Messages in this thread | | | Date | Tue, 26 Sep 2017 09:47:09 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] percpu: make this_cpu_generic_read() atomic w.r.t. interrupts |
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On Tue, 26 Sep 2017, Christopher Lameter wrote: > On Mon, 25 Sep 2017, Tejun Heo wrote: > > > Hello, > > > > On Mon, Sep 25, 2017 at 04:33:02PM +0100, Mark Rutland wrote: > > > Unfortunately, the generic this_cpu_read(), which is intended to be > > > irq-safe, is not: > > > > > > #define this_cpu_generic_read(pcp) \ > > > ({ \ > > > typeof(pcp) __ret; \ > > > preempt_disable_notrace(); \ > > > __ret = raw_cpu_generic_read(pcp); \ > > > preempt_enable_notrace(); \ > > > __ret; \ > > > }) > > > > I see. Yeah, that looks like the bug there. > > This is a single fetch operation of a value that needs to be atomic. It > really does not matter if an interrupt happens before or after that load > because it could also occur before or after the preempt_enable/disable > without the code being able to distinguish that case. > > The fetch of a scalar value from memory is an atomic operation and that is > required from all arches. There is an exception for double word fetches.
this_cpu_read_8() is a double word fetch on many 32bit architectures.
> Maybe we would need to special code that case but so far this does not > seem to have been an issue.
Just because nobody ran into problem with that it is a non issue? That's just hillarious.
It's obviously not correct and needs to be fixed _before_ someone has to go through the pain of debugging such a problem.
Thanks,
tglx
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