linux-kernel - Re: [RFC][PATCH] f2fs: Avoid print false deadlock messages.

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Date:	Thu, 16 May 2013 10:41:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	majianpeng <majianpeng@...il.com>
Cc:	Jaegeuk Kim <jaegeuk.kim@...sung.com>, mingo@...hat.com,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-f2fs <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [RFC][PATCH] f2fs: Avoid print false deadlock messages.

On Thu, May 16, 2013 at 09:16:45AM +0800, majianpeng wrote:

> > There isn't. What you typically want to do is annotate the lock site.
> > In particular it looks like mutex_lock_all() is the offensive piece of
> > code (horrible function name though; the only redeeming thing being that
> > f2fs.h isn't likely to be included elsewhere).
> >
> > One thing you can do here is modify it to look like:
> >
> > static inline void mutex_lock_all(struct f2fs_sb_info *sbi)
> > {
> > 	int i;
> >
> > 	for (i = 0; i < NR_GLOBAL_LOCKS; i++) {
> > 		/*
> > 		 * This is the only time we take multiple fs_lock[]
> > 		 * instances; the order is immaterial since we
> > 		 * always hold cp_mutex, which serializes multiple
> > 		 * such operations.
> > 		 */
> > 		mutex_lock_nest_lock(&sbi->fs_lock[i], &sbi->cp_mutex);
> > 	}
> > }
> >
> > That tells the lock validator that it is ok to lock multiple instances
> > of the fs_lock[i] class because the lock order is guarded by cp_mutex.
> > While your patch also works, it has multiple down-sides; its easy to get
> > out of sync when you modify NR_GLOBAL_LOCKS; also it consumes more
> > static lockdep resources (lockdep has to allocate all its resources
> > from static arrays since allocating memory also uses locks -- recursive
> > problem).
> >
> Yes, but there is a problem if fs_block[] met deadlock. How to find which one?
> Because the lock->name is the same.

The most useful part of the lockdep report are the call traces that tell you
where locks where acquired; the names are secondary. That is, while they are at
times helpful in finding the right lock site, they're rarely _that_ important.

Remember, your code will very likely not have the exact number hardcoded either.
It'll be a variable. So having the number in the lockdep output will not help
you find the offending code any sooner.

Suppose there's another site that acquires two fs_block[] locks; currently this
would generate another such warning as this thread started with -- lockdep
doesn't look at lock instances but at classes; and it cannot differentiate
between two locks of the same class and thus reports the possible deadlock.

The way to find the offending code is to look at the "locks held" section of
the lockdep report along with the call traces.

Once you find the way in which the two locks nest the specific numbers are
irrelevant. Your aim then is to prove your locking scheme is indeed deadlock
free and then tell lockdep about it.

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