x86/alternatives: Acquire pte lock with interrupts enabled

pte lock is never acquired in-IRQ context so it does not require interrupts
to be disabled. The lock is a regular spinlock which cannot be acquired
with interrupts disabled on RT.

RT complains about pte_lock() in __text_poke() because it's invoked after
disabling interrupts.

__text_poke() has to disable interrupts as use_temporary_mm() expects
interrupts to be off because it invokes switch_mm_irqs_off() and uses
per-CPU (current active mm) data.

Move the PTE lock handling outside the interrupt disabled region.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by; Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20200813105026.bvugytmsso6muljw@linutronix.de

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c826cdd..34a1b85 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -874,8 +874,6 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
         */
        BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));
 
-       local_irq_save(flags);
-
        /*
         * Map the page without the global bit, as TLB flushing is done with
         * flush_tlb_mm_range(), which is intended for non-global PTEs.
@@ -892,6 +890,8 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
         */
        VM_BUG_ON(!ptep);
 
+       local_irq_save(flags);
+
        pte = mk_pte(pages[0], pgprot);
        set_pte_at(poking_mm, poking_addr, ptep, pte);
 
@@ -941,8 +941,8 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
         */
        BUG_ON(memcmp(addr, opcode, len));
 
-       pte_unmap_unlock(ptep, ptl);
        local_irq_restore(flags);
+       pte_unmap_unlock(ptep, ptl);
        return addr;
 }