On Mon, Feb 27, 2012 at 9:22 AM, Oleg Nesterov [off-list ref] wrote:

SYNCHRONOUS_MASK just tells dequeue_signal() "pick them first".
This is needed to make sure that the handler for, say SIGSEGV,
can use ucontext->ip as a faulting addr.

It's desireable to have these signals handled first just so that the thread
state that provoked the signal is not obscured by an unrelated asynchronous
signal having its handler setup done beforehand.