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<concept id="GUID-314FAEB5-946C-4090-B6AA-1BEEC9BE8EFB" xml:lang="en"><title>Common

Error Patterns - Case Studies</title><shortdesc>Describes a few common error patterns that were found while testing

code designed for use in an SMP environment.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>During development and testing, a number of areas of code have been identified

that demonstrate some of the SMP common error patterns. These case studies

are described below to illustrate how these errors can be identified and typical

code fixes.</p>

<section id="C"><title>Relying on linear execution</title><p>The ETEL client

API makes use of an unusual implementation class called <codeph>CPtrHolder</codeph>.

Each ETEL client sub-session (derived from <codeph>RTelSubSessionBase</codeph>)

owns a <codeph>CPtrHolder</codeph>. This class stores a dynamic array of descriptors,

each of which corresponds to the IPC data used for a specific subset of ETEL

client/server messages. This allows the server to read/write data from/to

valid client descriptors when there are multiple outstanding messages from

a particular client sub-session. </p> <p>A problem occurred when the same

client sub-session sent two asynchronous messages of the same type (therefore

using the same <codeph>CPtrHolder</codeph> descriptor) to the ETEL server

in a short space of time. The problem did not manifest itself in a single

core environment because the ETEL server always ran at a higher priority than

the ETEL client, and so the first message was always completed before the

client sent the second message. However, in an SMP environment, the server

runs on a different CPU to the client and had not started processing the first

message by the time the client sent the second message. The <codeph>CPtrHolder</codeph> descriptor

was updated with data from the second message but the server was still expecting

to read data from the first message. As a result, the first message was completed

by the server with a <codeph>KErrCorrupt</codeph> error code. </p> <p>The

solution was to identify which messages could suffer from this problem and

send a synchronous <codeph>EEtelFlushInterfaceQueue</codeph> message to the

server immediately after the asynchronous request. This guaranteed that the

server would have read the data from the first asynchronous message by the

time the second asynchronous message was sent. </p></section>

<section id="D"><title>Relying on thread suspension when making asynchronous

calls</title><p>In a way, the "Non-synchronized IPC calls" category is a subset

of this category. Whenever code makes assumptions about which order threads

are executed based on thread priority, there is a potential problem. In most

cases, it is best to think of thread priority as nothing more than a hint

to the scheduler. In an environment where multiple processors are being activated

and deactivated dynamically it is not safe to make any assumptions concerning

which threads will run at a given time. </p><p>Consider the case below where

a mutex is acquired by two threads with different priorities: </p><codeblock xml:space="preserve">// High priority thread (T1)    

iMutex.Wait();

DoFirstThing();         

iMutex.Signal();         

// Low priority thread (T2)

iMutex.Wait();

DoSecondThing();

iMutex.Signal();</codeblock><p>Even on a single processor system it is unwise

to assume that T1 would acquire the lock before T2. For example, T1 could

be suspended because of a page fault. While it is suspended, the scheduler

could run T2, causing it to acquire the lock and execute <codeph>DoSecondThing()</codeph>,

which is not the desired behaviour. In an SMP context both threads could be

running concurrently, making it even harder to predict which thread will acquire

the lock first. While a mutex may be used to prevent concurrent access to

a shared resource, it cannot be used to force two threads to perform actions

in a certain sequence. The solution is to use an <xref href="GUID-AED27A76-3645-3A04-B80D-10473D9C5A27.dita"><apiname>RSemaphore</apiname></xref> initialised

with a value of 0, allowing one thread to suspend until another thread signals

that it is safe to proceed. We would use this in the above example as follows: </p><codeblock xml:space="preserve">// T1 

DoFirstThing();

iSemaphore.Signal();

// T2

iSemaphore.Wait();

DoSecondThing();

</codeblock><p>Now, T2 waits for explicit notification from T1. We are completely

agnostic about the relative thread priorities. Should T2 execute first, the

semaphore would be decremented and the thread blocked before executing <codeph>DoSecondThing()</codeph>.

It can only proceed once T1 has executed <codeph>DoFirstThing()</codeph> and

signalled the semaphore. </p><p><b>Case study: Thread <codeph>EwSrv.exe</codeph> panics

with <codeph>E32USER-CBase 46</codeph> on H4. </b></p><p>This is another case

of a bad assumption about thread priorities but with a different result. The

problem manifested itself on an SMP environment when pressing keys a lot within

an edit window. The edit window cursor is an animation that periodically generates

redraw events within the window server. The scheduling of these redraw events

was altered by the events generated by the key presses, which led to a stray

event panic (<codeph>E32USER-CBase 46</codeph>) in the window server main

thread. As this is a critical thread, the kernel then faulted. Like many stray

event panics, this problem was difficult to diagnose but easy to fix. </p><p>The

window server has a mechanism for redrawing animations involving a separate

low-priority thread (called the kickback thread) encapsulated within an active

object that controls that thread, called <codeph>CKickBack</codeph>. Kickbacks

from the window server main thread are executed through this function: </p><codeblock xml:space="preserve">void CWindowServer::CDefaultAnimationScheduler::CKickBack::RequestKickBack()

    {

    if (!IsActive())

        {

        TRequestStatus * status = &idleStatus;

        iIdleThread.RequestComplete(status, KErrNone);

        iStatus = KRequestPending;

        SetActive();

        }

    }

</codeblock><p>The kickback thread is signalled using a handle to that thread

(<codeph>iIdleThread</codeph>) and the <codeph>iIdleStatus</codeph> semaphore.

The <codeph>CKickBack</codeph> active object is then set active and waits

for the kickback thread to signal back to the main thread using the <codeph>iStatus</codeph> semaphore. </p><p>After

construction, the kickback thread spends its whole time in an endless loop,

waiting for <codeph>iIdleStatus</codeph> to be signalled from the window server

main thread: </p><codeblock xml:space="preserve">void CWindowServer::CDefaultAnimationScheduler::CKickBack::Loop()

    {

    FOREVER

        {

        User::WaitForRequest(iIdleStatus);

        TRequestStatus * status = &iStatus;

        iWservThread.RequestComplete(status,KErrNone);

        }

    }

</codeblock><p>Once <codeph>iIdleStatus</codeph> is completed, the kickback

thread resumes execution, immediately signals completion of the <codeph>iStatus</codeph> semaphore

back to the window server main thread (via the <codeph>iWservThread RThread</codeph> handle),

then waits for the next kickback. </p><p>Back in the window server thread,

the <codeph>CKickBack</codeph> active object is run, which simply calls back

a function that was supplied at construction time: </p><codeblock xml:space="preserve">void CWindowServer::CDefaultAnimationScheduler::CKickBack::RunL()

       {

       iCallBack.CallBack();

       }

</codeblock><p>The time between the initial <codeph>RequestKickBack()</codeph> and <codeph>CKickBack::RunL()</codeph> should

be very short but there is an opportunity for higher priority threads (or

active objects in the window server) to get some CPU time in-between. </p><p>The

problem occurs after the call to <codeph>RequestComplete()</codeph> in <codeph>RequestKickBack()</codeph>.

On a single core environment, the subsequent assignment <codeph>iStatus=KRequestPending</codeph> will

always happen before the kickback thread can resume execution (because the

main thread has a higher priority than the kickback thread). On an SMP environment

however, the kickback thread can resume execution in parallel with the main

thread, on a different CPU. It's possible for the<codeph> RequestComplete()</codeph> on <codeph>iStatus</codeph> in

the <codeph>Loop()</codeph> function of the kickback thread to occur before

the<codeph> iStatus=KRequestPending </codeph>assigment in the main thread.

If this happens then the <codeph>KErrNone</codeph> value of <codeph>iStatus</codeph> will

be overwritten by <codeph>KRequestPending</codeph>, and the <codeph>CKickBack</codeph> active

object will end up being active but with an <codeph>iStatus</codeph> of <codeph>KRequestPending</codeph>.

The active scheduler will then receive the signal from the kickback thread

and iterate through its active objects, looking for the one that is active

but not pending. It will get to the end of the list without finding a valid

active object, causing a stray event panic. </p><p>The simple solution is

to bring forward the assignment of <codeph>iStatus</codeph> in <codeph>RequestKickBack()</codeph> to

before the call to <codeph>RequestComplete()</codeph>. </p><codeblock xml:space="preserve">void CWindowServer::CDefaultAnimationScheduler::CKickBack::RequestKickBack()

    {

    if (!IsActive())

        {

        iStatus = KRequestPending;

        SetActive();

        TRequestStatus * status = &idleStatus;

        iIdleThread.RequestComplete(status, KErrNone);

        }

    }

</codeblock><p>Stray event panics can be particularly difficult to track down

because the call stack at the point it panics does not show the root cause

of the problem. </p></section>

<section id="GUID-A959F5DF-74C9-4F7B-B549-A971684E056E"><title>Unsafe assumption

of Publish and Subscribe message ordering</title><p>The Publish and Subscribe

mechanism is one of the standard IPC mechanisms available in Symbian platform.

There are unique SMP error patterns associated with its use. </p><p><b>Case

study: <codeph>t_InitialiseLocale</codeph> fails on X86 SMP Hardware </b></p><p>The

Generic OS Services utility, <filepath>initialiselocale.exe</filepath> uses

a Publish and Subscribe property to indicate to subscribers that locale initialisation

is complete. It also uses the Rendezvous mechanism to synchronise with threads

that are waiting for <filepath>initialiselocale.exe</filepath> to start. The

problem manifested itself in the test program <filepath>t_initialiselocale.exe</filepath>,

which first rendezvous with <filepath>initialiseLocale.exe</filepath> and

then checks whether it has initialised using the P&S property. Here are

the respective code fragments: </p><p>From <filepath>os/ossrv/lowlevellibsandfws/apputils/initlocale/src/initialiselocale.cpp</filepath>:</p><codeblock xml:space="preserve">void CExtendedLocaleManager::ConstructL()

    {

    TUid LocalePropertyUid;

    LocalePropertyUid.iUid = KUidLocalePersistProperties ;

    ...

    //now call RProcess::Rendezvous to indicate initialiseLocale is completed with no error

    RProcess().Rendezvous(KErrNone);

    // Set property to indicate initialisation complete

    User::LeaveIfError(RProperty::Set(LocalePropertyUid, EInitialisationState, ELocaleInitialisationComplete));

    // All ready to go! Start the active sheduler.

    CActiveScheduler::Start();

    }

</codeblock><p>From <filepath>os/ossrv/lowlevellibsandfws/apputils/initlocale/test/t_initialiselocale.cpp</filepath>:</p><codeblock xml:space="preserve">void TestLocaleChanges()

    {

    RProcess process;

    ...

    TInt r = process.Create(KInitialiseLocaleExeName, KNullDesC);

    if(r == KErrNone)

        {

        TRequestStatus stat;

        process.Rendezvous(stat);

        process.Resume(); // Start the process going

        //wait for the locale initialisation to complete first before testing

        User::WaitForRequest(stat);

        TEST2((stat.Int()==KErrNone)||(stat.Int()==KErrAlreadyExists),ETrue);

        TInt flag;

        TUid initLocaleUid=TUid::Uid(KUidLocalePersistProperties);

        r=RProperty::Get(initLocaleUid,(TUint)EInitialisationState,flag);

        TEST2(r,KErrNone);

        //check for the P&S flag that indicates localeinitialisation complete

        TEST2(flag,ELocaleInitialisationComplete); //!!! Test failed !!!

        ...

</codeblock><p>The problem manifested itself in an SMP environment because

the <codeph>t_initialiselocale</codeph> main thread reached the test marked

"Test failed" before the setting of this property in the <codeph>InitialiseLocale</codeph> main

thread was published. Therefore, the test failed. The solution was to simply

move the setting of the <codeph>KUidLocalePersistProperties</codeph> property

in <codeph>InitialiseLocale</codeph> to before the call to <codeph>Rendezvous()</codeph>.

It could then be guaranteed that the property would be published by the time <codeph>t_initialiselocale</codeph> had

rendezvoused with <codeph>InitialiseLocale</codeph>. </p><codeblock xml:space="preserve">void CExtendedLocaleManager::ConstructL()

    {

    TUid LocalePropertyUid;

    LocalePropertyUid.iUid = KUidLocalePersistProperties ;

    ...

    // Set property to indicate initialisation complete

    User::LeaveIfError(RProperty::Set(LocalePropertyUid, EInitialisationState, ELocaleInitialisationComplete));

    //now call RProcess::Rendezvous to indicate initialiseLocale is completed with no error

    RProcess().Rendezvous(KErrNone);

    // All ready to go! Start the active sheduler.

    CActiveScheduler::Start();

    }

</codeblock></section>

<section id="E"><title>Relying on thread priority for execution order</title><p><b>Case

study: Priority dependence in the file server in <codeph>T_CFSSIMPLE</codeph> </b></p><p>The

file server has a main thread, a thread for each drive and a disconnect thread

(plus some other ones not relevant to this discussion). All requests are received

by the main thread and each drive has a lock associated with it. </p><p>When

an <codeph>RFs</codeph> session is closed, the main thread passes the request

to the disconnect thread, and the disconnect thread eventually ends up running <codeph>TFsSessionDisconnect::DoRequestL()</codeph>.

Closing the session is asynchronous within the file server even though it

appears to be synchronous to the client. There may be outstanding requests

on any number of drives so every drive thread needs to be told to cancel any

pending requests before the session can be entirely deleted by the file server. </p><p>Before

the fix,<codeph> TFsSessionDisconnect::DoRequestL</codeph>() looped over the

drive array and did the following for each drive: </p><ol>

<li id="F"><p>Lock the drive.</p></li>

<li id="G"><p>Check whether the drive exists.</p></li>

<li id="H"><p>If it does exist, send a request to the drive thread to clean

up any pending requests that are were going to be handled for the session

being closed.</p></li>

<li id="I"><p>Unlock the drive.</p></li>

<li id="J"><p>Wait for the cleanup request to complete. </p></li>

<li id="K"><p>Check whether write caching is enabled on that drive.</p></li>

<li id="L"><p>If it is enabled, send a request to the drive thread to flush

any dirty data.</p></li>

<li id="M"><p>Wait for the flush request to complete.</p></li>

</ol><p>The problem occurred when the <codeph>T_CFSSIMPLE</codeph> test application

(after performing its tests) unmounted a drive just before closing its <codeph>RFs</codeph> session.

Unmounting a drive is also asynchronous from the file server's perspective

- the main thread has to send a request to the drive thread so that it shuts

itself down cleanly. While unmounting a drive, the drive is locked. </p><p>The

disconnect thread is normally the highest priority thread in the file server,

even higher than the main thread, so usually nothing else gets to issue any

new requests while this is happening. However, in an SMP environment (or when

the crazy scheduler is being used), it's possible for an unmount request to

happen at the same time as a session disconnect. </p><p>If one of the drives

is unmounted at the same time as the session disconnect, then the unmount

sets the drive as not present and kills the associated thread. Step 3 above

is safe because it acquires the lock on the drive before modifying it. The

disconnect thread may get there first and send the cleanup request; then the

drive thread unmounts itself (since the lock is released) and the cleanup

request completes with <codeph>KErrNotReady</codeph> (which is ignored by

the code). Alternatively, the unmount request may get there first; then the

disconnect thread never thinks the drive is there in the first place so does

not issue the cleanup request. </p><p>The problem was in step 7 - this part

of the code didn't lock the drive or check whether it existed. It assumed

that because the drive existed earlier in the loop it still continued to exist.

Since step 4 unlocked the drive, this was not necessarily true - there was

an opportunity for it to be unmounted and a flush request sent to a non-existent

drive, resulting in a panic. </p><p>The solution was to protect step 7 with

a lock and a "check whether drive exists" operation like in step 3. Here is

the actual code change: </p><p>Before: </p><p>From <filepath>os/kernelhwsrv/userlibandfileserver/fileserver/sfile/sf_ses.cpp</filepath></p><codeblock xml:space="preserve">if (TFileCacheSettings::Flags(i) &amp; (EFileCacheWriteEnabled | EFileCacheWriteOn))

    {

    // Flush dirty data pR->Set(FlushDirtyDataOp,aRequest->Session());

    pR->SetDriveNumber(i);

    pR->Status()=KRequestPending;

    pR->Dispatch();

    User::WaitForRequest(pR->Status()); // check request completed or cancelled (by file system dismount)

    }

__ASSERT_ALWAYS(pR->Status().Int()==KErrNone||pR->Status().Int()==KErrCancel,Fault(ESessionDisconnectThread2));

__THRD_PRINT2(_L("Flush dirty data on drive %d r=%d"),i,pR->Status().Int());

}

</codeblock><p>After: </p><codeblock xml:space="preserve">if (TFileCacheSettings::Flags(i) &amp; (EFileCacheWriteEnabled | EFileCacheWriteOn))

    {

    FsThreadManager::LockDrive(i);

    if(!FsThreadManager::IsDriveAvailable(i,EFalse)||FsThreadManager::IsDriveSync(i,EFalse))

        {

        FsThreadManager::UnlockDrive(i);

        continue;

        }

    // Flush dirty data

    pR->Set(FlushDirtyDataOp,aRequest->Session());

    pR->SetDriveNumber(i);

    pR->Status()=KRequestPending;

    pR->Dispatch();

    FsThreadManager::UnlockDrive(i);

    User::WaitForRequest(pR->Status());

    }

// check request completed or cancelled (by file system dismount which completes requests with KErrNotReady)

__ASSERT_ALWAYS(pR->Status().Int()==KErrNone||pR->Status().Int()==KErrNotReady,Fault(ESessionDisconnectThread2));

__THRD_PRINT2(_L("Flush dirty data on drive %d r=%d"),i,pR->Status().Int());

}

</codeblock><p><b>Case study: Unexpected running of low priority active object</b> </p><p>There

is a common pattern used in test code (and possibly also production code)

where: </p><ol>

<li id="N"><p>A low priority active object is used and immediately set "ready

to run". (Typically <xref href="GUID-619AF4A9-4DAF-3FA4-A704-717DB30B5389.dita"><apiname>CAsyncOneShot</apiname></xref> is used for the active

object.)</p></li>

<li id="O"><p>The active scheduler is nested.</p></li>

<li id="P"><p>One or more high priority active objects are started (probably

several times), which all make requests on a server. </p></li>

restarted. This continues until there is no more work for the high priority

active object(s) to do.</p></li>

scheduler.</p></li>

</ol><p>In a single core environment, the server always processes the client

requests before the client can run any other code (because the server runs

in a higher priority thread). When the client thread runs, the high priority

active objects are already "ready to run" and get serviced before the low

priority active object. </p><p>This pattern breaks under SMP conditions because

the client thread can continue to run on a different CPU at the same time

that the server is processing the client requests. At this point, the only

active object in the client thread that is "ready to run" is the low priority

active object. This object gets serviced and the active scheduler is stopped

earlier than expected. </p><p>For example, the window server test code function <codeph>WaitForRedrawsToFinish()</codeph> suffered

from the problem described above: </p><codeblock xml:space="preserve">EXPORT_C void WaitForRedrawsToFinish()

    {

    CStopTheScheduler* ps=new CStopTheScheduler(ETlibRedrawActivePriority-1);

    if(ps)

        {

        ps->Call();

        CActiveScheduler::Start();

        delete ps;

        }

    }

</codeblock><p>This creates a low priority active object and then nests the

active scheduler. The low priority active object used was defined like this: </p><codeblock xml:space="preserve">class CStopTheScheduler : public CAsyncOneShot

    {

public:

    CStopTheScheduler(TInt aPriority); //Pure virtual function from CActive void RunL();

    };

CStopTheScheduler::CStopTheScheduler(TInt aPriority) : CAsyncOneShot(aPriority)

    {}

void CStopTheScheduler::RunL()

    {

    CActiveScheduler::Stop();

    }

</codeblock><p>This just un-nests the active scheduler when it runs. Before <codeph>WaitForRedrawsToFinish()</codeph> is

called, there is already an outstanding request on the window server for it

to notify the test thread that there is a redraw event ready. If there is

a redraw ready, then a high priority active object called <codeph>CTRedraw</codeph> is

serviced, which processes the redraw event and immediately requests notification

of the next redraw from the server. </p><codeblock xml:space="preserve">EXPORT_C void CTRedraw::RunL()

    {

    TWsRedrawEvent redraw;

    iWs->GetRedraw(redraw);

    if (redraw.Handle()!=0 && redraw.Handle()!=ENullWsHandle)

        ((CTWin*)redraw.Handle())->Redraw(redraw.Rect());

    iWs->RedrawReady(&iStatus);

    SetActive();

    }

</codeblock><p>This is a simplified version of the code.</p><p>The intention

was that <codeph>CTRedraw::RunL()</codeph> would run many times, as long as

there were outstanding redraw events, before <codeph>CStopTheScheduler::RunL()</codeph> was

able to run. When there were no more outstanding events, <codeph>CStopTheScheduler::RunL()</codeph>would

run, which then allowed the code after <codeph>WaitForRedrawsToFinish()</codeph> to

run. </p><p>In an SMP environment though, every time <xref href="GUID-62F4FF8B-5E02-3312-8DB8-F8B2D2E7FC47.dita"><apiname>WaitForRedrawsToFinish()</apiname></xref> was

called, only one window was redrawn, even though redraw events for several

windows were waiting. <xref href="GUID-869360A0-8C14-3AB3-8FC8-94E12E7F21D7.dita#GUID-869360A0-8C14-3AB3-8FC8-94E12E7F21D7/GUID-7F533463-F972-3B53-9AAD-061EF10DCD20"><apiname>CTRedraw::RunL()</apiname></xref> only ran once before <xref href="GUID-B394E21C-942B-3A5D-AA57-1CE892826911.dita#GUID-B394E21C-942B-3A5D-AA57-1CE892826911/GUID-53EA2F9F-9730-3A8A-B206-96D1223D2FA6"><apiname>CStopTheScheduler::RunL()</apiname></xref> managed

to run. </p><p>There are at least two ways to fix this problem: </p><ol>

<li id="S"><p>After the call to <xref href="GUID-E3F0CB70-58E4-32FD-9828-71DF2F9976D3.dita"><apiname>RedrawReady()</apiname></xref>, add another

synchronous call to the window server. This solution is undesirable as the

additional IPC compromises the speed of the test code.</p></li>

<li id="T"><p>Modify the class <codeph>CStopTheScheduler</codeph> so that

in its<codeph> RunL()</codeph> it does: </p><codeblock xml:space="preserve">void CStopTheScheduler::RunL()

    {

    //Here a synchronous call to WSERV is made to make sure the asynchronous call has finished being processed

    if (iClient->RedrawHandler()->iStatus==KRequestPending)    //Check to see if the redraw active object is now ready to run

        CActiveScheduler::Stop();    //If it is not ready to run un-nest the active scheduler

    else

        Call(); //If it is ready to run set this active object ready to run again

    }

</codeblock><p>This was the solution that was chosen. </p></li>

</ol></section>

<section id="W"><title>Premature recreation of named objects</title><p>This

category of problems relates to the asynchronous cleanup of objects owned

by the kernel. On the user-side this involves classes derived from <codeph>RHandleBase</codeph>,

corresponding to a <codeph>DObject</codeph> in the kernel. These problems

take the following form: </p><ul>

<li><p>1a. An <codeph>RHandleBase</codeph> derived object is created user-side.</p></li>

<li><p>1b. A corresponding <codeph>DObject</codeph> is created kernel-side.</p></li>

<li><p>2. Something is done with that object.</p></li>

<li><p>3a. All user-side handles to that object are destroyed.</p></li>

<li><p>3b. Deletion of the corresponding <codeph>DObject</codeph> is initiated.</p></li>

<li><p>4a. A second <codeph>RHandleBase</codeph> derived object with the same

name is created user-side.</p></li>

<li><p>4b. A second corresponding <codeph>DObject</codeph> with the same name

is created kernel-side.</p></li>

</ul><p>The problem is that all <codeph>DObject</codeph> names must be unique

and 3b is not guaranteed to complete before 4b starts, therefore 4b (and 4a) <i>may</i> fail

with <codeph>KErrAlreadyExists</codeph>. </p><p>This most commonly manifests

itself during thread cleanup and so this case warrants more detailed discussion.

When a thread either exits or is killed externally, the thread itself runs

an exit handler inside the kernel. This runs at a slightly higher priority

than normal application threads and does part of the cleanup, including completing

log-ons. The last thing the exit handler does is to queue a DFC which gets

executed by the supervisor thread inside the kernel. This completes the cleanup,

including freeing the thread stacks and closing the <codeph>DThread</codeph> object

itself. If no other threads or processes have open handles on the <codeph>DThread</codeph>,

it is completely deleted at this point. </p><p>On a single processor system,

the following code would always work provided that the thread running it has

a lower priority than the supervisor thread and there are no undertakers in

the system (which is true for a lot of test code): </p><codeblock xml:space="preserve">RThread t;

TInt r = t.Create(KName, ...);

assert(r==KErrNone);

TRequestStatus s;

t.Logon(s);

assert(s.Int()==KRequestPending);

t.Resume();

User::WaitForRequest(s);

// read exit information for thread and do something with it

t.Close(); // thread actually disappears here

r = t.Create(KName,...); // recreate with same name

assert(r==KErrNone); 

</codeblock><p>The exit handler and supervisor would both run ahead of the

calling thread, so by the time the <codeph>t.Close()</codeph> line is executed

there would only be one handle remaining on the <codeph>DThread</codeph>.

If there are undertakers in the system, the supervisor thread exit handler

would open a handle on the exiting thread for each undertaker. The thread

would not then go away until all those handles were closed and the code above

would potentially fail. </p><p>On an SMP system the code above would clearly

fail, since regardless of thread priorities or undertakers, the supervisor

thread could run on a separate CPU from the thread calling <codeph>Close()</codeph> and

there is no guarantee that on return from <codeph>Close()</codeph> the thread

would have been destroyed. </p><p>A similar issue exists where, instead of

recreating the thread with the same name, a kernel heap check is performed

to ensure all kernel memory has been cleaned up. </p><p>There are three possibilities

to fix this problem: </p><ol>

<li id="X"><p>Recreate the thread with a different name.</p></li>

the heap check). </p><p>This is almost always a bad idea as there is no way

of knowing how long to wait and this may be different on any future system.</p></li>

</ol><p>In general, option 1 is preferred. For code that is likely to encounter

the problem in practice, APIs should be provided to aid with creating objects

with a unique name. For example, the <codeph>TDynamicDfcQue</codeph> and <codeph>Kern::DynamicDfcQCreate</codeph> APIs

are provided to help with creating DFC threads with unique names. </p><p>Using

a different name is easy in the above code example but it's not possible when

checking for kernel memory leaks (as in a lot of test code). In this case,

solution 3 can be employed, with the aid of <codeph>RHandleBase::NotifyDestruction </codeph>(in <filepath>e32cmn.h</filepath>)

and <codeph>CLOSE_AND_WAIT</codeph> (in <filepath>e32test.h</filepath>). This

solution is acceptable in a closed system, where there is control over all

handles to the object being recreated but in general there is no way of knowing

whether some component has an open handle to a thread (or what component it

might be). Any component can open a thread by name and hold the handle forever,

jamming the system while it waits for the handle to be closed. Therefore,

in practice you should always attempt to recreate it with a different name

(option 1). </p><p><b>Case study: <codeph>t_NamedPlugins</codeph> fails on

X86 SMP Hardware</b> </p><p>This is a classic example of the thread recreation

problem, with solution 3 employed as the fix.</p><p>From <filepath>os/ossrv/lowlevellibsandfws/apputils/tsrc/t_namedplugins.cpp</filepath>:</p><codeblock xml:space="preserve">LOCAL_C void NamedPluginsPanicConditionsThread()

    {

    ...

    _LIT(KName, "NamedPlugins_Panic_Thread");

    RThread thread;

    ...

    // Test EBafPanicBadResourceFileFormat

    TInt rc = thread.Create(KName,...);

    TEST(rc==KErrNone);

    ...

    thread.Close();

    // Test EBafPanicBadArrayPosition

    rc = thread.Create(KName,...);

    TEST(rc==KErrNone); //!!! Test failed !!!

    ...

</codeblock><p>The simple solution was to change the <codeph>thread.Close()</codeph> call

to <codeph>CLOSE_AND_WAIT(thread)</codeph> instead. This is acceptable for

test code because it is known that there will be no other threads holding

on to a handle to the <codeph>NamedPlugins</codeph> thread. </p></section>

</conbody><related-links>

<link href="GUID-E55F9286-F586-4665-93D8-86F1E7BE2C7C.dita"><linktext>SMP Developer

Tips</linktext></link>

</related-links></concept>