Sunday, January 17, 2016

On interrupts handling by a kernel

This is an excerpt from Jonathan S. Shapiro's article "Design Note: Kernel Interrupt and Concurrency Management" 

With the exception of the interval timer interrupt and (on SMP systems) the interprocessor interrupt, Coyotos does not handle interrupts directly. Also, the kernel does not support interrupt nesting. Our working assumptions are:
  • The longest kernel path is very short (units of microseconds).
  • We can therefore defer doing anything about interrupts until we are just about to exit the kernel, when kernel state is not ``in flight.''
  • Since we are going to defer interrupt handling to the scheduler, the kernel does not need to deal with nested interrupts.
  • It is preferable, and not much unduly expensive, to prioritize interrupt handlers using the kernel scheduler rather than hardware priorities.
  • If it were not for the need to deal with the interval timer interrupt and IPI issues, we could hypothetically run with interrupts entirely disabled in the kernel.
Ignoring the interval timer and the interprocessor interrupt, the primary action taken by the kernel in response to an interrupt is to wake up the relevant driver process by moving it from a per-interrupt stall queue to the ready queue. A preemption occurs only if the awakened process is higher priority than the currently executing process. On hardware platforms where a prompt hardware-level acknowledge is required to avoid interrupt controller lockup, the interrupt handler takes care of this as well.

Tuesday, January 5, 2016

On importance of FileObject->Vpb pointer

 An easily overlooked issue in FS(file systems) and FS filters development for Windows is Vpb pointer in FILE_OBJECT structure, i.e. Volume Parameters Block ( struct VPB ). It is responsibility of FS or FS filter ( in case of stacked FS, i.e. FSD-over-FSD ) to initialize this pointer to a correct value. It is especially important in case of FSD over FSD implementations as some underlying FSs check the VPB->ReferenceCount pointer on their PnP path and both upper FSD and lower FSD share the same VPB pointer. The kernel references Vpb in IopParseDevice routine that calls IRP_MJ_CREATE dispatch routine, but doesn't set Vpb pointer for FileObject sent with create Irp, and dereferences Vpb in IopDeleteFile just after IRP_MJ_CLOSE completes but IopDeleteFile fetches the pointer from FileObject->Vpb that is set by FS when processing IRP_MJ_CREATE. If FS or FS filter fails to set Vpb pointer PnP safe remove for some file systems would be impossible as Vpb->ReferenceCount never drops below a checked value.

 I encountered this problem when working on my own implementation for FSD-over-FSD FS filter. The problem manifested itself on FAT32 , NTFS was clean from this issue.

 Below are two call stacks for 32 bit Windows 8, the first is for a case when Vpb->ReferenceCount is bumped and the second for a case when it is decremented.