Messages in this thread | | | Date | Sun, 26 Jun 2011 03:55:03 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 0/3 v3] rcu: Detect rcu uses under extended quiescent state |
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On Sat, Jun 25, 2011 at 06:13:15PM -0700, Paul E. McKenney wrote: > 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?
Ah right. In fact rcu_read_lock_heald() shouldn't expect to have preemption disabled, at least not in PREEMPT_RCU.
So yeah, looks good.
> > See below for the splat due to tracing while in dyntick-idle mode. > Might this explain some otherwise mysterious crashes when tracing is > enabled?
May be.
So this is using a tracepoint in dynticks idle mode. There are various ways to solve this:
- move the tracepoint call out of that place, in an rcu safe place - call rcu_exit_nohz() / rcu_enter_nohz() there. But we need to know if the tracepoint if activated before that, or this will impact the tracing off case too. - split out the rcu extended qs from tick stop logic (https://patchwork.kernel.org/patch/850542/) That looks like a big change just to fix such a bug but anyway it is going to be needed for the nohz cpuset patches I'm working on. Once that's split, rcu_enter_nohz() can be called later after the tick has been stopped, like right before we hlt the cpu.
> > Thanx, Paul > > ------------------------------------------------------------------------ > > [ 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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