Messages in this thread | | | Date | Fri, 5 May 2006 21:50:29 +0930 | From | Alan Modra <> | Subject | Re: as bug (was: Re: smp/up alternatives crash when CONFIG_HOTPLUG_CPU) |
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On Fri, May 05, 2006 at 11:45:54AM +0300, Denis Vlasenko wrote: > > .section .smp_altinstr_replacement,"awx" > .section .sched.text,"ax",@progbits > call _spin_unlock # > 661: > 2: jle 2b # > 662: > .section .smp_altinstr_replacement,"awx" > .fill 662b-661b,1,0x42
gas should give a better error message here, but really, gas shouldn't be expected to assemble this. In essence, you have forward references in that expression for the .fill length..
Some background: Gas is a single pass assembler. It emits code and data into "frags", buffers containing some fixed number of bytes and possibly a variable length tail. The variable length part allows various features, notably that of variable length instructions. Symbols are defined relative to their frags. Until the frag addresses are finalized, an expresion involving subtraction of two symbols in different frags cannot be evaluated correctly. With the testcase above you have exactly that situation. The x86 "jle" instruction can be two sizes, either 6 bytes or 2 bytes depending on the offset needed, and gas doesn't have the smarts to recognize that the "jle" above is just 2 bytes. Instead, it assumes a variable size, putting the "jle" in its own frag. This means that label "661" and "662" are in separate frags with "661" at offset 5 in its frag, and "662" at offset 0.
Since you define the ".smp_altinstr_replacement" section before the ".sched.text section", gas tries to finalize ".smp_altinstr_replacement" first. When it tries to calculate the fill size using (<base addr "662" frag>+<offset "662">) - (<base addr "661" frag>+<offset "661">) the frag base addresses have not yet been set, and zero is used. ie. gas tries to assemble ".fill -5,1,0x42".
A workaround is to ensure that the ".sched.text" section is defined before ".smp_altinst_replacement".
-- Alan Modra IBM OzLabs - Linux Technology Centre - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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