Re: [PATCH v3] ring-buffer: Race when writing and swapping cpu buffer in parallel

["Petr Mládek" <pmladek@suse.cz>]

On Fri 2014-07-18 17:34:43, Petr Mládek wrote:
> On Wed 2014-07-16 12:43:56, Steven Rostedt wrote:
> > On Wed, 16 Jul 2014 10:58:04 +0200
> > Petr Mladek <pmladek@suse.cz> wrote:
> > 
> > 
> > > +/**
> > > + * ring_buffer_swap_cpu - swap a CPU buffer between two ring buffers
> > > + * @buffer_a: One buffer to swap with
> > > + * @buffer_b: The other buffer to swap with
> > > + *
> > > + * This function is useful for tracers that want to take a "snapshot"
> > > + * of a CPU buffer and has another back up buffer lying around.
> > > + * It is expected that the tracer handles the cpu buffer not being
> > > + * used at the moment.
> > > + */
> > > +int ring_buffer_swap_cpu(struct ring_buffer *buffer_a,
> > > +			 struct ring_buffer *buffer_b, int cpu)
> > > +{
> > > +	struct ring_buffer_swap_info rb_swap_info = {
> > > +		.buffer_a = buffer_a,
> > > +		.buffer_b = buffer_b,
> > > +	};
> > > +	int ret;
> > > +
> > > +	/*
> > > +	 * Swap the CPU buffer on the same CPU. Recording has to be fast
> > > +	 * and and this helps to avoid memory barriers.
> > > +	 */
> > > +	ret = smp_call_function_single(cpu, ring_buffer_swap_this_cpu,
> > > +				       (void *)&rb_swap_info, 1);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	return rb_swap_info.ret;
> > 
> > We need to check if the cpu is on the current CPU and if so, just call
> > the function directly. Otherwise this can't be done from interrupt
> > disabled context.
> 
> I see, my testing was not good enough :-(
> 
> So, I tried to use:
> 
> 	if (cpu == smp_processor_id())
> 		ring_buffer_swap_this_cpu(&rb_swap_info);
> 	else
> 		ret = smp_call_function_single(cpu, ring_buffer_swap_this_cpu,
> 					       (void *)&rb_swap_info, 1);
> 
> It solved the problem with enabled IRQSOFF_TRACER and
> FTRACE_STARTUP_TEST because there the swap was called from the same CPU.
> 
> But there is still the problem when the function is called from another
> CPU. I manage to trigger it by:
> 
>      echo 1 >/sys/kernel/debug/tracing/per_cpu/cpu0/snapshot
> 
> It produces:
> 
> [ 1594.060650] ------------[ cut here ]------------
> [ 1594.060664] WARNING: CPU: 3 PID: 1558 at kernel/smp.c:242 smp_call_function_single+0xa4/0xb0()
> [ 1594.060666] Modules linked in:
> [ 1594.060673] CPU: 3 PID: 1558 Comm: bash Not tainted 3.16.0-rc1-2-default+ #2404
> [ 1594.060676] Hardware name: Intel Corporation S2600CP/S2600CP, BIOS RMLSDP.86I.R3.27.D685.1305151734 05/15/2013
> [ 1594.060679]  00000000000000f2 ffff880815b93db8 ffffffff818d34e6 ffff880815b93df8
> [ 1594.060685]  ffffffff810cf28c ffff880813658150 0000000000000001 ffff880815b93e48
> [ 1594.060691]  ffffffff8118b7e0 0000000000000000 0000000000000002 ffff880815b93e08
> [ 1594.060696] Call Trace:
> [ 1594.060705]  [<ffffffff818d34e6>] dump_stack+0x6a/0x7c
> [ 1594.060713]  [<ffffffff810cf28c>] warn_slowpath_common+0x8c/0xc0
> [ 1594.060720]  [<ffffffff8118b7e0>] ? ring_buffer_size+0x40/0x40
> [ 1594.060725]  [<ffffffff810cf2da>] warn_slowpath_null+0x1a/0x20
> [ 1594.060730]  [<ffffffff81149cc4>] smp_call_function_single+0xa4/0xb0
> [ 1594.060735]  [<ffffffff8118c72f>] ring_buffer_swap_cpu+0x5f/0x70
> [ 1594.060742]  [<ffffffff811981ea>] update_max_tr_single+0x8a/0x180
> [ 1594.060747]  [<ffffffff8119843a>] tracing_snapshot_write+0x15a/0x1a0
> [ 1594.060754]  [<ffffffff8123cf95>] vfs_write+0xd5/0x180
> [ 1594.060759]  [<ffffffff8123d969>] SyS_write+0x59/0xc0
> [ 1594.060766]  [<ffffffff818d8569>] system_call_fastpath+0x16/0x1b
> [ 1594.060769] ---[ end trace 662a3aa81711f30e ]---
> 
> 
> No clever idea comes to my mind now. Maybe Monday will bring some
> fresh thinking.
> 
> I think about using IPI but this is what smp_call_function_single()
> does and it warns about possible deadlocks. I am not sure if it is
> because it is a generic function or if it is dangerous even in this
> particular situation.

I have two more ideas but both are ugly :-(


1. I wonder if we really need to call ring_buffer_swap_cpu() with IRQs
   disabled. It is used "only" in update_max_tr_single().

   The disabled IRQs might be needed only inside __update_max_tr()
   when we do something with "current" task.

   Otherwise, update_max_tr_single() is already called with IRQs
   disabled from:

       + tracing_snapshot_write() - here the IRQs are disabled only to
		call the function update_max_tr_single()/

       + check_critical_timing() - it looks to me the IRQs could get
		enabled before calling update_max_tr_single()



2. Go back, do the swap on any CPU, and do memory barriers via IPI.

   I wonder if the needed memory barrier in rb_reserve_next_event()
   could be avoided by calling IPI from ring_buffer_swap_cpu().

   I mean that rb_reserve_next_event() will include the current check
   for swapped ring buffer without barriers. But
   ring_buffer_swap_cpu() will interrupt the affected CPU and
   basically do the barrier there only when needed.

   But I am not sure how this is different from calling
   smp_call_function_single() from ring_buffer_swap_cpu().
   And I am back on the question why it is dangerous with disabled
   interrupts. I can't find any clue in git history. And I miss this
   part of the picture :-(


Any pointers or ideas are welcome.


Best Regards,
Petr