Messages in this thread |  | | Date | Mon, 16 Feb 2015 21:25:40 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH 2/8] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager_fpu() |
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On Fri, Feb 06, 2015 at 03:01:59PM -0500, riel@redhat.com wrote: > From: Oleg Nesterov <oleg@redhat.com> > > unlazy_fpu()->__thread_fpu_end() doesn't look right if use_eager_fpu(). > Unconditional __thread_fpu_end() is only correct if we know that this > thread can't return to user-mode and use FPU. > > Fortunately it has only 2 callers. fpu_copy() checks use_eager_fpu(), > and init_fpu(current) can be only called by the coredumping thread via > regset->get(). But it is exported to modules, and imo this should be > fixed anyway. > > And if we check use_eager_fpu() we can use __save_fpu() like fpu_copy() > and save_init_fpu() do. > > - It seems that even !use_eager_fpu() case doesn't need the unconditional > __thread_fpu_end(), we only need it if __save_init_fpu() returns 0.
I can follow so far.
> - It is still not clear to me if __save_init_fpu() can safely nest with > another save + restore from __kernel_fpu_begin(). If not, we can use > kernel_fpu_disable() to fix the race.
Well, my primitive understanding would say no, not safely, for the simple reason that we have only one XSAVE state area per thread. However, __kernel_fpu_begin() is called with preemption disabled so ... I guess I'm still not seeing the race.
Btw, what is kernel_fpu_disable()? Can't find it here.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply. --
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