From: Al Viro <viro@ZenIV.linux.org.uk>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
linux-mm@kvack.org, mgorman@suse.de,
kamezawa.hiroyu@jp.fujitsu.com, dhillf@gmail.com,
hughd@google.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race
Date: Tue, 28 Feb 2012 00:02:28 +0000 [thread overview]
Message-ID: <20120228000228.GE23916@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20120227151135.7d4076c6.akpm@linux-foundation.org>
On Mon, Feb 27, 2012 at 03:11:35PM -0800, Andrew Morton wrote:
> This patch comes somewhat out of the blue and I'm unsure what's going on.
>
> You say there's some (potential?) deadlock with mmap, but it is
> undescribed. Have people observed this deadlock? Has it caused
> lockdep warnings? Please update the changelog to fully describe the
> bug.
There's one simple rule: never, ever take ->i_mutex under ->mmap_sem.
E.g. in any ->mmap() (obvious - mmap(2) calls that under ->mmap_sem) or
any ->release() of mappable file (munmap(2) does fput() under ->mmap_sem
and that will call ->release() if no other references are still around).
Hugetlbfs is slightly unusual since it takes ->i_mutex in read() - usually
that's done in write(), while read() doesn't bother with that. In either
case you do copying to/from userland buffer while holding ->i_mutex, which
nests ->mmap_sem within it.
> Also, the new truncate_sem is undoumented. This leaves readers to work
> out for themselves what it might be for. Please let's add code
> comments which completely describe the race, and how this lock prevents
> it.
>
> We should also document our locking rules.
Hell, yes. I've spent the last couple of weeks crawling through VM-related
code and locking in there is _scary_. "Convoluted" doesn't even begin to
cover it, especially when it gets to "what locks are required when accessing
this field" ;-/ Got quite a catch out of that trawl by now...
> When should code take this
> lock? What are its ranking rules with respect to i_mutex, i_mmap_mutex
> and possibly others?
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next prev parent reply other threads:[~2012-02-28 0:02 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-26 18:19 [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race Aneesh Kumar K.V
2012-02-27 23:11 ` Andrew Morton
2012-02-28 0:02 ` Al Viro [this message]
2012-02-28 10:15 ` Aneesh Kumar K.V
2012-02-28 12:17 ` Hillf Danton
2012-02-29 11:04 ` Aneesh Kumar K.V
2012-02-29 14:42 ` Hillf Danton
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