[PATCH] powerpc: Fix deadlock with mmu_hash_lock in hash_page_sync

[Benjamin Herrenschmidt]

hash_page_sync() takes and releases the low level mmu hash
lock in order to sync with other processors disposing of page
tables. Because that lock can be needed to service hash misses
triggered by interrupt handler, taking it must be done with
interrupts off. However, hash_page_sync() appear to be called
with interrupts enabled, thus causing occasional deadlocks.

We fix it by making sure hash_page_sync() masks interrupts while
holding the lock.

Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
---

Please test and report asap thought it's probably too late for
2.6.25, it can still go into stable later on.

Index: linux-work/arch/powerpc/mm/hash_low_32.S
===================================================================
--- linux-work.orig/arch/powerpc/mm/hash_low_32.S	2008-03-31 08:42:56.000000000 +1100
+++ linux-work/arch/powerpc/mm/hash_low_32.S	2008-03-31 08:45:05.000000000 +1100
@@ -44,6 +44,9 @@ mmu_hash_lock:
 #ifdef CONFIG_SMP
 	.text
 _GLOBAL(hash_page_sync)
+	mfmsr   r10
+	rlwinm  r0,r10,0,17,15          /* clear bit 16 (MSR_EE) */
+	mtmsr   r0
 	lis	r8,mmu_hash_lock at h
 	ori	r8,r8,mmu_hash_lock at l
 	lis	r0,0x0fff
@@ -60,8 +63,9 @@ _GLOBAL(hash_page_sync)
 	eieio
 	li	r0,0
 	stw	r0,0(r8)
-	blr	
-#endif
+	mtmsr	r10
+	blr
+#endif /* CONFIG_SMP */
 
 /*
  * Load a PTE into the hash table, if possible.