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.
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.
J
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