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Date: Sun, 6 Jan 2019 20:19:41 -0800
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] lockdep: Add debug printk() for downgrade_write()
 warning.
Message-Id: <20190106201941.49f6dc4a4d2e9d15b575f88a@linux-foundation.org>
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References: <1546771139-9349-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
	<e1a38e21-d5fe-dee3-7081-bc1a12965a68@i-love.sakura.ne.jp>
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Sender: owner-linux-mm@kvack.org
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To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: linux-mm@kvack.org, Dmitry Vyukov <dvyukov@google.com>, Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Will Deacon <will.deacon@arm.com>

On Sun, 6 Jan 2019 19:56:59 +0900 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:

> syzbot is frequently hitting downgrade_write(&mm->mmap_sem) warning from
> munmap() request, but I don't know why it is happening. Since lockdep is
> not printing enough information, let's print more. This patch is meant for
> linux-next.git only and will be removed after the problem is solved.
> 
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -50,6 +50,7 @@
>  #include <linux/random.h>
>  #include <linux/jhash.h>
>  #include <linux/nmi.h>
> +#include <linux/rwsem.h>
>  
>  #include <asm/sections.h>
>  
> @@ -3550,6 +3551,24 @@ static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
>  	curr->lockdep_depth = i;
>  	curr->curr_chain_key = hlock->prev_chain_key;
>  
> +#if defined(CONFIG_RWSEM_XCHGADD_ALGORITHM) && defined(CONFIG_DEBUG_AID_FOR_SYZBOT)
> +	if (hlock->read && curr->mm) {
> +		struct rw_semaphore *sem = container_of(lock,
> +							struct rw_semaphore,
> +							dep_map);
> +
> +		if (sem == &curr->mm->mmap_sem) {
> +#if defined(CONFIG_RWSEM_SPIN_ON_OWNER)
> +			pr_warn("mmap_sem: hlock->read=%d count=%ld current=%px, owner=%px\n",
> +				hlock->read, atomic_long_read(&sem->count),
> +				curr, READ_ONCE(sem->owner));
> +#else
> +			pr_warn("mmap_sem: hlock->read=%d count=%ld\n",
> +				hlock->read, atomic_long_read(&sem->count));
> +#endif
> +		}
> +	}
> +#endif
>  	WARN(hlock->read, "downgrading a read lock");
>  	hlock->read = 1;
>  	hlock->acquire_ip = ip;

I tossed it in there.

But I wonder if anyone is actually running this code.  Because

--- a/lib/Kconfig.debug~info-task-hung-in-generic_file_write_iter
+++ a/lib/Kconfig.debug
@@ -2069,6 +2069,12 @@ config IO_STRICT_DEVMEM
 
 	  If in doubt, say Y.
 
+config DEBUG_AID_FOR_SYZBOT
+       bool "Additional debug code for syzbot"
+       default n
+       help
+         This option is intended for testing by syzbot.
+