Re: [patch V3 06/22] bpf/trace: Remove redundant preempt_disable from trace_call_bpf()

[Alexei Starovoitov]

On Mon, Feb 24, 2020 at 09:42:52PM +0100, Thomas Gleixner wrote:
> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:
> > On Mon, Feb 24, 2020 at 03:01:37PM +0100, Thomas Gleixner wrote:
> >> --- a/kernel/trace/bpf_trace.c
> >> +++ b/kernel/trace/bpf_trace.c
> >> @@ -83,7 +83,7 @@ unsigned int trace_call_bpf(struct trace
> >>  	if (in_nmi()) /* not supported yet */
> >>  		return 1;
> >>  
> >> -	preempt_disable();
> >> +	cant_sleep();
> >>  
> >>  	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> >>  		/*
> >> @@ -115,7 +115,6 @@ unsigned int trace_call_bpf(struct trace
> >>  
> >>   out:
> >>  	__this_cpu_dec(bpf_prog_active);
> >> -	preempt_enable();
> >
> > My testing uncovered that above was too aggressive:
> > [   41.533438] BUG: assuming atomic context at kernel/trace/bpf_trace.c:86
> > [   41.534265] in_atomic(): 0, irqs_disabled(): 0, pid: 2348, name: test_progs
> > [   41.536907] Call Trace:
> > [   41.537167]  dump_stack+0x75/0xa0
> > [   41.537546]  __cant_sleep.cold.105+0x8b/0xa3
> > [   41.538018]  ? exit_to_usermode_loop+0x77/0x140
> > [   41.538493]  trace_call_bpf+0x4e/0x2e0
> > [   41.538908]  __uprobe_perf_func.isra.15+0x38f/0x690
> > [   41.539399]  ? probes_profile_seq_show+0x220/0x220
> > [   41.539962]  ? __mutex_lock_slowpath+0x10/0x10
> > [   41.540412]  uprobe_dispatcher+0x5de/0x8f0
> > [   41.540875]  ? uretprobe_dispatcher+0x7c0/0x7c0
> > [   41.541404]  ? down_read_killable+0x200/0x200
> > [   41.541852]  ? __kasan_kmalloc.constprop.6+0xc1/0xd0
> > [   41.542356]  uprobe_notify_resume+0xacf/0x1d60
> 
> Duh. I missed that particular callchain.
> 
> > The following fixes it:
> >
> > commit 7b7b71ff43cc0b15567b60c38a951c8a2cbc97f0 (HEAD -> bpf-next)
> > Author: Alexei Starovoitov <ast@xxxxxxxxxx>
> > Date:   Mon Feb 24 11:27:15 2020 -0800
> >
> >     bpf: disable migration for bpf progs attached to uprobe
> >
> >     trace_call_bpf() no longer disables preemption on its own.
> >     All callers of this function has to do it explicitly.
> >
> >     Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> >
> > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> > index 18d16f3ef980..7581f5eb6091 100644
> > --- a/kernel/trace/trace_uprobe.c
> > +++ b/kernel/trace/trace_uprobe.c
> > @@ -1333,8 +1333,15 @@ static void __uprobe_perf_func(struct trace_uprobe *tu,
> >         int size, esize;
> >         int rctx;
> >
> > -       if (bpf_prog_array_valid(call) && !trace_call_bpf(call, regs))
> > -               return;
> > +       if (bpf_prog_array_valid(call)) {
> > +               u32 ret;
> > +
> > +               migrate_disable();
> > +               ret = trace_call_bpf(call, regs);
> > +               migrate_enable();
> > +               if (!ret)
> > +                       return;
> > +       }
> >
> > But looking at your patch cant_sleep() seems unnecessary strong.
> > Should it be cant_migrate() instead?
> 
> Yes, if we go with the migrate_disable(). OTOH, having a
> preempt_disable() in that uprobe callsite should work as well, then we
> can keep the cant_sleep() check which covers all other callsites
> properly. No strong opinion though.

ok. I went with preempt_disable() for uprobes. It's simpler.
And pushed the whole set to bpf-next.
In few days we'll send it to Dave for net-next and on the way
to Linus's next release. imo it's a big milestone.
Thank you for the hard work to make it happen.