- Fun debugging story; unreachable mapped network drives caused the logon process to stall for ~5s on every boot.
The story unfolds the week before last when I made a trip to Orlando to speak at Microsoft’s TechEd North America conference. While I was there I began to experience five minute black-screen delays when I logged on to my laptop’s Windows 7 installation:
I’d typically chalk up an isolated delay like this to networking issues, common at conferences and with hotel WiFi, but I hit the issue consistently switching between the laptop’s Windows 8 installation, where I was doing testing and presentations, and the Windows 7 installation, where I have my development tools. Being locked out of your computer for that long is annoying to say the least.
After I rebooted, I logged on, waited for five minutes looking at a black screen, then finally got to my desktop, where I ran Process Monitor again and saved the boot log. Instead of scanning the several million events that had been captured, which would have been like looking for a needle in a haystack, I used this Process Monitor filter to look for operations that took more than one second, and hence might have caused the slow down:
My eye was drawn to a pattern of the same four threads sampled every second throughout the entire black-screen period. The stack of the third thread was much more interesting. It was many frames deep, including calls into functions of the Multiple UNC Provider (MUP) and Distributed File System Client (DFSC) drivers, both related to accessing file servers:
The next frame’s function, WNetRestoreAllConnectionsW, combined with the deeper calls into file server functions, led me to conclude that Winlogon was trying to restore file server drive letter mappings before proceeding to launch my shell and give me access to the desktop. I quickly opened Explorer, recalling that I had two drives mapped to network shares hosted on computers inside the Microsoft network, one to my development system and another to the internal Sysinternals share where I publish pre-release versions of the tools. While at the conference I was not on the intranet, so Winlogon was unable to reconnect them during the logon and was eventually – after many minutes – giving up:
. Confident I’d solved the mystery, I right-clicked on each share and disconnected it. I rebooted the laptop to verify my fix (workaround to be precise), and to my immense satisfaction, the logon proceeded to the desktop within a few seconds. The case was closed! As for why the delays were unusually long, I haven’t had the time – or need - to investigate further. The point of this story isn’t to highlight this particular issue, but illustrate the use of the Sysinternals tools and troubleshooting techniques to solve problems.