Messages in this thread | | | Date | Sat, 25 Jun 2011 18:13:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 0/3 v3] rcu: Detect rcu uses under extended quiescent state |
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On Fri, Jun 24, 2011 at 01:20:49PM +0200, Frederic Weisbecker wrote: > On Thu, Jun 23, 2011 at 08:53:11PM -0700, Paul E. McKenney wrote: > > On Fri, Jun 24, 2011 at 01:12:37AM +0200, Frederic Weisbecker wrote: > > > This time I have no current practical cases to fix. Those I fixed > > > in previous versions were actually using rcu_dereference_raw(), which > > > is legal in extended qs. > > > > > > Frederic Weisbecker (3): > > > rcu: Detect illegal rcu dereference in extended quiescent state > > > rcu: Inform the user about dynticks idle mode on PROVE_RCU warning > > > rcu: Warn when rcu_read_lock() is used in extended quiescent state > > > > > > include/linux/rcupdate.h | 68 +++++++++++++++++++++++++++++++++++++++------- > > > kernel/lockdep.c | 4 +++ > > > kernel/rcupdate.c | 4 +++ > > > kernel/rcutiny.c | 13 +++++++++ > > > kernel/rcutree.c | 14 +++++++++ > > > 5 files changed, 93 insertions(+), 10 deletions(-) > > > > Queued, thank you, Frederic! > > > > I have also applied your approach to SRCU, and I applied the following > > to simplify the code a bit -- please let me know if there are any > > problems with this approach. > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > rcu: Remove one layer of abstraction from PROVE_RCU checking > > > > Simplify things a bit by substituting the definitions of the single-line > > rcu_read_acquire(), rcu_read_release(), rcu_read_acquire_bh(), > > rcu_read_release_bh(), rcu_read_acquire_sched(), and > > rcu_read_release_sched() functions at their call points. > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > Yeah looks good. Thanks!
And I thought that you might be amused by the following. Hmmm... I wonder how I am going to use event tracing for the portions of RCU that execute while in dyntick-idle mode...
But first... It turns out that rcu_check_extended_qs() is sometimes called with preemption enabled (for example, in CONFIG_TREE_PREEMPT_RCU), which causes smp_processor_id() to complain. One way to fix this would be to write rcu_check_extended_qs() as follows:
bool rcu_check_extended_qs(void) { struct rcu_dynticks *rdtp;
preempt_disable(); rdtp = &__get_cpu_var(rcu_dynticks); if (atomic_read(&rdtp->dynticks) & 0x1) { preempt_enable(); return false; } preempt_enable(); return true; } EXPORT_SYMBOL_GPL(rcu_check_extended_qs);
Does the above make sense, or is there a higher-level bug that should be addressed in a different way?
See below for the splat due to tracing while in dyntick-idle mode. Might this explain some otherwise mysterious crashes when tracing is enabled?
Thanx, Paul
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[ 0.449600] =============================== [ 0.449605] [ INFO: suspicious RCU usage. ] [ 0.449610] ------------------------------- [ 0.449616] /usr/local/autobench/var/tmp/build/arch/powerpc/include/asm/trace.h:122 suspicious rcu_dereference_check() usage! [ 0.449626] [ 0.449627] other info that might help us debug this: [ 0.449628] [ 0.449636] [ 0.449637] rcu_scheduler_active = 1, debug_locks = 0 [ 0.449644] rcu is in extended quiescent state! [ 0.449650] no locks held by kworker/0:0/0. [ 0.449655] [ 0.449656] stack backtrace: [ 0.449662] Call Trace: [ 0.449671] [c0000000e66d7b20] [c00000000001352c] .show_stack+0x70/0x184 (unreliable) [ 0.449684] [c0000000e66d7bd0] [c0000000000b1ef0] .lockdep_rcu_suspicious+0xe8/0x110 [ 0.449697] [c0000000e66d7c70] [c000000000044fc0] .__trace_hcall_exit+0x1e4/0x218 [ 0.449709] [c0000000e66d7d20] [c000000000045c40] .plpar_hcall_norets+0xb4/0xd0 [ 0.449720] [c0000000e66d7d90] [c000000000047cd4] .pseries_dedicated_idle_sleep+0x1b0/0x22c [ 0.449731] [c0000000e66d7e40] [c000000000016004] .cpu_idle+0x144/0x22c [ 0.449743] [c0000000e66d7ed0] [c0000000006572cc] .start_secondary+0x378/0x384 [ 0.449754] [c0000000e66d7f90] [c000000000009268] .start_secondary_prolog+0x10/0x14
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