Check for use of RCU from dyntick-idle mode

Hello!

This set of patches adds checks for use of RCU from "extended quiescent states" such as dyntick-idle mode. Such use is grossly illegal because RCU by definition ignores CPUs that are in extended quiescent states. In the case of dyntick-idle mode, the only way for RCU to avoid ignoring such CPUs would be to wake them up periodically, which would defeat the whole purpose of dyntick-idle mode.

The good news is that Frederic got this effort started. The bad news is that there are several cases where RCU read-side critical sections appear in dyntick-idle mode. Frederic is working on a solution, but because there is no point in giving warnings in cases where there is no known fix, I will not push these patches until a fix is available. (The most likely fix is to move the code that informs RCU of entry to and exit from dyntick-idle mode into the arch-specific idle loops, which would legitimize current RCU usage -- and potentially simplify RCU's quiescent-state detection.)

The individual patches are as follows:

1. Make rcu_read_lock_held(), rcu_read_lock_sched_held(),
and rcu_read_lock_bh_held() invoke a new rcu_check_extended_qs() function to check for calls from extended quiescent states, courtesy of Frederic Weisbecker.
2. Make lockdep RCU splats call out use from within extended
quiescent states, also courtesy of Frederic Weisbecker.
3. Make rcu_read_lock(), rcu_read_lock_sched(), and
rcu_read_lock_bh() also check for (illegal) use from within an extended quiescent state, again courtesy of Frederic Weisbecker.
4. Remove an unnecessary level of abstraction from the PROVE_RCU
checking.
5. Warn when srcu_read_lock() and srcu_read_lock_held() are called
from within an extended quiescent state.
6. Make srcu_read_lock_held() invoke the debug_lockdep_rcu_enabled()
common function that determines whether or not RCU lockdep splats are appropriate.

These changes are also available on the subject-to-rebase branch in the git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git eqscheck.2011.07.08a

Thanx, Paul

------------------------------------------------------------------------

 b/include/linux/rcupdate.h |   36 +++++++++++++++
 b/include/linux/srcu.h     |   25 ++++++----
 b/kernel/lockdep.c         |   20 ++++++++
 b/kernel/rcupdate.c        |   17 ++++++-
 b/kernel/rcutiny.c         |   14 ++++++
 b/kernel/rcutree.c         |   16 ++++++
 include/linux/rcupdate.h   |  105 +++++++++++++++++++++------------------------
 include/linux/srcu.h       |    5 +-
 8 files changed, 171 insertions(+), 67 deletions(-)