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i387: ptrace breaks the lazy-fpu-restore logic
Starting from 7e16838 "i387: support lazy restore of FPU state" we assume that fpu_owner_task doesn't need restore_fpu_checking() on the context switch, its FPU state should match what we already have in the FPU on this CPU. However, debugger can change the tracee's FPU state, in this case we should reset fpu.last_cpu to ensure fpu_lazy_restore() can't return true. Change init_fpu() to do this, it is called by user_regset->set() methods. Reported-by: Jan Kratochvil <email@example.com> Suggested-by: Linus Torvalds <firstname.lastname@example.org> Signed-off-by: Oleg Nesterov <email@example.com> Link: http://lkml.kernel.org/r/20120416204815.GB24884@redhat.com Cc: <firstname.lastname@example.org> v3.3 Signed-off-by: H. Peter Anvin <email@example.com>
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