We fixed this bug before, but it didn't take.  It may have been the case
that the problem was first noticed to occur in a CONFIG_REGPARM compile.
But it's not regparm functions that need not to make tail calls, it's
asmlinkage functions called with a user pt_regs frame on the stack
supplying their arguments.  prevent_tail_call probably doesn't do anything
at all in regparm functions (your argument registers are going to be
clobbered, period).  It was a braino to conditionalize that definition in
the first place.
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>