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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
This commit is contained in:
Mathias Agopian 2010-04-27 16:11:38 -07:00
parent fd4a232374
commit 0a8cd0689a
2 changed files with 13 additions and 26 deletions
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4
include/private/surfaceflinger/SharedBufferStack.h
View File

@ -167,6 +167,7 @@ protected:
SharedBufferStack* const mSharedStack;
const int mNumBuffers;
const int mIdentity;
int32_t computeTail() const;
friend struct Update;
friend struct QueueUpdate;
@ -259,8 +260,6 @@ private:
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 @@ private:
};
int32_t tail;
int32_t undoDequeueTail;
// statistics...
nsecs_t mDequeueTime[NUM_BUFFER_MAX];
};

35
libs/surfaceflinger_client/SharedBufferStack.cpp
View File

@ -179,7 +179,7 @@ String8 SharedBufferBase::dump(char const* prefix) const
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 @@ String8 SharedBufferBase::dump(char const* prefix) const
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 @@ ssize_t SharedBufferServer::StatusUpdate::operator()() {
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 @@ ssize_t SharedBufferClient::dequeue()
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 @@ status_t SharedBufferClient::undoDequeue(int buf)
UndoDequeueUpdate update(this);
status_t err = updateCondition( update );
if (err == NO_ERROR) {
tail = computeTail();
tail = undoDequeueTail;
}
return err;
}