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Message-ID: <20120502213238.GA12153@linux.vnet.ibm.com>
Date: Wed, 2 May 2012 14:32:38 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Hugh Dickins <hughd@...gle.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>,
"Paul E. McKenney" <paul.mckenney@...aro.org>,
linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: linux-next ppc64: RCU mods cause __might_sleep BUGs
On Wed, May 02, 2012 at 01:49:54PM -0700, Paul E. McKenney wrote:
> On Wed, May 02, 2012 at 01:25:30PM -0700, Hugh Dickins wrote:
> > On Tue, 1 May 2012, Paul E. McKenney wrote:
> > > > > > > On Mon, 2012-04-30 at 15:37 -0700, Hugh Dickins wrote:
> > > > > > > >
> > > > > > > > BUG: sleeping function called from invalid context at include/linux/pagemap.h:354
> > > > > > > > in_atomic(): 0, irqs_disabled(): 0, pid: 6886, name: cc1
> > > > > > > > Call Trace:
> > > > > > > > [c0000001a99f78e0] [c00000000000f34c] .show_stack+0x6c/0x16c (unreliable)
> > > > > > > > [c0000001a99f7990] [c000000000077b40] .__might_sleep+0x11c/0x134
> > > > > > > > [c0000001a99f7a10] [c0000000000c6228] .filemap_fault+0x1fc/0x494
> > > > > > > > [c0000001a99f7af0] [c0000000000e7c9c] .__do_fault+0x120/0x684
> > > > > > > > [c0000001a99f7c00] [c000000000025790] .do_page_fault+0x458/0x664
> > > > > > > > [c0000001a99f7e30] [c000000000005868] handle_page_fault+0x10/0x30
> >
> > Got it at last. Embarrassingly obvious. __rcu_read_lock() and
> > __rcu_read_unlock() are not safe to be using __this_cpu operations,
> > the cpu may change in between the rmw's read and write: they should
> > be using this_cpu operations (or, I put preempt_disable/enable in the
> > __rcu_read_unlock below). __this_cpus there work out fine on x86,
> > which was given good instructions to use; but not so well on PowerPC.
>
> Thank you very much for tracking this down!!!
>
> > I've been running successfully for an hour now with the patch below;
> > but I expect you'll want to consider the tradeoffs, and may choose a
> > different solution.
>
> The thing that puzzles me about this is that the normal path through
> the scheduler would save and restore these per-CPU variables to and
> from the task structure. There must be a sneak path through the
> scheduler that I failed to account for.
Sigh... I am slow today, I guess. The preemption could of course
happen between the time that the task calculated the address of the
per-CPU variable and the time that it modified it. If this happens,
we are modifying some other CPU's per-CPU variable.
Given that Linus saw no performance benefit from this patchset, I am
strongly tempted to just drop this inlinable-__rcu_read_lock() series
at this point.
I suppose that the other option is to move preempt_count() to a
per-CPU variable, then use the space in the task_info struct.
But that didn't generate anywhere near as good of code...
Thanx, Paul
> But given your good work, this should be easy for me to chase down
> even on my x86-based laptop -- just convert from __this_cpu_inc() to a
> read-inc-delay-write sequence. And check that the underlying variable
> didn't change in the meantime. And dump an ftrace if it did. ;-)
>
> Thank you again, Hugh!
>
> Thanx, Paul
>
> > Hugh
> >
> > --- 3.4-rc4-next-20120427/include/linux/rcupdate.h 2012-04-28 09:26:38.000000000 -0700
> > +++ testing/include/linux/rcupdate.h 2012-05-02 11:46:06.000000000 -0700
> > @@ -159,7 +159,7 @@ DECLARE_PER_CPU(struct task_struct *, rc
> > */
> > static inline void __rcu_read_lock(void)
> > {
> > - __this_cpu_inc(rcu_read_lock_nesting);
> > + this_cpu_inc(rcu_read_lock_nesting);
> > barrier(); /* Keep code within RCU read-side critical section. */
> > }
> >
> > --- 3.4-rc4-next-20120427/kernel/rcupdate.c 2012-04-28 09:26:40.000000000 -0700
> > +++ testing/kernel/rcupdate.c 2012-05-02 11:44:13.000000000 -0700
> > @@ -72,6 +72,7 @@ DEFINE_PER_CPU(struct task_struct *, rcu
> > */
> > void __rcu_read_unlock(void)
> > {
> > + preempt_disable();
> > if (__this_cpu_read(rcu_read_lock_nesting) != 1)
> > __this_cpu_dec(rcu_read_lock_nesting);
> > else {
> > @@ -83,13 +84,14 @@ void __rcu_read_unlock(void)
> > barrier(); /* ->rcu_read_unlock_special load before assign */
> > __this_cpu_write(rcu_read_lock_nesting, 0);
> > }
> > -#ifdef CONFIG_PROVE_LOCKING
> > +#if 1 /* CONFIG_PROVE_LOCKING */
> > {
> > int rln = __this_cpu_read(rcu_read_lock_nesting);
> >
> > - WARN_ON_ONCE(rln < 0 && rln > INT_MIN / 2);
> > + BUG_ON(rln < 0 && rln > INT_MIN / 2);
> > }
> > #endif /* #ifdef CONFIG_PROVE_LOCKING */
> > + preempt_enable();
> > }
> > EXPORT_SYMBOL_GPL(__rcu_read_unlock);
> >
> > --- 3.4-rc4-next-20120427/kernel/sched/core.c 2012-04-28 09:26:40.000000000 -0700
> > +++ testing/kernel/sched/core.c 2012-05-01 22:40:46.000000000 -0700
> > @@ -2024,7 +2024,7 @@ asmlinkage void schedule_tail(struct tas
> > {
> > struct rq *rq = this_rq();
> >
> > - rcu_switch_from(prev);
> > + /* rcu_switch_from(prev); */
> > rcu_switch_to();
> > finish_task_switch(rq, prev);
> >
> > @@ -7093,6 +7093,10 @@ void __might_sleep(const char *file, int
> > "BUG: sleeping function called from invalid context at %s:%d\n",
> > file, line);
> > printk(KERN_ERR
> > + "cpu=%d preempt_count=%x preempt_offset=%x rcu_nesting=%x nesting_save=%x\n",
> > + raw_smp_processor_id(), preempt_count(), preempt_offset,
> > + rcu_preempt_depth(), current->rcu_read_lock_nesting_save);
> > + printk(KERN_ERR
> > "in_atomic(): %d, irqs_disabled(): %d, pid: %d, name: %s\n",
> > in_atomic(), irqs_disabled(),
> > current->pid, current->comm);
> > --
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