Problem with commit deda2e81961e96be4f2c09328baca4710a2fd1a0
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From: Larry Finger on 12 Aug 2010 16:20 Guys, With the above commit, building an i386 version of the kernel results in the following from the build: kernel/built-in.o: In function `logarithmic_accumulation': /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to `__umoddi3' /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to `__udivdi3' make: *** [.tmp_vmlinux1] Error 1 Reverting the patch allows the system to build correctly. Larry -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Linus Torvalds on 12 Aug 2010 16:30 On Thu, Aug 12, 2010 at 1:18 PM, Larry Finger <Larry.Finger(a)lwfinger.net> wrote: > > With the above commit, building an i386 version of the kernel results in the > following from the build: > > kernel/built-in.o: In function `logarithmic_accumulation': > /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to > `__umoddi3' > /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to > `__udivdi3' > make: *** [.tmp_vmlinux1] Error 1 > > Reverting the patch allows the system to build correctly. Damn. It's your compiler turning a while-loop into a divide. Which likely isn't even an optimization, but whatever. John: I think that while-loop needs to be something like if (raw_nsecs >= NSEC_PER_SEC) { u64 raw_secs = raw_nsecs; raw_nsecs = do_div(raw_secs, NSEC_PER_SEC); raw_time.tv_sec += taw_secs; } raw_time.tc_nsec = raw_nsecs; which is sad and overly complicated, but the simple thing seems to get messed up by the compiler. Untested. Maybe I got the complex do_div() semantics wrong. Somebody needs to check. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Larry Finger on 12 Aug 2010 16:50 On 08/12/2010 03:26 PM, Linus Torvalds wrote: > On Thu, Aug 12, 2010 at 1:18 PM, Larry Finger <Larry.Finger(a)lwfinger.net> wrote: >> >> With the above commit, building an i386 version of the kernel results in the >> following from the build: >> >> kernel/built-in.o: In function `logarithmic_accumulation': >> /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to >> `__umoddi3' >> /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to >> `__udivdi3' >> make: *** [.tmp_vmlinux1] Error 1 >> >> Reverting the patch allows the system to build correctly. > > Damn. It's your compiler turning a while-loop into a divide. Which > likely isn't even an optimization, but whatever. > > John: I think that while-loop needs to be something like > > if (raw_nsecs >= NSEC_PER_SEC) { > u64 raw_secs = raw_nsecs; > raw_nsecs = do_div(raw_secs, NSEC_PER_SEC); > raw_time.tv_sec += taw_secs; > } > raw_time.tc_nsec = raw_nsecs; > > which is sad and overly complicated, but the simple thing seems to get > messed up by the compiler. > > Untested. Maybe I got the complex do_div() semantics wrong. Somebody > needs to check. I'll try it. The system with the problem has gcc (SUSE Linux) 4.3.2 [gcc-4_3-branch revision 141291]. That is on a 64-bit system cross compiling with ARCH=i386. The real 32-bit system with gcc (SUSE Linux) 4.5.0 20100604 [gcc-4_5-branch revision 160292] builds OK. It just wasn't finished when I wrote the first message. Larry -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: john stultz on 12 Aug 2010 17:00 On Thu, 2010-08-12 at 15:18 -0500, Larry Finger wrote: > Guys, > > With the above commit, building an i386 version of the kernel results in the > following from the build: > > kernel/built-in.o: In function `logarithmic_accumulation': > /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to > `__umoddi3' > /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to > `__udivdi3' > make: *** [.tmp_vmlinux1] Error 1 > > Reverting the patch allows the system to build correctly. Ugh. I'm surprised it picks *this* loop to optimize instead of the similar one right above. I'm guessing its the local raw_nsecs value, but whatever. Also surprised Jason's testing didn't hit this issue, but its probably a gcc version thing. Regardless, I clearly need to give i386 more love in my testing. My profuse apologies. As suggested by Linus, here's the do_div explicit version. It builds ok on i386 & x86_64, but I have not yet tested it. Larry, Jason: Could you verify it works for you (and avoids the original issue)? thanks -john >From 70b106aaaa1de81a635bbd7ea6edc244ba098d7e Mon Sep 17 00:00:00 2001 From: John Stultz <johnstul(a)us.ibm.com> Date: Thu, 12 Aug 2010 13:45:28 -0700 Subject: [PATCH] time: Workaround gcc loop optimization that causes 64bit div errors Some versions of gcc apparently aggressively optimize the raw time accumulation loop, replacing it with a divide. On 32bit systems, this causes the following link errors: undefined reference to `__umoddi3' undefined reference to `__udivdi3' This patch replaces the accumulation loop with a do_div, as suggested by Linus. Signed-off-by: John Stultz <johnstul(a)us.ibm.com> --- kernel/time/timekeeping.c | 7 ++++--- 1 files changed, 4 insertions(+), 3 deletions(-) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index dc54b72..d0ef5aa 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -710,9 +710,10 @@ static cycle_t logarithmic_accumulation(cycle_t offset, int shift) /* Accumulate raw time */ raw_nsecs = timekeeper.raw_interval << shift; raw_nsecs += raw_time.tv_nsec; - while (raw_nsecs >= NSEC_PER_SEC) { - raw_nsecs -= NSEC_PER_SEC; - raw_time.tv_sec++; + if (raw_nsecs >= NSEC_PER_SEC) { + u64 raw_secs = raw_nsecs; + raw_nsecs = do_div(raw_secs, NSEC_PER_SEC); + raw_time.tv_sec += raw_secs; } raw_time.tv_nsec = raw_nsecs; -- 1.6.0.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Larry Finger on 12 Aug 2010 23:20
On 08/12/2010 03:52 PM, john stultz wrote: > > Ugh. I'm surprised it picks *this* loop to optimize instead of the > similar one right above. I'm guessing its the local raw_nsecs value, but > whatever. Also surprised Jason's testing didn't hit this issue, but its > probably a gcc version thing. > > Regardless, I clearly need to give i386 more love in my testing. > My profuse apologies. > > As suggested by Linus, here's the do_div explicit version. It builds ok > on i386 & x86_64, but I have not yet tested it. > > Larry, Jason: Could you verify it works for you (and avoids the original > issue)? This one builds for me with both compilers. It appears to run OK. As to the original issue - I don't think I ever saw the problem. I'll leave that question for Jason. Larry -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |