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Tracing uses rcu_irq_enter() as a way to make sure that RCU is watching when it needs to use rcu_read_lock() and friends. This is because tracing can happen as RCU is about to enter user space, or about to go idle, and RCU does not watch for RCU read side critical sections as it makes the transition. There is a small location within the RCU infrastructure that rcu_irq_enter() itself will not work. If tracing were to occur in that section it will break if it tries to use rcu_irq_enter(). Originally, this happens with the stack_tracer, because it will call save_stack_trace when it encounters stack usage that is greater than any stack usage it had encountered previously. There was a case where that happened in the RCU section where rcu_irq_enter() did not work, and lockdep complained loudly about it. To fix it, stack tracing added a call to be disabled and RCU would disable stack tracing during the critical section that rcu_irq_enter() was inoperable. This solution worked, but there are other cases that use rcu_irq_enter() and it would be a good idea to let RCU give a way to let others know that rcu_irq_enter() will not work. For example, in trace events. Another helpful aspect of this change is that it also moves the per cpu variable called in the RCU critical section into a cache locale along with other RCU per cpu variables used in that same location. I'm keeping the stack_trace_disable() code, as that still could be used in the future by places that really need to disable it. And since it's only a static inline, it wont take up any kernel text if it is not used. Link: http://lkml.kernel.org/r/20170405093207.404f8deb@gandalf.local.home Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.