On 04/27/2015 07:18 AM, Heiko Carstens wrote: > On Sat, Apr 25, 2015 at 08:50:49AM -0400, Rik van Riel wrote: >> On 04/25/2015 05:43 AM, Heiko Carstens wrote: >>> ...the READ_ONCE() doesn't give you any guarantees about reading >>> tsk->acct_timexpd in an atomic way. >>> Well, actually you don't need atomic semantics, but only to make sure that >>> the read access happens with a single instruction, since you want to protect >>> against interrupts. >>> But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still >>> result >>> in two instructions on 32 bit architectures. >>> (or isn't there currently no 32 bit architecture with 64 bit cputime_t >>> left?) >> >> Even if there is (maybe some ARM system?), can we even guarantee >> that a single instruction to read 64 bits exists on such a system? > > I wouldn't bet on it. I can only talk for s390 and there is an instruction > available which would do that. But since s390 is now a 64 bit only > architecture > it doesn't matter anyway. > For other architectures I'd say: no, you can't rely on that.
So what can I do to move forward with this patch? It speeds up syscall entry / exit by 7% when nohz_full is enabled on a CPU... Should I have the irq block compiled in only when sizeof(cputime_t) > sizeof(long) ? -- All rights reversed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
