[patch] POSIX advisory mode lock panic fix by Dfly

[Matthew Dillon]

:Matt, consider the following lock situation:
:--rrx-xrr-rrr-
:You want to acquire:
:-----r-------- ==> one range needed
:-----x-------- ==> no range needed
:-----rr------- ==> one range can be freed (not absolute sure, I got this case)
:---xxxxx------ ==> two ranges can be freed
:-----------w-- ==> two additional locks needed
:
:Esp. the forth case is difficult two handle if you want to merging
:allocation and refcounting and still avoid a second run the list.

    It looks like a simple, single-run iteration to me.  The one thing you
    don't want to do is embed any resource limit checking in the middle
    of the loop, if that is what you mean.

:>     I noticed one issue with the code, and that is the fact that since the
:>     malloc() can block and the locking operations appear to be called
:>     without a locked vnode (which is how we want to call them), there is
:>     a race where the lock list can get corrupted if malloc() blocks.
:
:Thanks, yes. I'll fix that in the second iteration, the patch is complex
:enough for now and the problem isn't new. How you want to protect this in
:the long run? IMO locking either the inode or the lockf is necessary, but
:this can of course be a simple token.

    Well, for now an independant token (just use one from the token pool) 
    will be sufficient.  Not optimal, but sufficient.  But you still have to
    pre-allocate the lock structures you think you might need because you
    still can't afford to block in the middle.

:>     I think the solution is to pre-allocate two lock structures at the
:>     beginning and then free whichever ones we wind up not using at 
:>     the end.  If you are worried about performance, you can implement a
:>     simple linked list in the per-cpu globaldata to cache free structures.
:>     For a first implementation, though, I'd simply use malloc().
:
:I thought about adding a reference count to lockf and increment it on all
:changes, but your solution is cleaner :)
:
:Joerg

    Definitely wouldn't work anyway.  Remember that ultimately we need to
    find an MP solution to this, which is likely going to mean using a
    token, which does not guarentee atomicy through a blocking condition.
    So all blocking conditions must be removed.

					-Matt
					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>