Re: test_and_set_bit() not atomic forever? [cli/sti in char/vt.c [patch]]
Richard Gooch (Richard.Gooch@atnf.CSIRO.AU)
Sun, 31 May 1998 10:04:38 +1000
Andrew Derrick Balsa writes:
> Hi Andrea,
>
> Andrea Arcangeli wrote:
> >=20
> > On Fri, 29 May 1998, Pavel Machek wrote:
> >=20
> > >Don't think so. Well, it is safe on i386, but on other architectures,
> > >test_and_set_bit is not guaranteed to be atomic. [And I'm afraid that
> >=20
> > Arggh I thought that test_and_set_bit() was atomic on all ports (since =
> I
> > looked only its i386 implementation)!
>
> test_and_set is *by* *definition* an atomic operation. However, some
> processors don't have an opcode that implements atomic test_and_set. On
> these processors, we either use another atomic instruction to build a
> function that implements atomic test_and_set, or, if the architecture is
> not SMP capable, just forget about the atomicity requirement.
Hold on: even if the machine has a single CPU, you still need to
guarantee atomicity because of interrupt handlers. On a single CPU
machine you could do this by disabling interrupts. But you can't
just ignore the whole issue because you only have one CPU.
Regards,
Richard....
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