Messages in this thread | | | Date | Thu, 5 Jan 2012 02:45:20 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH] rcu: Improve detection of illegal synchronize_rcu() call from RCU read side |
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On Wed, Jan 04, 2012 at 01:30:35PM -0800, Paul E. McKenney wrote: > On Wed, Jan 04, 2012 at 08:03:39PM +0100, Frederic Weisbecker wrote: > > Actually for the case of RCU, the wait_for_completion() called by synchronize_rcu() > > has a might_sleep() call that triggers a warning in this case. > > > > But in the case of SMP with 1 online CPU, the rcu_blocking_is_gp() > > checks returns right away on rcutree. So probably we need this? > > I modified this to push the might_sleep() down into the > rcu_blocking_is_gp() function, queued the result, and retained your > Signed-off-by. (Please let me know if there is any problem with this.) > > This does work for TREE_PREEMPT_RCU and for synchronize_rcu_bh() in > TREE_RCU, but not for synchronize_sched() in TREE_RCU. This is because > rcu_read_lock() and rcu_read_unlock() are no-ops in the TREE_RCU case.
Not sure about that. This calls preempt_disable() which, in any case with CONFIG_DEBUG_ATOMIC_SLEEP, handles the preempt count. And that even if !CONFIG_PREEMPT.
> > So I queued up a separate patch using rcu_lockdep_assert() to check for > illegal RCU grace period within the same-type RCU read-side critical > section, including for SRCU. This is also a partial solution, as it > does not handle things like this: > > void foo(void) > { > mutex_lock(&my_mutex); > . . . > synchronize_srcu(&my_srcu); > . . . > mutex_unlock(&my_mutex); > } > > void bar(void) > { > int idx; > > idx = rcu_read_lock(&m_srcu); > . . . > mutex_lock(&my_mutex); > . . . > mutex_unlock(&my_mutex); > . . . > srcu_read_unlock(&m_srcu, idx); > } > > This can be extended into a chain of locks and a chain of SRCU instances. > For an example of the latter, consider an SRCU-A read-side critical > section containing an SRCU-B grace period, an SRCU-B read-side critical > section containing an SRCU-C grace period, and so on, with the SRCU-Z > read-side critical section containing an RCU-A grace period.
Heh! Indeed...
> But it > is OK to hold a mutex across one SRCU read-side critical section while > acquiring that same mutex within another same-flavor SRCU read-side > critical section. So the analogy with reader-writer locking only goes > so far. > > At the moment, a full solution seems to require some surgery on lockdep > itself, but perhaps there is a better way.
Ok.
> > > rcutiny seems to be fine with the cond_resched() call, but srcu needs > > a special treatment. > > For the moment, I just applied rcu_lockdep_assert() everywhere -- zero > cost on non-lockdep kernels, and fully handles all of the RCU simple > self-deadlock cases.
So, for RCU I'm not sure this is useful given the might_sleep() things. But for srcu it is.
Thanks.
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