Re: [PATCH 2/4] set_restore_sigmask TIF_SIGPENDING

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, 28 Mar 2008, Roland McGrath wrote:
>
> Set TIF_SIGPENDING in set_restore_sigmask.  This lets arch code take
> TIF_RESTORE_SIGMASK out of the set of bits that will be noticed on
> return to user mode.  On some machines those bits are scarce, and we
> can free this unneeded one up for other uses.

Hmm. That probably means that TIF_RESTORE_SIGMASK shouldn't be a "TIF" 
flag at all, but a "TS" ("thread status") flag.

The TS flags are faster, because they are thread-synchronous and do not 
need atomic accesses (ie they are purely thread-local in setting, testing 
and clearing).

Of course, it may well not be worth it. Unlike the TIF flags, the TS flags 
have been architecture-specific and I don't think all architectures even 
do them (x86 uses them for FP state bits and stuff like that).

I guess TIF_RESTORE_SIGMASK is never *so* performance-critical that we'd 
care about the difference between a single cycle (approx) for a non-atomic 
"or" into memory and an atomic bitop (~50 cycles or so). 

			Linus