Messages in this thread | | | Date | Fri, 9 May 2008 18:38:23 -0400 | From | Mathieu Desnoyers <> | Subject | Re: Using markers w/ preemptible RCU in early code |
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* Eduard - Gabriel Munteanu (eduard.munteanu@linux360.ro) wrote: > Hello, > > I'm working on the kmemtrace GSoC project and I'm having a little > problem. > > My code inserts probes during early code (just after kmem_cache_init()). > This works on the classic RCU. But on the preemptible RCU, this > triggers BUG()s (supposedly on each probe, there are two), although the > kernel runs fine: > > > BUG: scheduling while atomic: swapper/0/0x00000002 > > Pid: 0, comm: swapper Not tainted 2.6.25-00002-gf80e324-dirty #16 > > > > Call Trace: > > [<ffffffff802365f5>] __schedule_bug+0x65/0x70 > > [<ffffffff804f8bb0>] thread_return+0x346/0x566 > > [<ffffffff802734ba>] ? get_marker+0x23a/0x260 > > [<ffffffff8025fbc6>] ? put_online_cpus+0x46/0x70 > > [<ffffffff80270c88>] __synchronize_sched+0x48/0x80 > > [<ffffffff802741a2>] marker_probe_register+0x152/0x660 > > [<ffffffff8029af10>] ? kmemtrace_probe_alloc+0x0/0x1b0 > > [<ffffffff8029ad6d>] kmemtrace_init+0x4d/0xd0 > > [<ffffffff80693c05>] start_kernel+0x205/0x300 > > [<ffffffff806931b2>] _sinittext+0x1b2/0x200 > > The is triggered by the following code in marker_probe_register(): > #ifdef CONFIG_PREEMPT_RCU > synchronize_sched(); /* Until we have the call_rcu_sched() */ > #endif > > Since preemption and SMP are disabled during early code, we could do > something like: > diff --git a/kernel/marker.c b/kernel/marker.c > index 005b959..84964dc 100644 > --- a/kernel/marker.c > +++ b/kernel/marker.c > @@ -23,6 +23,7 @@ > #include <linux/rcupdate.h> > #include <linux/marker.h> > #include <linux/err.h> > +#include <linux/hardirq.h> > > extern struct marker __start___markers[]; > extern struct marker __stop___markers[]; > @@ -672,7 +673,9 @@ int marker_probe_register(const char *name, const char *format, > /* write rcu_pending before calling the RCU callback */ > smp_wmb(); > #ifdef CONFIG_PREEMPT_RCU > - synchronize_sched(); /* Until we have the call_rcu_sched() */ > + /* We are not preemptible when registering probes in early code */ > + if (likely(preemptible())) > + synchronize_sched(); /* Until we have the call_rcu_sched() */ > #endif > call_rcu(&entry->rcu, free_old_closure);
I think call_rcu can become a problem too in that case. It is responsible for freeing the old closure and I don't think it will be executed if rcu or the scheduler is not active. Therefore, we should also do something like :
if (likely(preemptible())) call_rcu(&entry->rcu, free_old_closure); else free_old_closure(&entry->rcu); (modulo passing the right parameters..)
All the synchronize_sched, cal_rcu and rcu_barriers should be changed. I think the rcu_barriers should also have this kind of test to check for early boot use.
Mathieu
> end: > > Is this the best approach? marker_probe_register() isn't a hot path, so > that check won't mess up the performance. Anyway, this works for me. > > > Eduard > > (forgot LKML, added now.) >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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