Re: [PATCH] VFS: br_write_lock locks on possible CPUs other thanonline CPUs

[Al Viro]

On Mon, Dec 19, 2011 at 11:36:15AM +0800, mengcong wrote:
> In a heavily loaded system, when frequently turning on and off CPUs, the
> kernel will detect soft-lockups on multiple CPUs. The detailed bug report
> is at https://lkml.org/lkml/2011/8/24/185.
> 
> The root cause is that brlock functions, i.e. br_write_lock() and
> br_write_unlock(), only locks/unlocks the per-CPU spinlock of CPUs that
> are online, which means, if one online CPU is locked and then goes
> offline, any later unlocking operation happens during its offline state
> will not touch it; and when it goes online again, it has the incorrect
> brlock state. This has been verified in current kernel.
> 
> I can reproduce this bug on the intact 3.1 kernel. After my patch applied, 
> I've ran an 8-hours long test(test script provided by the bug reporter), 
> and no soft lockup happened again.

Argh...  OK, that's seriously nasty.  I agree that this is broken, but
your patch makes br_write_lock() very costly on kernels build with
huge number of possible CPUs, even when it's run on a box with few
CPUs ;-/

That sucks.  Worse, AFAICS, the only way to prevent on-/off-line status
changes is blocking (and both directions are bad - if the thing goes online
between br_write_lock() and br_write_unlock(), we'll get spin_unlock without
spin_lock).  And I really don't want to make vfsmount_lock writers blocking -
we *probably* could get away with that, but it'll suck very badly.  Especially
since we'll have that nested inside namespace_sem...

Alternative is to do get_online_cpus/put_online_cpus around the stuff in
fs/namespace.c, putting it *outside* everything but actual IO.  We can
do that (since right now vfsmount_lock is non-blocking and the only
potentially blocking operations under namespace_sem is kmalloc()), but
I'm not particulary comfortable doing that - I never played with the code
in kernel/cpu.c and I don't know if there's anything subtle to watch out
for.

The same issue exists for lg_global_lock_online(), but that beast is
never used (and the only remaining user of lg_global_lock() is
hardly time-critical - with Miklos' patches it's only done on
mount -o remount,force,ro).

Nick, any comments?  That's your code...
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