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Date: Thu, 10 Dec 2015 21:56:48 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Kees Cook <keescook@chromium.org>
Cc: Willy Tarreau <w@1wt.eu>, Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, yalin wang <yalin.wang2010@gmail.com>,
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        Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing
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On Thu, Dec 10, 2015 at 01:45:09PM -0800, Kees Cook wrote:
> > but generally you need ->f_lock.  And in situations where the bit can
> > go only off->on, check it lockless, skip the whole thing entirely if it's
> > already set and grab the spinlock otherwise.
> 
> And I can take f_lock safely under mmap_sem?

Are you asking whether it's safe to take a spinlock under an rwsem?