fix a race condition in undoDequeue(), where 'tail' could be computed incorrectly.
in the undoDequeue() case, 'tail' was recalculated from 'available' and 'head'
however there was a race between this and retireAndLock(), which could cause
'tail' to be recalculated wrongly.
the interesting thing though is that retireAndLock() shouldn't have any impact
on the value of 'tail', which is client-side only attribute.
we fix the race by saving the value of 'tail' before dequeue() and restore it
in the case of undoDequeue(), since we know it doesn't depend on retireAndLock().
Change-Id: I4bcc4d16b6bc4dd93717ee739c603040b18295a0
diff --git a/include/private/surfaceflinger/SharedBufferStack.h b/include/private/surfaceflinger/SharedBufferStack.h
index 6ace5bc..39ef3a1 100644
--- a/include/private/surfaceflinger/SharedBufferStack.h
+++ b/include/private/surfaceflinger/SharedBufferStack.h
@@ -167,6 +167,7 @@
SharedBufferStack* const mSharedStack;
const int mNumBuffers;
const int mIdentity;
+ int32_t computeTail() const;
friend struct Update;
friend struct QueueUpdate;
@@ -259,8 +260,6 @@
friend struct Condition;
friend struct DequeueCondition;
friend struct LockCondition;
-
- int32_t computeTail() const;
struct QueueUpdate : public UpdateBase {
inline QueueUpdate(SharedBufferBase* sbb);
@@ -288,6 +287,7 @@
};
int32_t tail;
+ int32_t undoDequeueTail;
// statistics...
nsecs_t mDequeueTime[NUM_BUFFER_MAX];
};
diff --git a/libs/surfaceflinger_client/SharedBufferStack.cpp b/libs/surfaceflinger_client/SharedBufferStack.cpp
index f1aec8a..6745704 100644
--- a/libs/surfaceflinger_client/SharedBufferStack.cpp
+++ b/libs/surfaceflinger_client/SharedBufferStack.cpp
@@ -179,7 +179,7 @@
char buffer[SIZE];
String8 result;
SharedBufferStack& stack( *mSharedStack );
- int tail = (mNumBuffers + stack.head - stack.available + 1) % mNumBuffers;
+ int tail = computeTail();
snprintf(buffer, SIZE,
"%s[ head=%2d, available=%2d, queued=%2d, tail=%2d ] "
"reallocMask=%08x, inUse=%2d, identity=%d, status=%d\n",
@@ -189,6 +189,12 @@
return result;
}
+int32_t SharedBufferBase::computeTail() const
+{
+ SharedBufferStack& stack( *mSharedStack );
+ return (mNumBuffers + stack.head - stack.available + 1) % mNumBuffers;
+}
+
// ============================================================================
// conditions and updates
// ============================================================================
@@ -297,32 +303,12 @@
SharedBufferClient::SharedBufferClient(SharedClient* sharedClient,
int surface, int num, int32_t identity)
- : SharedBufferBase(sharedClient, surface, num, identity), tail(0)
+ : SharedBufferBase(sharedClient, surface, num, identity),
+ tail(0), undoDequeueTail(0)
{
tail = computeTail();
}
-int32_t SharedBufferClient::computeTail() const
-{
- SharedBufferStack& stack( *mSharedStack );
- // we need to make sure we read available and head coherently,
- // w.r.t RetireUpdate.
- int32_t newTail;
- int32_t avail;
- int32_t head;
- do {
- avail = stack.available;
- head = stack.head;
- } while (stack.available != avail);
- newTail = head - avail + 1;
- if (newTail < 0) {
- newTail += mNumBuffers;
- } else if (newTail >= mNumBuffers) {
- newTail -= mNumBuffers;
- }
- return newTail;
-}
-
ssize_t SharedBufferClient::dequeue()
{
SharedBufferStack& stack( *mSharedStack );
@@ -350,6 +336,7 @@
int dequeued = tail;
tail = ((tail+1 >= mNumBuffers) ? 0 : tail+1);
+ undoDequeueTail = dequeued;
LOGD_IF(DEBUG_ATOMICS, "dequeued=%d, tail=%d, %s",
dequeued, tail, dump("").string());
@@ -363,7 +350,7 @@
UndoDequeueUpdate update(this);
status_t err = updateCondition( update );
if (err == NO_ERROR) {
- tail = computeTail();
+ tail = undoDequeueTail;
}
return err;
}