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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:8063e41d2f
("tracing: Change syscall_*regfunc() to check PF_KTHREAD and use for_each_process_thread()")' and3472eaa1f1
("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>
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@ -5713,7 +5713,6 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
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{
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int i;
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int ret = 0;
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unsigned long flags;
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int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
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struct task_struct *g, *t;
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@ -5729,7 +5728,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
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}
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}
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read_lock_irqsave(&tasklist_lock, flags);
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read_lock(&tasklist_lock);
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do_each_thread(g, t) {
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if (start == end) {
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ret = -EAGAIN;
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@ -5747,7 +5746,7 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
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} while_each_thread(g, t);
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unlock:
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read_unlock_irqrestore(&tasklist_lock, flags);
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read_unlock(&tasklist_lock);
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free:
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for (i = start; i < end; i++)
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kfree(ret_stack_list[i]);
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