Re: frequent lockups in 3.18rc4

[Linus Torvalds]

Side note: I think I've found a real potential lockup bug in
fs/namespace.c, but afaik it could only trigger with the RT patches.

I'm looking at what lxsetattr() does, since you had that
lxsetattr-only lockup. I doubt it's really related to lxsetattr(), but
whatever. The generic code does that mnt_want_write/mnt_drop_write
dance adound the call to setxattr, and that in turn does

        while (ACCESS_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
                cpu_relax();

with preemption explicitly disabled. It's waitingo for
mnt_make_readonly() to go away if it is racing with it.

But mnt_make_readonly() doesn't actually explicitly disable preemption
while it sets that MNT_WRITE_HOLD bit. Instead, it depends on
lock_mount_hash() to disable preemption for it. Which it does, because
it is a seq-writelock, which uses a spinlock, which will disable
preemption.

Except it won't with the RT patches, I guess. So it looks like you could have:\

 - mnt_make_readonly() sets that bit
 - gets preempted with the RT patches
 - we run mnt_want_write() on all CPU's, which disables preemption and
waits for the bit to be cleared
 - nothing happens.

This is clearly not what happens in your lockup, but it does seem to
be a potential issue for the RT kernel.

Added Al and Thomas to the cc, for fs/namespace.c and RT kernel
respectively. Maybe the RT patches already fix this, I didn't actually
check.

                     Linus
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