On 2023-10-26 14:50, Linus Torvalds wrote:
On Thu, 26 Oct 2023 at 08:36, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
asm volatile("xchg %b0,%1"
which has an implicit lock prefix (xchg with a memory operand is a
special-case):
Yeah, this is why we do "percpu_xchg()" - which does not want locked
semantics - as a "cmpxchg" loop.
Steven, check out
arch/x86/include/asm/percpu.h
for a rough implementation of a 'xchg()' without SMP coherency, just
cpu-local one (ie atomic wrt being preempted by the kernel, but not
atomic wrt other CPU's accessing the same variable concurrently)
Actually Steven does not need a xchg to test-and-set a single bit which
is only accessed concurrently between kernel and userspace from the same
thread. Either "bts" or "andb" should work fine.
Thanks,
Mathieu
Linus
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
