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Date: Thu, 26 May 2022 10:03:13 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
linux-toolchains@...r.kernel.org
Subject: Re: objtool warning, branch, patch, and .config (GCC bug)
+ Josh + toolchains
Start of thread (including .config) here:
https://lore.kernel.org/all/20220525173332.GA4008581@paulmck-ThinkPad-P17-Gen-1/T/#u
On Wed, May 25, 2022 at 10:34:44AM -0700, Paul E. McKenney wrote:
> On Wed, May 25, 2022 at 10:33:32AM -0700, Paul E. McKenney wrote:
> > Hello, Peter,
> >
> > Please see below for a .config that emits this warning:
> >
> > $ make -j32 kernel/rcu/{rcuscale,rcu_segcblist,rcutorture,refscale,srcutree,sync,tree,update}.o kernel/torture.o
> > kernel/rcu/update.o: warning: objtool: rcu_tasks_trace_pertask() falls through to next function rcu_tasks_trace_postscan.cold()
> > kernel/rcu/update.o: warning: objtool: rcu_tasks_trace_postscan() falls through to next function rcu_early_boot_tests()
> >
> > The commit where I see it is for-peterz.2022.05.25a on -rcu.
> >
> > If I apply this patch, it goes away. Sometimes it goes away of its own
> > accord, no idea why.
>
> And the compiler:
>
> $ gcc -v
> Using built-in specs.
> COLLECT_GCC=gcc
> COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
> OFFLOAD_TARGET_NAMES=nvptx-none:hsa
> OFFLOAD_TARGET_DEFAULT=1
> Target: x86_64-linux-gnu
> Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.4.0-1ubuntu1~20.04.1' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-9-Av3uEd/gcc-9-9.4.0/debian/tmp-nvptx/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
> Thread model: posix
> gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
Right, I so have a Debian based 9.4 but it reproduces just fine. The
offending code gen is:
...
00000000000059f0 <rcu_tasks_trace_pertask>:
59f0: 48 85 ff test %rdi,%rdi
59f3: 74 40 je 5a35 <rcu_tasks_trace_pertask+0x45>
59f5: 65 48 8b 04 25 00 00 00 00 mov %gs:0x0,%rax 59fa: R_X86_64_32S current_task
59fe: 48 39 c7 cmp %rax,%rdi
5a01: 74 32 je 5a35 <rcu_tasks_trace_pertask+0x45>
5a03: c6 87 41 04 00 00 00 movb $0x0,0x441(%rdi)
5a0a: f0 83 44 24 fc 00 lock addl $0x0,-0x4(%rsp)
5a10: c7 87 3c 04 00 00 ff ff ff ff movl $0xffffffff,0x43c(%rdi)
5a1a: 8b 87 3c 04 00 00 mov 0x43c(%rdi),%eax
5a20: 83 f8 ff cmp $0xffffffff,%eax
5a23: 74 11 je 5a36 <rcu_tasks_trace_pertask+0x46>
5a25: 0f b6 87 58 04 00 00 movzbl 0x458(%rdi),%eax
5a2c: 84 c0 test %al,%al
5a2e: 0f 85 00 00 00 00 jne 5a34 <rcu_tasks_trace_pertask+0x44> 5a30: R_X86_64_PC32 .text.unlikely+0x30e
5a34: c3 ret
5a35: c3 ret
5a36: e9 65 fb ff ff jmp 55a0 <trc_wait_for_one_reader.part.0>
5a3b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
0000000000005a40 <rcu_tasks_trace_postscan>:
5a40: 41 54 push %r12
5a42: 49 89 fc mov %rdi,%r12
5a45: 55 push %rbp
5a46: bd ff ff ff ff mov $0xffffffff,%ebp
5a4b: 53 push %rbx
5a4c: 65 48 8b 1c 25 00 00 00 00 mov %gs:0x0,%rbx 5a51: R_X86_64_32S current_task
5a55: 89 ef mov %ebp,%edi
5a57: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 5a5a: R_X86_64_32S __cpu_online_mask
5a5e: e8 00 00 00 00 call 5a63 <rcu_tasks_trace_postscan+0x23> 5a5f: R_X86_64_PLT32 cpumask_next-0x4
5a63: 3b 05 00 00 00 00 cmp 0x0(%rip),%eax # 5a69 <rcu_tasks_trace_postscan+0x29> 5a65: R_X86_64_PC32 nr_cpu_ids-0x4
5a69: 89 c5 mov %eax,%ebp
5a6b: 73 50 jae 5abd <rcu_tasks_trace_postscan+0x7d>
5a6d: 89 ef mov %ebp,%edi
5a6f: e8 00 00 00 00 call 5a74 <rcu_tasks_trace_postscan+0x34> 5a70: R_X86_64_PLT32 idle_task-0x4
5a74: 48 89 c7 mov %rax,%rdi
5a77: 48 85 c0 test %rax,%rax
5a7a: 74 d9 je 5a55 <rcu_tasks_trace_postscan+0x15>
5a7c: 48 39 d8 cmp %rbx,%rax
5a7f: 74 d4 je 5a55 <rcu_tasks_trace_postscan+0x15>
5a81: c6 80 41 04 00 00 00 movb $0x0,0x441(%rax)
5a88: f0 83 44 24 fc 00 lock addl $0x0,-0x4(%rsp)
5a8e: c7 80 3c 04 00 00 ff ff ff ff movl $0xffffffff,0x43c(%rax)
5a98: 8b 80 3c 04 00 00 mov 0x43c(%rax),%eax
5a9e: 83 f8 ff cmp $0xffffffff,%eax
5aa1: 74 10 je 5ab3 <rcu_tasks_trace_postscan+0x73>
5aa3: 0f b6 87 58 04 00 00 movzbl 0x458(%rdi),%eax
5aaa: 84 c0 test %al,%al
5aac: 74 a7 je 5a55 <rcu_tasks_trace_postscan+0x15>
5aae: e9 00 00 00 00 jmp 5ab3 <rcu_tasks_trace_postscan+0x73> 5aaf: R_X86_64_PC32 .text.unlikely+0x311
5ab3: 4c 89 e6 mov %r12,%rsi
5ab6: e8 e5 fa ff ff call 55a0 <trc_wait_for_one_reader.part.0>
5abb: eb 98 jmp 5a55 <rcu_tasks_trace_postscan+0x15>
5abd: e8 00 00 00 00 call 5ac2 <rcu_tasks_trace_postscan+0x82> 5abe: R_X86_64_PLT32 cpus_read_unlock-0x4
5ac2: 5b pop %rbx
5ac3: 5d pop %rbp
5ac4: 41 5c pop %r12
5ac6: e9 00 00 00 00 jmp 5acb <rcu_tasks_trace_postscan+0x8b> 5ac7: R_X86_64_PLT32 synchronize_rcu-0x4
5acb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
...
0000000000000312 <rcu_tasks_trace_pertask.cold>:
312: 8b 47 14 mov 0x14(%rdi),%eax
0000000000000315 <rcu_tasks_trace_postscan.cold>:
315: 8b 47 14 mov 0x14(%rdi),%eax
0000000000000318 <rcu_early_boot_tests>:
...
Which is just bloody weird/broken if you ask me. What's worse, GCC-10
does the same. Only when I use GCC-11 do I get sensible code again:
00000000000002ef <rcu_tasks_trace_postscan.cold>:
2ef: 8b 4f 14 mov 0x14(%rdi),%ecx
2f2: 8b 97 88 05 00 00 mov 0x588(%rdi),%edx
2f8: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 2fb: R_X86_64_32S .rodata+0x40
2ff: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 302: R_X86_64_32S .rodata.str1.8+0x5e0
306: e8 00 00 00 00 call 30b <rcu_tasks_trace_postscan.cold+0x1c> 307: R_X86_64_PLT32 _printk-0x4
30b: e9 00 00 00 00 jmp 310 <rcu_tasks_trace_pertask.cold> 30c: R_X86_64_PC32 .text+0x5721
0000000000000310 <rcu_tasks_trace_pertask.cold>:
310: 8b 4f 14 mov 0x14(%rdi),%ecx
313: 8b 97 88 05 00 00 mov 0x588(%rdi),%edx
319: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 31c: R_X86_64_32S .rodata+0x40
320: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 323: R_X86_64_32S .rodata.str1.8+0x5e0
327: e9 00 00 00 00 jmp 32c <rcu_early_boot_tests> 328: R_X86_64_PLT32 _printk-0x4
> > ------------------------------------------------------------------------
> >
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index 3f7601eca554d..0104bb37a034b 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -1388,8 +1388,6 @@ static void trc_wait_for_one_reader(struct task_struct *t,
> >
> > // The current task had better be in a quiescent state.
> > if (t == current) {
> > - if (READ_ONCE(t->trc_needreport))
> > - pr_info("%s(P%d/%d) is currently running task.\n", __func__, t->pid, task_cpu(t));
> > rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
> > WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
> > return;
> > @@ -1398,8 +1396,6 @@ static void trc_wait_for_one_reader(struct task_struct *t,
> > // Attempt to nail down the task for inspection.
> > get_task_struct(t);
> > if (!task_call_func(t, trc_inspect_reader, NULL)) {
> > - if (READ_ONCE(t->trc_needreport))
> > - pr_info("%s(P%d/%d) task_call_func() succeeded.\n", __func__, t->pid, task_cpu(t));
> > put_task_struct(t);
> > return;
> > }
> >
> > ------------------------------------------------------------------------
Yeah, so it moves that printk invocation into a .cold subfunction, which
is entirely reasonable, except it then completely mangles it, which is
less reasonable.
I suppose the next step is you upgrading your compiler to gcc-11 or
later and any inteterested tools person looking at fixing gcc<11 if they
still care enough about them.
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