irq_fpu_usable() is irreliable

[Jason A. Donenfeld]

Hi folks,

The availability of the FPU in kernel space, as you know, is
determined by this function:

bool irq_fpu_usable(void)
{
        return !in_interrupt() ||
                interrupted_user_mode() ||
                interrupted_kernel_fpu_idle();
}

My understanding is that the first check is !in_interrupt(), because
if `current` is valid - if we are in process context - then we have a
place to store the existing FPU regs in kernel_fpu_begin, to be
restored later in kernel_fpu_end. Recently I've been tracking down a
problem in which irq_fpu_usable() returns false, yet a stack trace
shows the first function is the syscall entry point. This leads me to
believe that in_interrupt() is not an adequate way of testing for a
valid `current`. In my particular problematic case, the reason
in_interrupt() was returning false is because a number of
rcu_read_lock_bh()s were being held; IOW this is occurring in the
ndo_start_xmit path of a network driver.

I therefore propose changing the function to this:

bool irq_fpu_usable(void)
{
        return (!in_irq() && !in_nmi()) ||
                interrupted_user_mode() ||
                interrupted_kernel_fpu_idle();
}

What would you think of that?

Thanks,
Jason
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