Messages in this thread | | | Date | Wed, 18 Apr 2012 07:11:55 -0700 | From | "Paul E. McKenney" <> | Subject | Re: New RCU related warning due to rcu_preempt_depth() changes |
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On Wed, Apr 18, 2012 at 07:29:19AM +0200, Sasha Levin wrote: > On Tue, Apr 17, 2012 at 6:45 PM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > On Tue, Apr 17, 2012 at 08:53:16AM -0700, Paul E. McKenney wrote: > >> On Tue, Apr 17, 2012 at 05:36:59PM +0200, Sasha Levin wrote: > >> > On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney > >> > <paulmck@linux.vnet.ibm.com> wrote: > >> > > On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote: > >> > >> Hi Paul, > >> > >> > >> > >> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and > >> > >> __rcu_read_unlock() to per-CPU variables") is causing the following > >> > >> warning (I've added the extra fields on the second line): > >> > >> > >> > >> [ 77.330920] BUG: sleeping function called from invalid context at > >> > >> mm/memory.c:3933 > >> > >> [ 77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0, > >> > >> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity > >> > >> [ 77.344135] no locks held by trinity/5669. > >> > >> [ 77.349644] Pid: 5669, comm: trinity Tainted: G W > >> > >> 3.4.0-rc3-next-20120417-sasha-dirty #83 > >> > >> [ 77.354401] Call Trace: > >> > >> [ 77.355956] [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210 > >> > >> [ 77.358811] [<ffffffff81198eaf>] might_fault+0x2f/0xa0 > >> > >> [ 77.361997] [<ffffffff810e3228>] schedule_tail+0x88/0xb0 > >> > >> [ 77.364671] [<ffffffff826a01d3>] ret_from_fork+0x13/0x80 > >> > >> > >> > >> As you can see, rcu_preempt_depth() returns 1 when running in that > >> > >> context, which looks pretty odd. > >> > > > >> > > Ouch!!! > >> > > > >> > > So it looks like I missed a place where I need to save and restore > >> > > the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special > >> > > variables. My (probably hopelessly naive) guess is that I need to add > >> > > a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to > >> > > make rcu_switch_from() take the task_struct pointer as an argument, > >> > > passing in prev. > >> > > > >> > > Does this make sense, or am I still missing something here? > >> > > >> > I've let the test run for a bit more, and it appears that I'm getting > >> > this warning from lots of different sources, would this > >> > schedule_tail() fix all of them? > >> > >> If I understand the failure correctly, yes. If the task switches without > >> RCU paying attention, the nesting count for both the outgoing and the > >> incoming tasks can get messed up. The messed-up counts could easily > >> cause problems downstream. > >> > >> Of course, there might well be additional bugs. > >> > >> I will put a speculative patch together and send it along. > > > > And here it is, testing just started. > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > rcu: Add RCU context switching to schedule_tail() > > > > The new rcu_read_lock_nesting and rcu_read_unlock_special per-CPU > > variables must be saved and restored at every context switch, including > > those involving schedule_tail(). This commit therefore adds the saving > > and restoring to schedul_tail(). > > > > Reported-by: Sasha Levin <levinsasha928@gmail.com> > > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > Looks good here.
Very good! I have added your Tested-by.
Thanx, Paul
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