Re: [v2 PATCH] arm64: replace read_lock to rcu lock in call_break_hook

[Shi, Yang]

On 10/1/2015 3:15 PM, Shi, Yang wrote:
> On 10/1/2015 2:27 PM, Paul E. McKenney wrote:
> > On Thu, Oct 01, 2015 at 01:53:51PM -0700, Shi, Yang wrote:
> > > On 10/1/2015 10:08 AM, Steven Rostedt wrote:
> > > > On Thu,  1 Oct 2015 09:37:37 -0700
> > > > Yang Shi <yang.shi@xxxxxxxxxx> wrote:
> > > >
> > > > > BUG: sleeping function called from invalid context at
> > > > > kernel/locking/rtmutex.c:917
> > > > > in_atomic(): 0, irqs_disabled(): 128, pid: 342, name: perf
> > > > > 1 lock held by perf/342:
> > > > >   #0:  (break_hook_lock){+.+...}, at: [<ffffffc0000851ac>]
> > > > > call_break_hook+0x34/0xd0
> > > > > irq event stamp: 62224
> > > > > hardirqs last  enabled at (62223): [<ffffffc00010b7bc>]
> > > > > __call_rcu.constprop.59+0x104/0x270
> > > > > hardirqs last disabled at (62224): [<ffffffc0000fbe20>]
> > > > > vprintk_emit+0x68/0x640
> > > > > softirqs last  enabled at (0): [<ffffffc000097928>]
> > > > > copy_process.part.8+0x428/0x17f8
> > > > > softirqs last disabled at (0): [<          (null)>]           (null)
> > > > > CPU: 0 PID: 342 Comm: perf Not tainted 4.1.6-rt5 #4
> > > > > Hardware name: linux,dummy-virt (DT)
> > > > > Call trace:
> > > > > [<ffffffc000089968>] dump_backtrace+0x0/0x128
> > > > > [<ffffffc000089ab0>] show_stack+0x20/0x30
> > > > > [<ffffffc0007030d0>] dump_stack+0x7c/0xa0
> > > > > [<ffffffc0000c878c>] ___might_sleep+0x174/0x260
> > > > > [<ffffffc000708ac8>] __rt_spin_lock+0x28/0x40
> > > > > [<ffffffc000708db0>] rt_read_lock+0x60/0x80
> > > > > [<ffffffc0000851a8>] call_break_hook+0x30/0xd0
> > > > > [<ffffffc000085a70>] brk_handler+0x30/0x98
> > > > > [<ffffffc000082248>] do_debug_exception+0x50/0xb8
> > > > > Exception stack(0xffffffc00514fe30 to 0xffffffc00514ff50)
> > > > > fe20:                                     00000000 00000000
> > > > > c1594680 0000007f
> > > > > fe40: ffffffff ffffffff 92063940 0000007f 0550dcd8 ffffffc0
> > > > > 00000000 00000000
> > > > > fe60: 0514fe70 ffffffc0 000be1f8 ffffffc0 0514feb0 ffffffc0
> > > > > 0008948c ffffffc0
> > > > > fe80: 00000004 00000000 0514fed0 ffffffc0 ffffffff ffffffff
> > > > > 9282a948 0000007f
> > > > > fea0: 00000000 00000000 9282b708 0000007f c1592820 0000007f
> > > > > 00083914 ffffffc0
> > > > > fec0: 00000000 00000000 00000010 00000000 00000064 00000000
> > > > > 00000001 00000000
> > > > > fee0: 005101e0 00000000 c1594680 0000007f c1594740 0000007f
> > > > > ffffffd8 ffffff80
> > > > > ff00: 00000000 00000000 00000000 00000000 c1594770 0000007f
> > > > > c1594770 0000007f
> > > > > ff20: 00665e10 00000000 7f7f7f7f 7f7f7f7f 01010101 01010101
> > > > > 00000000 00000000
> > > > > ff40: 928e4cc0 0000007f 91ff11e8 0000007f
> > > > >
> > > > > call_break_hook is called in atomic context (hard irq disabled), so
> > > > > replace
> > > > > the sleepable lock to rcu lock and replace relevant list operations
> > > > > to rcu
> > > > > version.
> > > > >
> > > > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> > > > > ---
> > > > > v1-> v2
> > > > > Replace list operations to rcu version.
> > > > >
> > > > >   arch/arm64/kernel/debug-monitors.c | 10 +++++-----
> > > > >   1 file changed, 5 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/arch/arm64/kernel/debug-monitors.c
> > > > > b/arch/arm64/kernel/debug-monitors.c
> > > > > index cebf786..cf0e4fc 100644
> > > > > --- a/arch/arm64/kernel/debug-monitors.c
> > > > > +++ b/arch/arm64/kernel/debug-monitors.c
> > > > > @@ -276,14 +276,14 @@ static DEFINE_RWLOCK(break_hook_lock);
> > > > >   void register_break_hook(struct break_hook *hook)
> > > > >   {
> > > > >       write_lock(&break_hook_lock);
> > > > > -    list_add(&hook->node, &break_hook);
> > > > > +    list_add_rcu(&hook->node, &break_hook);
> > > > >       write_unlock(&break_hook_lock);
> > > > >   }
> > > > >
> > > > >   void unregister_break_hook(struct break_hook *hook)
> > > > >   {
> > > > >       write_lock(&break_hook_lock);
> > > > > -    list_del(&hook->node);
> > > > > +    list_del_rcu(&hook->node);
> > > > >       write_unlock(&break_hook_lock);
> > > > >   }
> > > >
> > > > Shouldn't there be a synchronize_rcu() somewhere?
> > >
> > > So far kgdb is the only user of unregister_break_hook in mainline
> > > kernel.
> > >
> > > Just read Documentation/RCU/checklist.txt, it says:
> > >
> > > Note that synchronize_rcu() -only- guarantees to wait until
> > > all currently executing rcu_read_lock()-protected RCU read-side
> > > critical sections complete.
> > >
> > > For kgdb, the unregister is just called in kgdb_arch_exit by
> > > kgdb_unregister_io_module, which is called when rmmod kgdb module.
> > >
> > > The break point handler is done synchronously. So, it sounds should
> > > be not a problem without calling synchronize_rcu().
> >
> > OK, I will bite...  What does "synchronously" mean here?  Unless you
> > have somehow guaranteed that all current readers in call_break_hook()
> > are done between the time you call unregister_break_hook() to remove a
> > given break_hook structure and the time you call register_break_hook()
> > to add that same structure back in, you have a problem.
>
> For kgdb usecase, this might be guaranteed.
>
> Generally, kgdb module is loaded then register_break_hook() is called.
> Then connect kgdb from host or via kdb, set breakpoint, wait for the
> break point is hit, run some commands to debug. Then finish debug, rmmod
> kgdb which will call unregister_break_hook().
>
> It sounds the current readers in call_break_hook() could be done during
> the time otherwise I won't be able to continue my debug when break point
> is hit.
>
> > What you have now only protects against invoking register_break_hook()
> > on newly allocated and initialized break_hook structure.  But the only
> > calls to register_break_hook() that I see in v4.2 use compile-time
> > initialized structures.  So the only failure from using non-RCU list
> > primitives would be due to the list_head's ->next pointer initialization.
> > This could momentarily make the list appear to have only the new element,
> > but not the old element.
> >
> > Unless you do a series of register_break_hook() and
> > unregister_break_hook()
> > calls, in which case a previously deleted structure could momentarily
> > appear to already (or still) be in the list.

This might be a problem. Just thought of the below senario.

1. load kgdb module
2. do some debugging
3. unload kgdb module <-- the hook pointer may be still in the list
4. load kgdb module again <-- there may be two hook pointers
5. do some debugging <-- it may call them twice

Although my test didn't catch this problem, it still sounds like a 
potential issue.

Preparing for v3 with synchronize_rcu() added.

Thanks,
Yang

> They are called in series:
>
> In kgdb_arch_init
>      register_break_hook(&kgdb_brkpt_hook);
>          register_break_hook(&kgdb_compiled_brkpt_hook);
>
> In kgdb_arch_exit
>      unregister_break_hook(&kgdb_brkpt_hook);
>          unregister_break_hook(&kgdb_compiled_brkpt_hook);
>
> Yang
>
>
>
> > Are those the sorts of failures you are seeing?
> >
> >                             Thanx, Paul
> >
> > > Yang
> > >
> > > > -- Steve
> > > >
> > > > > @@ -292,11 +292,11 @@ static int call_break_hook(struct pt_regs
> > > > > *regs, unsigned int esr)
> > > > >       struct break_hook *hook;
> > > > >       int (*fn)(struct pt_regs *regs, unsigned int esr) = NULL;
> > > > >
> > > > > -    read_lock(&break_hook_lock);
> > > > > -    list_for_each_entry(hook, &break_hook, node)
> > > > > +    rcu_read_lock();
> > > > > +    list_for_each_entry_rcu(hook, &break_hook, node)
> > > > >           if ((esr & hook->esr_mask) == hook->esr_val)
> > > > >               fn = hook->fn;
> > > > > -    read_unlock(&break_hook_lock);
> > > > > +    rcu_read_unlock();
> > > > >
> > > > >       return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
> > > > >   }

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