Commit 6112a300 authored May 17, 2016 by Soumya PN Committed by Steven Rostedt May 20, 2016

ftrace: Don't disable irqs when taking the tasklist_lock read_lock

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 a list of threads. Here the lock
is 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().

A similar change was made in commits: 8063e41d ("tracing: Change
syscall_*regfunc() to check PF_KTHREAD and use for_each_process_thread()")'
and 3472eaa1 ("sched: normalize_rt_tasks(): Don't use _irqsave for
tasklist_lock, use task_rq_lock()")'

Link: http://lkml.kernel.org/r/1463500874-77480-1-git-send-email-soumya.p.n@hpe.com

Signed-off-by: Soumya PN <soumya.p.n@hpe.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

parent 91e6f1ce

kernel/trace/ftrace.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -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;

		@@ -5729,7 +5728,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
		}
		}

		read_lock_irqsave(&tasklist_lock, flags);
		read_lock(&tasklist_lock);
		do_each_thread(g, t) {
		if (start == end) {
		ret = -EAGAIN;
		@@ -5747,7 +5746,7 @@ 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]);

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