So the fault is from

{{{

LFDS710_PAL_ASSERT( (lfds710_pal_uint_t) ss->top % LFDS710_PAL_ATOMIC_ISOLATION_IN_BYTES == 0 );

}}}

Where the ASSERT fails. So ss->top == 0, which means that ss>top % 0x80 == 0. So this is not correct, something weird here ...

{{{

include

int
main(int argc, char **argv) {
printf("%d\n", !(0 % 128 == 0));
return 0;
}

}}}

Which correctly prints 0, and the assert macro is:

{{{

define LFDS710_PAL_ASSERT( expression ) if( !(expression) ) LFDS710_MISC_DELIBERATELY_CRASH;

}}}

So give that if will execute the code if nonzero, something funky is happening here.

Ahhhh I see. I was mislead by gdb. {0x0, 0x0} is because it knows the array is of fixed size. So what's actually happening is that ss->top % LFDS710_PAL_ATOMIC_ISOLATION_IN_BYTES is checking the address of the array, but gdb "helpfully" is just showing the content.

This yields

{{{
(gdb) print &ss->top
$3 = (struct lfds710_stack_element * volatile(*)[2]) 0x6054e0
(gdb) print 0x6054e0 % 128
$4 = 96
}}}

This then triggers the assert, because the library won't be able to CAS on an address that isn't aligned a 128byte boundary.

Curiously, this doesn't affect a build that uses ASAN ...

This appears to be an issue between Fedora gcc vs Rhel GCC

Affects - Fedora: gcc-6.2.1-2.fc26.x86_64

Does not affect - Rhel: gcc-4.8.5-4.el7.x86_64

Okay, its not an issue in GCC. It's a flaw in my understanding of the alignment operator. This only affects stack allocated elements. It was by chance that previously on el7 this would allocate on a 128 byte alignment. It just so happens that gcc 6 produces different code, and thus the issue.

So as we are mallocing on the heap, the stack state and queue states for lfds, these must be aligned to a 128 byte boundary, but malloc doesn't guarantee this! How does malloc know the struct wants alignment at runtime. It doesn't.

So we need to use the posix_memalign functions to satisfy this.

This means we'll need to add a similar memalign function to slapi_ch_ to satisfy this.

attachment
0001-Ticket-63-segfault-in-thr_stack_create.patch

commit a11fb3a4848d0a04753abd27b651fa9d0e0bfd15
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To ssh://git.fedorahosted.org/git/nunc-stans.git
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