netdev - Re: [PATCH bpf-next 2/4] bpf: fix lockdep false positive in stackmap

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Message-ID: <20190130101530.GE2278@hirez.programming.kicks-ass.net>
Date:   Wed, 30 Jan 2019 11:15:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     davem@...emloft.net, daniel@...earbox.net, edumazet@...gle.com,
        jannh@...gle.com, netdev@...r.kernel.org, kernel-team@...com,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH bpf-next 2/4] bpf: fix lockdep false positive in stackmap

On Tue, Jan 29, 2019 at 08:04:56PM -0800, Alexei Starovoitov wrote:
> Lockdep warns about false positive:

This is not a false positive, and you probably also need to use
down_read_non_owner() to match this up_read_non_owner().

{up,down}_read() and {up,down}_read_non_owner() are not only different
in the lockdep annotation; there is also optimistic spin stuff that
relies on 'owner' tracking.

> [   11.211460] ------------[ cut here ]------------
> [   11.211936] DEBUG_LOCKS_WARN_ON(depth <= 0)
> [   11.211985] WARNING: CPU: 0 PID: 141 at ../kernel/locking/lockdep.c:3592 lock_release+0x1ad/0x280
> [   11.213134] Modules linked in:
> [   11.213413] CPU: 0 PID: 141 Comm: systemd-journal Not tainted 5.0.0-rc3-00018-g2fa53f892422-dirty #476
> [   11.214191] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014
> [   11.214954] RIP: 0010:lock_release+0x1ad/0x280
> [   11.217036] RSP: 0018:ffff88813ba03f50 EFLAGS: 00010086
> [   11.217516] RAX: 000000000000001f RBX: ffff8881378d8000 RCX: 0000000000000000
> [   11.218179] RDX: ffffffff810d3e9e RSI: 0000000000000001 RDI: ffffffff810d3eb3
> [   11.218851] RBP: ffff8881393e2b08 R08: 0000000000000002 R09: 0000000000000000
> [   11.219504] R10: 0000000000000000 R11: ffff88813ba03d9d R12: ffffffff8118dfa2
> [   11.220162] R13: 0000000000000086 R14: 0000000000000000 R15: 0000000000000000
> [   11.220717] FS:  00007f3c8cf35780(0000) GS:ffff88813ba00000(0000) knlGS:0000000000000000
> [   11.221348] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   11.221822] CR2: 00007f5825d92080 CR3: 00000001378c8005 CR4: 00000000003606f0
> [   11.222381] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   11.222951] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [   11.223508] Call Trace:
> [   11.223705]  <IRQ>
> [   11.223874]  ? __local_bh_enable+0x7a/0x80
> [   11.224199]  up_read+0x1c/0xa0
> [   11.224446]  do_up_read+0x12/0x20
> [   11.224713]  irq_work_run_list+0x43/0x70
> [   11.225030]  irq_work_run+0x26/0x50
> [   11.225310]  smp_irq_work_interrupt+0x57/0x1f0
> [   11.225662]  irq_work_interrupt+0xf/0x20
> 
> since rw_semaphore is released in a different task vs task that locked the sema.
> It is expected behavior.
> Silence the warning by using up_read_non_owner().
> 
> Fixes: bae77c5eb5b2 ("bpf: enable stackmap with build_id in nmi context")
> Acked-by: Martin KaFai Lau <kafai@...com>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
>  kernel/bpf/stackmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index d43b14535827..4b79e7c251e5 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -44,7 +44,7 @@ static void do_up_read(struct irq_work *entry)
>  	struct stack_map_irq_work *work;
>  
>  	work = container_of(entry, struct stack_map_irq_work, irq_work);
> -	up_read(work->sem);
> +	up_read_non_owner(work->sem);
>  	work->sem = NULL;
>  }
>  
> -- 
> 2.20.0
>