RFC: No more deadlock detection for POSIX locks

[Matthew Wilcox <willy@debian.org>]

The deadlock detection for posix locks really isn't worth anything
any more.  It was always slightly dubious, since a parent/child could
remove each other's locks (thanks, POSIX!).  But now it's really dubious
since we store the TID, not the PID of the requesting process, and any
thread can unlock a lock set by another thread.

Here's one situation in which it can falsely return -EDEADLK:

TID 1001, PID 1002 takes lock A
TID 1003, PID 1004 takes lock B
TID 1001, PID 1005 takes lock B, blocks
TID 1003, PID 1004 takes lock A, gets -EDEADLK.
Even though (1001,1002) isn't blocking on any lock and will release lock A
in the future.

So how about we just delete the nasty deadlock detection code?  I've never
been fond of the user-triggerable O(N^2) algorithm, and we're permitted
to not implement it (POSIX suggests applications set a timer to detect
deadlock themselves, so anyone writing a portable application is already
doing this).

Objections?

-- 
Revolutions do not require corporate support.