Messages in this thread | | | Date | Thu, 25 Feb 2016 07:37:24 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC v2 0/6] Track RCU dereferences in RCU read-side critical sections |
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On Thu, Feb 25, 2016 at 03:32:43PM +0100, Peter Zijlstra wrote: > On Tue, Feb 16, 2016 at 01:57:39PM +0800, Boqun Feng wrote: > > As a characteristic of RCU, read-side critical sections have a very > > loose connection with rcu_dereference()s, which is you can only be sure > > about an rcu_dereference() might be called in some read-side critical > > section, but if code gets complex, you may not be sure which read-side > > critical section exactly, this might be also an problem for some other > > locking mechanisms, that is the critical sections protecting data and > > the data accesses protected are not clearly correlated. > > > > In this series, we are introducing LOCKED_ACCESS framework and based on > > which, we implement the RCU_LOCKED_ACCESS functionality to give us a > > clear hint: which rcu_dereference() happens in which RCU read-side > > critical section. > > > > After this series applied, and if CONFIG_RCU_LOCKED_ACCESS=y, the proc > > file /proc/locked_access/rcu will show all relationships collected so > > far for rcu_read_lock() and their friends and rcu_dereference*(). > > But why !? What does this bring us, why do I want to even look at these > patches?
There were some complaints about the difficulty of figuring out what was being protected by a given rcu_read_lock() in cases where the corresponding rcu_dereference() is several function calls down, and especially in cases where the function calls are via pointers. These cases show up in a number of places, perhaps most prominently in networking.
Boqun's patches therefore use lockdep to make an association between each rcu_dereference() and the rcu_read_lock() protecting it.
Seem reasonable, or were the complaints just a flash in the pan?
Thanx, Paul
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