[PATCH] ftrace: As disabling interrupts is costly and write_lock variant of tasklist_lock is not held from interrupt context it is not necessary to disable interrupts.

[Soumya PN]

In ftrace.c inside the function alloc_retstack_tasklist()(which will be
invoked when function_graph tracing is on) the tasklist_lock is being
held as reader while iterating through list of threads. Here the lock
being held as reader with irqs disabled. The tasklist_lock is never
write_locked in interrupt context so it is safe to not disable interrupts
for the duration of read_lock in this block which, can be significant,
given the block of code iterates through all threads. Hence changing the
code to call read_lock() and read_unlock() instead of read_lock_irqsave()
and read_unlock_irqrestore(). Seen the same change is made in
"kernel/tracepoint.c" and "kernel/sched/core.c"files with commit ids
'commit 8063e41d2ffc ("tracing: Change syscall_*regfunc() to check
PF_KTHREAD and use for_each_process_thread()")'
'commit 3472eaa1f12e ("sched: normalize_rt_tasks(): Don't use _irqsave
for tasklist_lock, use task_rq_lock()")'

Done basic testing by enabling function graph tracing in x86_64.

Signed-off-by: Soumya PN <soumya.p.n@xxxxxxx>
---
 kernel/trace/ftrace.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index b1870fb..45bc72f 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5713,7 +5713,6 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
 {
 	int i;
 	int ret = 0;
-	unsigned long flags;
 	int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
 	struct task_struct *g, *t;
 
@@ -5728,8 +5727,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
 			goto free;
 		}
 	}
-
-	read_lock_irqsave(&tasklist_lock, flags);
+	read_lock(&tasklist_lock);
 	do_each_thread(g, t) {
 		if (start == end) {
 			ret = -EAGAIN;
@@ -5747,7 +5745,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
 	} while_each_thread(g, t);
 
 unlock:
-	read_unlock_irqrestore(&tasklist_lock, flags);
+	read_unlock(&tasklist_lock);
 free:
 	for (i = start; i < end; i++)
 		kfree(ret_stack_list[i]);
-- 
1.8.3.1