static int em_syscall(struct x86_emulate_ctxt *ctxt)
{
	const struct x86_emulate_ops *ops = ctxt->ops;
	struct desc_struct cs, ss;
	u64 msr_data;
	u16 cs_sel, ss_sel;
	u64 efer = 0;

	/* syscall is not available in real mode */
	if (ctxt->mode == X86EMUL_MODE_REAL ||
	    ctxt->mode == X86EMUL_MODE_VM86)
		return emulate_ud(ctxt);

	if (!(em_syscall_is_enabled(ctxt)))
		return emulate_ud(ctxt);

	ops->get_msr(ctxt, MSR_EFER, &efer);
	setup_syscalls_segments(ctxt, &cs, &ss);

	if (!(efer & EFER_SCE))
		return emulate_ud(ctxt);

	ops->get_msr(ctxt, MSR_STAR, &msr_data);
	msr_data >>= 32;
	cs_sel = (u16)(msr_data & 0xfffc);
	ss_sel = (u16)(msr_data + 8);

	if (efer & EFER_LMA) {
		cs.d = 0;
		cs.l = 1;
	}
	ops->set_segment(ctxt, cs_sel, &cs, 0, VCPU_SREG_CS);
	ops->set_segment(ctxt, ss_sel, &ss, 0, VCPU_SREG_SS);

	*reg_write(ctxt, VCPU_REGS_RCX) = ctxt->_eip;
	if (efer & EFER_LMA) {
#ifdef CONFIG_X86_64
		*reg_write(ctxt, VCPU_REGS_R11) = ctxt->eflags;

		ops->get_msr(ctxt,
			     ctxt->mode == X86EMUL_MODE_PROT64 ?
			     MSR_LSTAR : MSR_CSTAR, &msr_data);
		ctxt->_eip = msr_data;

		ops->get_msr(ctxt, MSR_SYSCALL_MASK, &msr_data);
		ctxt->eflags &= ~msr_data;
		ctxt->eflags |= EFLG_RESERVED_ONE_MASK;
#endif
	} else {
		/* legacy mode */
		ops->get_msr(ctxt, MSR_STAR, &msr_data);
		ctxt->_eip = (u32)msr_data;

		ctxt->eflags &= ~(EFLG_VM | EFLG_IF);
	}

	return X86EMUL_CONTINUE;
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.