Messages in this thread | | | Date | Tue, 26 Sep 2006 14:04:14 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17 |
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Hi,
Ok, so as far as I can see, we can only control the execution flow by modifying values in the output list of the asm.
Do you think the following would work ?
#define MARK_JUMP(name, format, args...) \ do { \ char condition; \ asm volatile( ".section .markers, \"a\";\n\t" \ ".long 0f;\n\t" \ ".previous;\n\t" \ "0:\n\t" \ "movb $0,%1;\n\t" \ : "+m" (__marker_sequencer), \ "=r" (condition) : ); \ if(unlikely(condition)) { \ MARK_CALL(name, format, ## args); \ } \ } while(0)
The jump is left to gcc, we only modify an immediate value (a byte) to change the selection. The is no memory load involved on the fast path :
... 6: b0 00 mov $0x0,%al 8: 84 c0 test %al,%al a: 75 07 jne 13 <my_open+0x13> c: b8 ff ff ff ff mov $0xffffffff,%eax 11: c9 leave 12: c3 ret 13: b8 01 00 00 00 mov $0x1,%eax 18: e8 fc ff ff ff call 19 <my_open+0x19> 1d: c7 44 24 0c 00 00 00 movl $0x0,0xc(%esp) 24: 00 25: c7 44 24 08 06 00 00 movl $0x6,0x8(%esp) 2c: 00 2d: c7 44 24 04 02 00 00 movl $0x2,0x4(%esp) 34: 00 35: c7 04 24 0c 00 00 00 movl $0xc,(%esp) 3c: ff 15 94 00 00 00 call *0x94 42: b8 01 00 00 00 mov $0x1,%eax 47: e8 fc ff ff ff call 48 <my_open+0x48> 4c: eb be jmp c <my_open+0xc>
Mathieu
* Jeremy Fitzhardinge (jeremy@goop.org) wrote: > Mathieu Desnoyers wrote: > >>Mathieu Desnoyers wrote: > >> > >>>To protect code from being preempted, the macros preempt_disable and > >>>preempt_enable must normally be used. Logically, this macro must make > >>>sure gcc > >>>doesn't interleave preemptible code and non-preemptible code. > >>> > >>> > >>No, it only needs to prevent globally visible side-effects from being > >>moved into/out of preemptable blocks. In practice that means memory > >>updates (including the implicit ones that calls to external functions > >>are assumed to make). > >> > >> > >>>Which makes me think that if I put barriers around my asm, call, asm > >>>trio, no > >>>other code will be interleaved. Is it right ? > >>> > >>> > >>No global side effects, but code with local side effects could be moved > >>around without changing the meaning of preempt. > >> > >>For example: > >> > >> int foo; > >> extern int global; > >> > >> foo = some_function(); > >> > >> foo += 42; > >> > >> preempt_disable(); > >> // stuff > >> preempt_enable(); > >> > >> global = foo; > >> foo += other_thing(); > >> > >>Assume here that some_function and other_function are extern, and so gcc > >>has no insight into their behaviour and therefore conservatively assumes > >>they have global side-effects. > >> > >>The memory barriers in preempt_disable/enable will prevent gcc from > >>moving any of the function calls into the non-preemptable region. But > >>because "foo" is local and isn't visible to any other code, there's no > >>reason why the "foo += 42" couldn't move into the preempt region. > >> > > > >I am not sure about this last statement. The same reference : > >http://developer.apple.com/documentation/DeveloperTools/gcc-4.0.1/gcc/Extended-Asm.html > > > (This is pretty old, and this is an area which changes quite a lot. You > should refer to something more recent; > http://www.cims.nyu.edu/cgi-systems/info2html?/usr/local/info(gcc)Top > for example, though in this case the quoted text looks the same.) > > >I am just wondering how gcc can assume that I will not modify variables on > >the > >stack from within a function with a memory clobber. If I would like to do > >some > >nasty things in my assembly code, like accessing directly to a local > >variable by > >using an offset from the stack pointer, I would expect gcc not to relocate > >this > >local variable around my asm volatile memory clobbered statement, as it > >falls > >under the category "access memory in an unpredictable fashion". > > > > That not really what it means. gcc is free to put local variables in > memory or register, and unless you pass the local to your asm as a > parameter, your code has no way of knowing how to find the current > location of the local. You could trash your stack frame from within the > asm if you like, but I don't think gcc is under any obligation to behave > in a deterministic way if you do. > > "Unpredictable" in this case means that the memory modified isn't easily > specified as a normal asm parameter. For example, if you have an asm > which does a memset(), the modified memory's size is a runtime variable > rather than a compile-time constant. Or perhaps your asm follows a > linked list and modifies memory as it traverses the list. > > > J > OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - 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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