  */
 int set_thread_tidr(struct task_struct *t)
 {
-	int rc;
-
 	if (!cpu_has_feature(CPU_FTR_P9_TIDR))
 		return -EINVAL;
 
@@ -1602,11 +1511,7 @@ 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;
-- 
2.14.3