linux-kernel - Re: [patch V3 06/22] bpf/trace: Remove redundant preempt_disable from trace_call

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Message-ID: <20200224194017.rtwjcgjxnmltisfe@ast-mbp>
Date:   Mon, 24 Feb 2020 11:40:20 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Sebastian Sewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Clark Williams <williams@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [patch V3 06/22] bpf/trace: Remove redundant preempt_disable
 from trace_call_bpf()

On Mon, Feb 24, 2020 at 03:01:37PM +0100, Thomas Gleixner wrote:
> Similar to __bpf_trace_run this is redundant because __bpf_trace_run() is
> invoked from a trace point via __DO_TRACE() which already disables
> preemption _before_ invoking any of the functions which are attached to a
> trace point.
> 
> Remove it and add a cant_sleep() check.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V3: New patch. Replaces the previous one which converted this to migrate_disable() 
> ---
>  kernel/trace/bpf_trace.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- 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

The following fixes it:

commit 7b7b71ff43cc0b15567b60c38a951c8a2cbc97f0 (HEAD -> bpf-next)
Author: Alexei Starovoitov <ast@...nel.org>
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@...nel.org>

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?
And two calls to __this_cpu*() replaced with this_cpu*() ?
If you can ack it I can fix it up in place and apply the whole thing.
That was the only issue I found.