Currently with fence-array, we have a potential deadlock situation. If we
fence_add_callback() on an array-fence, the array-fence's lock is aquired
first, and in it's ->enable_signaling() callback, it will install cb's on
it's array-member fences, so the array-member's lock is acquired second.
But in the signal path, the array-member's lock is acquired first, and the
array-fence's lock acquired second.
One approach to deal with this is avoid holding the fence's lock when
calling the cb. It is a bit intrusive and I haven't fixed up all the
other drivers that call directly or indirectly fence_signal_locked().
I guess the other option would be introduce a work-queue for array-fence?
Or??
lockdep splat:
======================================================
[ INFO: possible circular locking dependency detected ]
4.7.0-rc7+ #489 Not tainted
-------------------------------------------------------
surfaceflinger/2034 is trying to acquire lock:
(&(&array->lock)->rlock){......}, at: [<ffff00000858cddc>] fence_signal+0x5c/0xf8
but task is already holding lock:
(&(&obj->child_list_lock)->rlock){......}, at: [<ffff0000085902f8>] sw_sync_ioctl+0x228/0x3b0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&(&obj->child_list_lock)->rlock){......}:
[<ffff000008108924>] __lock_acquire+0x173c/0x18d8
[<ffff000008108e0c>] lock_acquire+0x4c/0x68
[<ffff000008ac6a6c>] _raw_spin_lock_irqsave+0x54/0x70
[<ffff00000858d05c>] fence_add_callback+0x3c/0x100
[<ffff00000858f100>] fence_array_enable_signaling+0x80/0x170
[<ffff00000858d0d8>] fence_add_callback+0xb8/0x100
[<ffff00000858f504>] sync_file_poll+0xd4/0xf0
[<ffff0000081fd3a0>] do_sys_poll+0x220/0x438
[<ffff0000081fd8d0>] SyS_ppoll+0x1b0/0x1d8
[<ffff000008084f30>] el0_svc_naked+0x24/0x28
-> #0 (&(&array->lock)->rlock){......}:
[<ffff000008104768>] print_circular_bug+0x80/0x2e0
[<ffff0000081089ac>] __lock_acquire+0x17c4/0x18d8
[<ffff000008108e0c>] lock_acquire+0x4c/0x68
[<ffff000008ac6a6c>] _raw_spin_lock_irqsave+0x54/0x70
[<ffff00000858cddc>] fence_signal+0x5c/0xf8
[<ffff00000858f268>] fence_array_cb_func+0x78/0x88
[<ffff00000858cb28>] fence_signal_locked+0x80/0xe0
[<ffff0000085903c8>] sw_sync_ioctl+0x2f8/0x3b0
[<ffff0000081faf6c>] do_vfs_ioctl+0xa4/0x790
[<ffff0000081fb6e4>] SyS_ioctl+0x8c/0xa0
[<ffff000008084f30>] el0_svc_naked+0x24/0x28
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&(&obj->child_list_lock)->rlock);
lock(&(&array->lock)->rlock);
lock(&(&obj->child_list_lock)->rlock);
lock(&(&array->lock)->rlock);
*** DEADLOCK ***
1 lock held by surfaceflinger/2034:
#0: (&(&obj->child_list_lock)->rlock){......}, at: [<ffff0000085902f8>] sw_sync_ioctl+0x228/0x3b0
---
drivers/dma-buf/fence.c | 22 ++++++++++++++--------
drivers/dma-buf/sw_sync.c | 2 +-
drivers/dma-buf/sync_debug.c | 16 +++++++++-------
include/linux/fence.h | 6 +++---
4 files changed, 27 insertions(+), 19 deletions(-)
diff --git a/drivers/dma-buf/fence.c b/drivers/dma-buf/fence.c
index cc05ddd..917f905 100644
--- a/drivers/dma-buf/fence.c
+++ b/drivers/dma-buf/fence.c
@@ -63,9 +63,9 @@ EXPORT_SYMBOL(fence_context_alloc);
*
* Unlike fence_signal, this function must be called with fence->lock held.
*/
-int fence_signal_locked(struct fence *fence)
+int fence_signal_locked(struct fence *fence, unsigned long flags)
{
- struct fence_cb *cur, *tmp;
+ struct fence_cb *cur;
int ret = 0;
lockdep_assert_held(fence->lock);
@@ -88,9 +88,12 @@ int fence_signal_locked(struct fence *fence)
} else
trace_fence_signaled(fence);
- list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
+ while (!list_empty(&fence->cb_list)) {
+ cur = list_first_entry(&fence->cb_list, struct fence_cb, node);
list_del_init(&cur->node);
+ spin_unlock_irqrestore(fence->lock, flags);
cur->func(fence, cur);
+ spin_lock_irqsave(fence->lock, flags);
}
return ret;
}
@@ -124,12 +127,15 @@ int fence_signal(struct fence *fence)
trace_fence_signaled(fence);
if (test_bit(FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)) {
- struct fence_cb *cur, *tmp;
+ struct fence_cb *cur;
spin_lock_irqsave(fence->lock, flags);
- list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
+ while (!list_empty(&fence->cb_list)) {
+ cur = list_first_entry(&fence->cb_list, struct fence_cb, node);
list_del_init(&cur->node);
+ spin_unlock_irqrestore(fence->lock, flags);
cur->func(fence, cur);
+ spin_lock_irqsave(fence->lock, flags);
}
spin_unlock_irqrestore(fence->lock, flags);
}
@@ -211,7 +217,7 @@ void fence_enable_sw_signaling(struct fence *fence)
spin_lock_irqsave(fence->lock, flags);
if (!fence->ops->enable_signaling(fence))
- fence_signal_locked(fence);
+ fence_signal_locked(fence, flags);
spin_unlock_irqrestore(fence->lock, flags);
}
@@ -266,7 +272,7 @@ int fence_add_callback(struct fence *fence, struct fence_cb *cb,
trace_fence_enable_signal(fence);
if (!fence->ops->enable_signaling(fence)) {
- fence_signal_locked(fence);
+ fence_signal_locked(fence, flags);
ret = -ENOENT;
}
}
@@ -366,7 +372,7 @@ fence_default_wait(struct fence *fence, bool intr, signed long timeout)
trace_fence_enable_signal(fence);
if (!fence->ops->enable_signaling(fence)) {
- fence_signal_locked(fence);
+ fence_signal_locked(fence, flags);
goto out;
}
}
diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
index 62e8e6d..2271f7f 100644
--- a/drivers/dma-buf/sw_sync.c
+++ b/drivers/dma-buf/sw_sync.c
@@ -146,7 +146,7 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
list_for_each_entry_safe(pt, next, &obj->active_list_head,
active_list) {
- if (fence_is_signaled_locked(&pt->base))
+ if (fence_is_signaled_locked(&pt->base, flags))
list_del_init(&pt->active_list);
}
diff --git a/drivers/dma-buf/sync_debug.c b/drivers/dma-buf/sync_debug.c
index 2dd4c3d..a7556d3 100644
--- a/drivers/dma-buf/sync_debug.c
+++ b/drivers/dma-buf/sync_debug.c
@@ -71,12 +71,13 @@ static const char *sync_status_str(int status)
return "error";
}
-static void sync_print_fence(struct seq_file *s, struct fence *fence, bool show)
+static void sync_print_fence(struct seq_file *s, struct fence *fence,
+ bool show, unsigned long flags)
{
int status = 1;
struct sync_timeline *parent = fence_parent(fence);
- if (fence_is_signaled_locked(fence))
+ if (fence_is_signaled_locked(fence, flags))
status = fence->status;
seq_printf(s, " %s%sfence %s",
@@ -124,13 +125,14 @@ static void sync_print_obj(struct seq_file *s, struct sync_timeline *obj)
list_for_each(pos, &obj->child_list_head) {
struct sync_pt *pt =
container_of(pos, struct sync_pt, child_list);
- sync_print_fence(s, &pt->base, false);
+ sync_print_fence(s, &pt->base, false, flags);
}
spin_unlock_irqrestore(&obj->child_list_lock, flags);
}
static void sync_print_sync_file(struct seq_file *s,
- struct sync_file *sync_file)
+ struct sync_file *sync_file,
+ unsigned long flags)
{
int i;
@@ -141,9 +143,9 @@ static void sync_print_sync_file(struct seq_file *s,
struct fence_array *array = to_fence_array(sync_file->fence);
for (i = 0; i < array->num_fences; ++i)
- sync_print_fence(s, array->fences[i], true);
+ sync_print_fence(s, array->fences[i], true, flags);
} else {
- sync_print_fence(s, sync_file->fence, true);
+ sync_print_fence(s, sync_file->fence, true, flags);
}
}
@@ -172,7 +174,7 @@ static int sync_debugfs_show(struct seq_file *s, void *unused)
struct sync_file *sync_file =
container_of(pos, struct sync_file, sync_file_list);
- sync_print_sync_file(s, sync_file);
+ sync_print_sync_file(s, sync_file, flags);
seq_puts(s, "\n");
}
spin_unlock_irqrestore(&sync_file_list_lock, flags);
diff --git a/include/linux/fence.h b/include/linux/fence.h
index 0d76305..073f380 100644
--- a/include/linux/fence.h
+++ b/include/linux/fence.h
@@ -220,7 +220,7 @@ static inline void fence_put(struct fence *fence)
}
int fence_signal(struct fence *fence);
-int fence_signal_locked(struct fence *fence);
+int fence_signal_locked(struct fence *fence, unsigned long flags);
signed long fence_default_wait(struct fence *fence, bool intr, signed long timeout);
int fence_add_callback(struct fence *fence, struct fence_cb *cb,
fence_func_t func);
@@ -239,13 +239,13 @@ void fence_enable_sw_signaling(struct fence *fence);
* This function requires fence->lock to be held.
*/
static inline bool
-fence_is_signaled_locked(struct fence *fence)
+fence_is_signaled_locked(struct fence *fence, unsigned long flags)
{
if (test_bit(FENCE_FLAG_SIGNALED_BIT, &fence->flags))
return true;
if (fence->ops->signaled && fence->ops->signaled(fence)) {
- fence_signal_locked(fence);
+ fence_signal_locked(fence, flags);
return true;
}
--
2.7.4
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