Microsoft KB Archive/100579

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Claiming Critical Sections on Timer Ticks


The information in this article applies to:

  • Microsoft Windows Software Development Kit (SDK) 3.1


A deadlock situation in which the system is apparently frozen but the mouse still moves on the screen.


Claiming a critical section from inside a timer (IRQ 0) interrupt handler may cause a deadlock in the following scenario:

  1. Virtual machine (VM) A claims the critical section.
  2. VM B receives a virtual timer interrupt, tries to claim the critical section itself, and gets suspended.
  3. VM A waits on a timer tick before it releases the critical section, but will no longer see timer interrupts. This deadlocks the machine.

Note that it is not uncommon for global terminate-and-stay-resident (TSR) programs to wait for timer interrupts to occur before resuming execution; in particular, many network drivers use this technique to periodically retry unsuccessful packet transmissions.


The workaround for this problem is to use the documented TimerCriticalSection entry in SYSTEM.INI, which will implicitly wrap the virtual interrupt into a critical section.

Alternatively, application code could call into a VxD to obtain the critical section status (using the Get_Crit_Sect_Status service) before attempting to claim the critical section in a timer tick.


VM A no longer receives timer interrupts in the above scenario because the virtual timer device (VTD) sets the V86 interrupt vector of each virtual machine to a small code stub that filters out certain timer interrupts. In particular, the code stub does not allow nested timer interrupts to be reflected into the same virtual machine. Thus, virtual machine B has a pending timer interrupt, and therefore, virtual machine A does not receive any timer interrupts but does not release the critical section because it waits for a timer interrupt, while VM B does not finish processing the timer interrupt because it is suspended on the critical section. This scenario will deadlock the two processes.

NeilSa discovered a bug in the VTD that initializes the code stub at the wrong time, causing a black screen of death under certain circumstances. The 1-byte fix for that bug involves relocating the VTD_Device_Init code to Init_Complete, but it will not fix the problem discussed in this article. dskbsweep

Additional query words: 3.10

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Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310

Last Reviewed: November 3, 1999
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