On Fri, 18 Nov 2016, John Stultz wrote: > From: Liav Rehana <liavr@xxxxxxxxxxxx> > > During the calculation of the nsec variable in the inline function > timekeeping_delta_to_ns, it may undergo a sign extension if its msb > is set just before the shift. The sign extension may, in some cases, > gain it a value near the maximum value of the 64-bit range. This is > bad when it is later used in a division function, such as > __iter_div_u64_rem, where the amount of loops it will go through to > calculate the division will be too large. One can encounter such a > problem, for example, when trying to connect through ftp from an > outside host to the operation system. When the OS is too overloaded, > delta will get a high enough value for the msb of the sum > delta * tkr->mult + tkr->xtime_nsec to be set, and so after the > shift the nsec variable will gain a value similar to > 0xffffffffff000000. Using a variable with such a value in the > inline function __iter_div_u64_rem will take too long, making the > ftp connection attempt seem to get stuck. > The following commit fixes that chance of sign extension, while > maintaining the type of the nsec variable as signed for other > functions that use this variable, for possible legit negative > time intervals. > > Thomas/Ingo: This is for tip:timers/urgent. Certainly not! My objections against this still stand. See: http://lkml.kernel.org/r/alpine.DEB.2.20.1609261956160.4915@nanos http://lkml.kernel.org/r/alpine.DEB.2.20.1609270929170.4891@nanos If we have legitimate use cases with a negative delta, then this patch breaks them no matter what. See the basic C course section in the second link. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html