[PATCH 46/45] sparc64: Fix MM refcount check in smp_flush_tlb_pending().

From: Chris Wright <chrisw@sous-sol.org>
To: linux-kernel@vger.kernel.org, stable@kernel.org
Cc: "Theodore Ts'o" <tytso@mit.edu>,
	Zwane Mwaikambo <zwane@arm.linux.org.uk>,
	Eugene Teo <eteo@redhat.com>,
	Justin Forbes <jmforbes@linuxtx.org>,
	Domenico Andreoli <cavokz@gmail.com>,
	Chris Wedgwood <reviews@ml.cw.f00f.org>, Jake Edge <jake@lwn.net>,
	Randy Dunlap <rdunlap@xenotime.net>,
	Michael Krufky <mkrufky@linuxtv.org>,
	alan@lxorguk.ukuu.org.uk, Chuck Ebbert <cebbert@redhat.com>,
	Dave Jones <davej@redhat.com>,
	Chuck Wolber <chuckw@quantumlinux.com>,
	akpm@linux-foundation.org, torvalds@linux-foundation.org,
	Willy Tarreau <w@1wt.eu>,
	Rodrigo Rubira Branco <rbranco@la.checkpoint.com>,
	davem@davemloft.net
Subject: [PATCH 46/45] sparc64: Fix MM refcount check in smp_flush_tlb_pending().
Date: Wed, 1 Apr 2009 23:57:40 -0700	[thread overview]
Message-ID: <20090402065740.GB18394@sequoia.sous-sol.org> (raw)
In-Reply-To: <20090331231045.719396245@sous-sol.org>

-stable review patch.  If anyone has any objections, please let us know.
---------------------

From: David S. Miller <davem@davemloft.net>

[ Upstream commit f9384d41c02408dd404aa64d66d0ef38adcf6479 ]

As explained by Benjamin Herrenschmidt:

> CPU 0 is running the context, task->mm == task->active_mm == your
> context. The CPU is in userspace happily churning things.
>
> CPU 1 used to run it, not anymore, it's now running fancyfsd which
> is a kernel thread, but current->active_mm still points to that
> same context.
>
> Because there's only one "real" user, mm_users is 1 (but mm_count is
> elevated, it's just that the presence on CPU 1 as active_mm has no
> effect on mm_count().
>
> At this point, fancyfsd decides to invalidate a mapping currently mapped
> by that context, for example because a networked file has changed
> remotely or something like that, using unmap_mapping_ranges().
>
> So CPU 1 goes into the zapping code, which eventually ends up calling
> flush_tlb_pending(). Your test will succeed, as current->active_mm is
> indeed the target mm for the flush, and mm_users is indeed 1. So you
> will -not- send an IPI to the other CPU, and CPU 0 will continue happily
> accessing the pages that should have been unmapped.

To fix this problem, check ->mm instead of ->active_mm, and this
means:

> So if you test current->mm, you effectively account for mm_users == 1,
> so the only way the mm can be active on another processor is as a lazy
> mm for a kernel thread. So your test should work properly as long
> as you don't have a HW that will do speculative TLB reloads into the
> TLB on that other CPU (and even if you do, you flush-on-switch-in should
> get rid of any crap here).

And therefore we should be OK.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
---
 arch/sparc/kernel/smp_64.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/sparc/kernel/smp_64.c
+++ b/arch/sparc/kernel/smp_64.c
@@ -1031,7 +1031,7 @@ void smp_fetch_global_regs(void)
  *    If the address space is non-shared (ie. mm->count == 1) we avoid
  *    cross calls when we want to flush the currently running process's
  *    tlb state.  This is done by clearing all cpu bits except the current
- *    processor's in current->active_mm->cpu_vm_mask and performing the
+ *    processor's in current->mm->cpu_vm_mask and performing the
  *    flush locally only.  This will force any subsequent cpus which run
  *    this task to flush the context from the local tlb if the process
  *    migrates to another cpu (again).
@@ -1074,7 +1074,7 @@ void smp_flush_tlb_pending(struct mm_str
 	u32 ctx = CTX_HWBITS(mm->context);
 	int cpu = get_cpu();
 
-	if (mm == current->active_mm && atomic_read(&mm->mm_users) == 1)
+	if (mm == current->mm && atomic_read(&mm->mm_users) == 1)
 		mm->cpu_vm_mask = cpumask_of_cpu(cpu);
 	else
 		smp_cross_call_masked(&xcall_flush_tlb_pending,
