Do yourself a favor, if you already have a weapon in hand (umbrella or cane), use it. Don't try and juggle one weapon while going for the other. You have distance, and a knife wielder isn't going to be able to deal with that if you're using the first weapon effectively. When you stop to try and draw your firearm, you become EXCEPTIONALLY vulnerable, especially if your method of concealment has your weapon underneath jacket/shirt, etc. Small of back carry is one of the better ways to conceal a gun, but lousey for getting it out at any rate of speed.