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SubjectRe: [BUG REPORT] Soft Lockup in smp_call_function_single+0xD8
On 1/25/16, Jeff Merkey <> wrote:
> On 1/25/16, Jeff Merkey <> wrote:
>> On 1/24/16, Jeff Merkey <> wrote:
>>> On 1/24/16, Jeff Merkey <> wrote:
>>>> If I single step with either kgdb, kgdb, or mdb kernel debuggers over
>>>> a sysret instruction anywhere in the OS, the system hard hangs in
>>>> smp_call_function_single after the debugger releases the system and it
>>>> resumes normal operation. The specific place the kernel hangs is in
>>>> the loop below. The softlockup detector will periodically detect
>>>> this condition when it occurs, but not always, most of the time the
>>>> system is just hung and unresponsive.
>>>> (2)> u smp_call_function_single+d8
>>>> <<<< hard hang in this loop with EDX=3
>>>> 0xffffffff810fce48 8B55E0 mov edx,DWORD PTR
>>>> [rbp-32]=0xCE037DC0
>>>> 0xffffffff810fce4b 83E201 and edx,0x1
>>>> 0xffffffff810fce4e 75F6 jne
>>>> smp_call_function_single+0xd6 (0xffffffff810fce46) (up)
>>>> <<<<<
>>>> 0xffffffff810fce50 EBC3 jmp
>>>> smp_call_function_single+0xa5 (0xffffffff810fce15) (up)
>>>> 0xffffffff810fce52 8B05E08EC700 mov eax,[oops_in_progress]=0x0
>>>> 0xffffffff810fce58 85C0 test eax,eax
>>>> 0xffffffff810fce5a 7585 jne
>>>> smp_call_function_single+0x71 (0xffffffff810fcde1) (up)
>>>> 0xffffffff810fce5c 803D8E0C9D0000 cmp [__warned.20610]=0x00,0x0
>>>> 0xffffffff810fce63 0F8578FFFFFF jne
>>>> smp_call_function_single+0x71 (0xffffffff810fcde1) (up)
>>>> 0xffffffff810fce69 BE24010000 mov esi,0x124
>>>> 0xffffffff810fce6e 48C7C796B08C81 mov rdi,0xffffffff818cb096
>>>> 0xffffffff810fce75 894DBC mov DWORD PTR [rbp-68]=0x0,ecx
>>>> 0xffffffff810fce78 488955C0 mov QWORD PTR
>>>> [rbp-64]=0xFFFFFFFFFFFFFF10,rdx
>>>> 0xffffffff810fce7c E8FF21F8FF call warn_slowpath_null
>>>> 0xffffffff810fce81 C605690C9D0001 mov [__warned.20610]=0x00,0x1
>>>> 0xffffffff810fce88 8B4DBC mov ecx,DWORD PTR [rbp-68]=0x0
>>>> 0xffffffff810fce8b 488B55C0 mov rdx,QWORD PTR
>>>> [rbp-64]=0xFFFFFFFFFFFFFF10
>>>> 0xffffffff810fce8f E94DFFFFFF jmp
>>>> smp_call_function_single+0x71 (0xffffffff810fcde1) (up)
>>>> 0xffffffff810fce94 E8A71EF8FF call __stack_chk_fail
>>>> 0xffffffff810fce99 0F1F8000000000 nop DWORD PTR [rax]=0x0
>>>> (2)> g
>>>> The stack backtrace when the bug occurs is:
>>>> smp_call_function_single+0xd8
>>>> unmap_page_range+0x613
>>>> flush_tlb_func+0x0
>>>> smp_call_function_many+215
>>>> native_flush_tlb_others+0x118
>>>> flush_tlb_mm_range+0x61
>>>> tlb_flush_mmu_tlbonly+0x6b
>>>> tlb_finish_mmu+0x14
>>>> unmap_region+0xe2
>>>> vma_rb_erase+0x10f
>>>> do_unmap+0x217
>>>> vm_unmap+0x41
>>>> SyS_munmap+0x22
>>>> entry_SYSCALL_64_fastpath+0x12
>>>> I traced through this code a bunch of times in just normal operations
>>>> without triggering the bug to get a feel for what it normally sees in
>>>> EDX and it looks like someone has coded a looping function that always
>>>> has EDX=0 in every case I saw in the except for when this bug occurs.
>>>> So the exact C code this maps fro objdump of kernel/smp.o is:
>>>> 469: e8 62 fe ff ff callq 2d0 <generic_exec_single>
>>>> 46e: 8b 55 e0 mov -0x20(%rbp),%edx
>>>> * previous function call. For multi-cpu calls its even more
>>>> interesting
>>>> * as we'll have to ensure no other cpu is observing our csd.
>>>> */
>>>> static void csd_lock_wait(struct call_single_data *csd)
>>>> {
>>>> while (smp_load_acquire(&csd->flags) & CSD_FLAG_LOCK)
>>>> 471: 83 e2 01 and $0x1,%edx
>>>> 474: 74 cf je 445 <smp_call_function_single+0xa5>
>>>> 476: f3 90 pause
>>>> <<<<<<<<<<
>>>> 478: 8b 55 e0 mov -0x20(%rbp),%edx
>>>> 47b: 83 e2 01 and $0x1,%edx
>>>> 47e: 75 f6 jne 476 <smp_call_function_single+0xd6>
>>>> <<<<<<<<<<<
>>>> 480: eb c3 jmp 445 <smp_call_function_single+0xa5>
>>>> * Can deadlock when called with interrupts disabled.
>>>> * We allow cpu's that are not yet online though, as no one else can
>>>> * send smp call function interrupt to this cpu and as such deadlocks
>>>> * can't happen.
>>>> */
>>>> WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
>>>> Each time this bug occurs csd->flags is always set to a value of 3 and
>>>> never changes. When the system is just running normally, it seems to
>>>> be 0 the rest of the time. Setting EDX=0 from the debugger console
>>>> clears the hang condition and the system seems to recover except the
>>>> system reports this error from the console when you attempt to load
>>>> programs, indicating the ability of the system to load shared objects
>>>> is fritzed.
>>>> #
>>>> # ls -l
>>>> /lib64/ version GLI not found << this error and no shared
>>>> objects will load
>>>> #
>>>> #
>>>> Jeff
>>> I am running down a trace of the MSR values for swapgs. Looks like it
>>> got nested somewhere down in the entry_64 code. If so, then this is
>>> just a symptom and not the sickness.
>>> Jeff
>> I got to the bottom of this one and its related to the function loop
>> locking up the system that is described above when code is allowed to
>> run on one processor but not the others and one of them is trying to
>> send an IPI.
>> You might want to consider some sort of timeout logic for that
>> function or ability for it to recover. The problem is caused by
>> holding all the processors except the target processor being traced
>> and letting it run for a trace session while the other processors are
>> held suspended. If I release the all processors each time I step
>> with the target processor the problem goes away. If I hold the
>> processors while stepping over a section of code this lockup will
>> occur and the processor the bug occurs on apparently never recovers
>> In the process of tracking down this bug I traced all of the swapgs
>> usage in all of the exception handlers and it works well. The check
>> in paranoid_entry and paranoid_exit seems to catch the cases where the
>> debugger is or has stepped over a swapgs instruction pointing to a gs
>> in userspace for both int1 and int3 exceptions, and recovers correctly
>> through paranoid_entry and paranoid exit with the correct gs and MSR
>> value in all the cases I traced.
>> I coded around this issue but it's probably a bug in using that while
>> loop without some way to determine a processor has gone away
>> temporarily. The solution is to let the processors run in between
>> each and every int1 trap and not hold a processor in focus mode, then
>> this code snippet that locks up in smp.c does not get hit.
>> Jeff
> I guess Linus has already been trying to fix this problem since he was
> the last person to touch that code. Looks like there is another case
> where csd_unlock does not get called or is delayed.
> I think I'll work on this one some more and see if I can fix it. It
> still shows up after stepping through a sysret even with my changes,
> its just harder to trigger. It crashes at exactly the same place in
> the stacktrace from the same source so there's a clue to start with.
> Jeff

This bug is confirmed to be an int1 recursion that happens because the
sysret code in entry_64.S allows return to userspace with the trap
flag set. The last person to try to fix this was from this commit --

for this reported problem.

I verified that I am seeing the same problem here, it also exposes a
bug in Linus code in smp.c. I have that fixed as well -- use a
spinlock and don't try to guess what order hardware will post bus
transactions for memory reads and writes. That code in smp.c is
busted on some systems.

I have a fix for this problem and Ingo needs to review this code
because I have set the trap flag and watched is cycle through this
code in entry_64.S while set and still end up executing a sysret when
its not supposed to and take the other return path.


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