Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17

From: Jeremy Fitzhardinge
Date: Mon Sep 25 2006 - 21:02:31 EST

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;

// stuff

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. Likewise, the assignment to "global" can't move out of the range between the preempt_enable and the call to other_thing().

So in your case, if your equivalent of the non-preemptable block is the call to the marker function, then there's a good chance that the compiler might decide to move some other code in there.

Now it might be possible to take the addresses of labels to inhibit code motion into a particular range:

__label__ before, after;
asm volatile("" : : "m" (*&&before), "m" (*&&after)); // gcc can't know what we're doing with the labels

before: ;
// stuff
after: ;

but that might be risky for several reasons: I don't know of any particular promises gcc makes in this circumstance; I suspect taking the address of a label will have a pretty severe inhibition on what optimisations gcc's is willing to use (it may prevent inlining altogether); and this looks pretty unusual, so there could be bugs.

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