Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Feb 13, 2009 at 06:41:35AM +0000, Al Viro wrote:

Aaaargh...

        /*
         * We don't have to hold all of the locks at the
         * same time here because we know that we're the
         * last reference to mnt and that no new writers
         * can come in.
         */
        for_each_possible_cpu(cpu) {
                struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
                if (cpu_writer->mnt != mnt)
                        continue;
                spin_lock(&cpu_writer->lock);

is *almost* OK.  Modulo SMP cache coherency.  We know that nothing should
be setting ->mnt to ours anymore, that's fine.  But we do not know if
we'd seen *earlier* change done on CPU in question (not the one we
are running __mntput() on).

I probably would still like to use milder solution in the long run, but for
now let's check if turning that into

                struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
                spin_lock(&cpu_writer->lock);
                if (cpu_writer->mnt != mnt) {
			spin_unlock(&cpu_writer->lock);
                        continue;
		}
prevents the problem, OK?