linux-kernel - Re: [PATCH next] trace/blktrace: fix task hung in blk_trace

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Date:   Sat, 2 Dec 2023 17:07:43 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     Edward Adam Davis <eadavis@...com>,
        syzbot+ed812ed461471ab17a0c@...kaller.appspotmail.com,
        akpm@...ux-foundation.org, axboe@...nel.dk, dvyukov@...gle.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, mhiramat@...nel.org,
        pengfei.xu@...el.com, syzkaller-bugs@...glegroups.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH next] trace/blktrace: fix task hung in blk_trace_ioctl

On Sat, 2 Dec 2023 17:19:25 +0800
Yu Kuai <yukuai1@...weicloud.com> wrote:

> Hi,
> 
> 在 2023/12/02 17:01, Edward Adam Davis 写道:
> > The reproducer involves running test programs on multiple processors separately,
> > in order to enter blkdev_ioctl() and ultimately reach blk_trace_ioctl() through
> > two different paths, triggering an AA deadlock.
> > 
> > 	CPU0						CPU1
> > 	---						---
> > 	mutex_lock(&q->debugfs_mutex)			mutex_lock(&q->debugfs_mutex)
> > 	mutex_lock(&q->debugfs_mutex)			mutex_lock(&q->debugfs_mutex)
> > 
> > 
> > The first path:
> > blkdev_ioctl()->
> > 	blk_trace_ioctl()->
> > 		mutex_lock(&q->debugfs_mutex)
> > 
> > The second path:
> > blkdev_ioctl()->				
> > 	blkdev_common_ioctl()->
> > 		blk_trace_ioctl()->
> > 			mutex_lock(&q->debugfs_mutex)  
> I still don't understand how this AA deadlock is triggered, does the
> 'debugfs_mutex' already held before calling blk_trace_ioctl()?

Right, I don't see where the mutex is taken twice. You don't need two
paths for an AA lock, you only need one.

> 
> > 
> > The solution I have proposed is to exit blk_trace_ioctl() to avoid AA locks if
> > a task has already obtained debugfs_mutex.
> > 
> > Fixes: 0d345996e4cb ("x86/kernel: increase kcov coverage under arch/x86/kernel folder")

How does it fix the above? I don't see how the above is even related to this.

-- Steve

> > Reported-and-tested-by: syzbot+ed812ed461471ab17a0c@...kaller.appspotmail.com
> > Signed-off-by: Edward Adam Davis <eadavis@...com>
> > ---
> >   kernel/trace/blktrace.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c