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From:Warren Young Date:October 25 2007 4:34pm
Subject:Re: Sample files -- BadQuery w/Errnum patch part 4
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Jim Wallace wrote:
>> 2. Why does the first example need the loop?  The second pass 
>> isn't actually needed to make it deadlock, is it?
> Not to get the deadlock, but to have it succeed, it must do the second
> pass 

Okay, I just didn't understand that you want it to show both states, 
success and failure.

There's an implied solution here that I wouldn't want to present in a 
MySQL++ example, because that implies an endorsement.  The examples are 
supposed to show good programming style and acceptable idioms.  "I'm 
doing it like the examples" should never be a defense for bad code.

In a real program, looping is no fix for this problem, because you can't 
know that a second try will always fix it.  The lock could be reacquired 
by another program by the time you loop back around.  And putting the 
whole thing in a while (1) or "for (i = 0; i < bignum; ++i)" loop isn't 
any good either.

I think we don't want the example to succeed.  It's intentionally 
demonstrating a programming bug, so it should fail.

> problem is that deadlock2 hangs waiting for the lock 1.  Once deadlock1
> tries to get lock 2 (which deadlock2 has), it immediately gets the
> deadlock exception, and rolls back, releasing lock1 so deadlock2
> completes ok.  And after deadlock2 completes, deadlock1 can retry and
> succeed.

Okay, so it's like with regular mutexes, where the order of acquisition 
matters.  Do it in the same order in all programs, and it can't 
deadlock, yes?

If we get rid of the loop and maybe add another pause point, is the only 
difference the order of lock acquisition?  If so, you could just add an 
option to examples/cmdline.cpp to switch between the modes.  It could be 
just a generic "mode switch" (-m?) taking an integer that means 
something only to some programs.  To examples/deadlock, -m1 says to 
acquire lock1 first, and -m2 lock2.  All other programs can ignore it 
for now.  The value can go in a global variable, like dtest_mode.
Thread
RELEASE: v2.3.2Warren Young11 Jul
  • Getting errnum() in exception?Jim Wallace20 Sep
    • Re: Getting errnum() in exception?Warren Young22 Sep
  • Building the head of SVN with MSVC?Jim Wallace25 Sep
    • Re: Building the head of SVN with MSVC?Warren Young26 Sep
  • Error num in BadQuery patchJim Wallace25 Sep
    • RE: Error num in BadQuery patchJim Wallace26 Sep
      • Re: Error num in BadQuery patchWarren Young26 Sep
    • Re: Error num in BadQuery patchWarren Young26 Sep
  • v2.3.2 and execute?Jim Wallace5 Oct
    • Re: v2.3.2 and execute?Warren Young5 Oct
  • Building svn tip on WindowsJim Wallace24 Oct
    • Re: Building svn tip on WindowsWarren Young25 Oct
  • Patch for better exception use -- BadQuery w/Errnum patch part 1Jim Wallace24 Oct
  • Better exception usage -- BadQuery w/Errnum patch part 2Jim Wallace24 Oct
  • Expose errnum() in query -- BadQuery w/Errnum patch part 3Jim Wallace24 Oct
    • Re: Expose errnum() in query -- BadQuery w/Errnum patch part 3Warren Young25 Oct
      • RE: Expose errnum() in query -- BadQuery w/Errnum patch part 3Jim Wallace25 Oct
  • BadQuery w/Errnum patch part 4Jim Wallace24 Oct
    • Re: BadQuery w/Errnum patch part 4Warren Young25 Oct
      • RE: BadQuery w/Errnum patch part 4Jim Wallace25 Oct
  • Sample files -- BadQuery w/Errnum patch part 4Jim Wallace24 Oct
    • Re: Sample files -- BadQuery w/Errnum patch part 4Warren Young25 Oct
      • RE: Sample files -- BadQuery w/Errnum patch part 4Jim Wallace25 Oct
        • Re: Sample files -- BadQuery w/Errnum patch part 4Warren Young25 Oct
          • RE: Sample files -- BadQuery w/Errnum patch part 4Jim Wallace25 Oct
          • RE: Sample files -- BadQuery w/Errnum patch part 4Jim Wallace25 Oct
            • Re: Sample files -- BadQuery w/Errnum patch part 4Warren Young25 Oct
              • RE: Sample files -- BadQuery w/Errnum patch part 4Jim Wallace25 Oct
      • RE: Sample files -- BadQuery w/Errnum patch part 4Jim Wallace25 Oct