lkml.org 
[lkml]   [2012]   [Jan]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] rcu: Improve detection of illegal synchronize_rcu() call from RCU read side
    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.


    \
     
     \ /
      Last update: 2012-01-05 02:47    [W:0.036 / U:0.308 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site