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Subject[PATCH RFC] ptr_ring: fix race conditions when resizing
Resizing currently drops consumer lock.  This can cause entries to be
reordered, which isn't good in itself. More importantly, consumer can
detect a false ring empty condition and block forever.

Further, nesting of consumer within producer lock is problematic for
tun, since it produces entries in an interrupt, which causes a
lock order reversal:

CPU0 CPU1
---- ----
consume:
lock(&(&r->consumer_lock)->rlock);
resize:
local_irq_disable();
lock(&(&r->producer_lock)->rlock);
lock(&(&r->consumer_lock)->rlock);
<Interrupt>
produce:
lock(&(&r->producer_lock)->rlock);

To fix, nest producer lock within consumer lock during resize,
and keep consumer lock during the whole swap operation.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
include/linux/ptr_ring.h | 36 +++++++++++++++++++++++++++++++-----
1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index 2052011..6c70444 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -111,6 +111,11 @@ static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
return 0;
}

+/*
+ * Note: resize (below) nests producer lock within consumer lock, so if you
+ * consume in interrupt or BH context, you must disable interrupts/BH when
+ * calling this.
+ */
static inline int ptr_ring_produce(struct ptr_ring *r, void *ptr)
{
int ret;
@@ -242,6 +247,11 @@ static inline void *__ptr_ring_consume(struct ptr_ring *r)
return ptr;
}

+/*
+ * Note: resize (below) nests producer lock within consumer lock, so if you
+ * call this in interrupt or BH context, you must disable interrupts/BH when
+ * producing.
+ */
static inline void *ptr_ring_consume(struct ptr_ring *r)
{
void *ptr;
@@ -357,7 +367,7 @@ static inline void **__ptr_ring_swap_queue(struct ptr_ring *r, void **queue,
void **old;
void *ptr;

- while ((ptr = ptr_ring_consume(r)))
+ while ((ptr = __ptr_ring_consume(r)))
if (producer < size)
queue[producer++] = ptr;
else if (destroy)
@@ -372,6 +382,12 @@ static inline void **__ptr_ring_swap_queue(struct ptr_ring *r, void **queue,
return old;
}

+/*
+ * Note: producer lock is nested within consumer lock, so if you
+ * resize you must make sure all uses nest correctly.
+ * In particular if you consume ring in interrupt or BH context, you must
+ * disable interrupts/BH when doing so.
+ */
static inline int ptr_ring_resize(struct ptr_ring *r, int size, gfp_t gfp,
void (*destroy)(void *))
{
@@ -382,17 +398,25 @@ static inline int ptr_ring_resize(struct ptr_ring *r, int size, gfp_t gfp,
if (!queue)
return -ENOMEM;

- spin_lock_irqsave(&(r)->producer_lock, flags);
+ spin_lock_irqsave(&(r)->consumer_lock, flags);
+ spin_lock(&(r)->producer_lock);

old = __ptr_ring_swap_queue(r, queue, size, gfp, destroy);

- spin_unlock_irqrestore(&(r)->producer_lock, flags);
+ spin_unlock(&(r)->producer_lock);
+ spin_unlock_irqrestore(&(r)->consumer_lock, flags);

kfree(old);

return 0;
}

+/*
+ * Note: producer lock is nested within consumer lock, so if you
+ * resize you must make sure all uses nest correctly.
+ * In particular if you consume ring in interrupt or BH context, you must
+ * disable interrupts/BH when doing so.
+ */
static inline int ptr_ring_resize_multiple(struct ptr_ring **rings, int nrings,
int size,
gfp_t gfp, void (*destroy)(void *))
@@ -412,10 +436,12 @@ static inline int ptr_ring_resize_multiple(struct ptr_ring **rings, int nrings,
}

for (i = 0; i < nrings; ++i) {
- spin_lock_irqsave(&(rings[i])->producer_lock, flags);
+ spin_lock_irqsave(&(rings[i])->consumer_lock, flags);
+ spin_lock(&(rings[i])->producer_lock);
queues[i] = __ptr_ring_swap_queue(rings[i], queues[i],
size, gfp, destroy);
- spin_unlock_irqrestore(&(rings[i])->producer_lock, flags);
+ spin_unlock(&(rings[i])->producer_lock);
+ spin_unlock_irqrestore(&(rings[i])->consumer_lock, flags);
}

for (i = 0; i < nrings; ++i)
--
MST
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