[PATCH v4 2/8] alpha: run check_mmu_context() from finish_arch_post_lock_switch()

[Magnus Lindholm]

check_mmu_context() clears asn_lock and acts on need_new_asn, but it
runs only as the tail of switch_to(), after alpha_switch_to() returns.
A newly forked task never gets there: its first context switch resumes
at ret_from_fork and reaches schedule_tail() without returning to the
code following alpha_switch_to(). New kernel threads take the same
path through schedule_tail().

asn_lock is left set on that CPU, so the new task can run with
interrupts enabled while shootdown IPIs keep taking the deferred path.
The handshake meant to complete those invalidations never runs.

Move check_mmu_context() to finish_arch_post_lock_switch(), which is
called by finish_task_switch() for both existing and newly created tasks.
Drop the now redundant call from switch_to().

The hook runs after finish_lock_switch() releases the rq lock and enables
interrupts. An IPI arriving before asn_lock is cleared retires the slot
through flush_tlb_other(); an IPI arriving afterwards can flush directly,
since PAL_swpctx has already installed the incoming context.

Set need_new_asn whether ev5_switch_mm() reuses or allocates an ASN. An
IPI in that interval can zero either slot. Without this, the allocated
case can return to user space with a live ASN but a zero context slot,
which a later context-based shootdown shortcut could mistake for an
inactive CPU. check_mmu_context() reloads only when the slot is zero.

Call the hook unconditionally. Alpha selects ARCH_NO_PREEMPT, and neither
kthread_use_mm() nor sched_force_init_mm() sleeps between the direct
switch and this hook. RCU_STRICT_GRACE_PERIOD can enable PREEMPT_COUNT,
but cannot make those callers migrate there. Testing preemptible() would
only leave asn_lock set in that configuration.

This requires the preceding direct-switch fix, "alpha: load the MMU
context when switch_mm() switches the current task". It installs the
borrowed context before kthread_use_mm() reaches this hook, so clearing
asn_lock cannot expose a targeted invalidate against the wrong context.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Magnus Lindholm <linmag7@xxxxxxxxx>
---
 arch/alpha/include/asm/mmu_context.h | 11 +++++++++--
 arch/alpha/include/asm/switch_to.h   |  1 -
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/alpha/include/asm/mmu_context.h b/arch/alpha/include/asm/mmu_context.h
index c5cf7dbe6161..e5a5737506db 100644
--- a/arch/alpha/include/asm/mmu_context.h
+++ b/arch/alpha/include/asm/mmu_context.h
@@ -161,8 +161,8 @@ ev5_switch_mm(struct mm_struct *prev_mm, struct mm_struct *next_mm,
 		next_mm->context[cpu] = mmc;
 	}
 #ifdef CONFIG_SMP
-	else
-		cpu_data[cpu].need_new_asn = 1;
+	/* A deferred shootdown can also invalidate a newly allocated ASN. */
+	cpu_data[cpu].need_new_asn = 1;
 #endif
 
 	/* Always update the PCB ASN.  Another thread may have allocated
@@ -191,6 +191,13 @@ do {								\
 #define check_mmu_context()  do { } while(0)
 #endif
 
+/* Alpha has no kernel preemption; these callers cannot migrate here. */
+#define finish_arch_post_lock_switch finish_arch_post_lock_switch
+static inline void finish_arch_post_lock_switch(void)
+{
+	check_mmu_context();
+}
+
 __EXTERN_INLINE void
 ev5_activate_mm(struct mm_struct *prev_mm, struct mm_struct *next_mm)
 {
diff --git a/arch/alpha/include/asm/switch_to.h b/arch/alpha/include/asm/switch_to.h
index 762b7f975310..35c4b2c9d992 100644
--- a/arch/alpha/include/asm/switch_to.h
+++ b/arch/alpha/include/asm/switch_to.h
@@ -9,7 +9,6 @@ extern struct task_struct *alpha_switch_to(unsigned long, struct task_struct *);
 #define switch_to(P,N,L)						 \
   do {									 \
     (L) = alpha_switch_to(virt_to_phys(&task_thread_info(N)->pcb), (P)); \
-    check_mmu_context();						 \
   } while (0)
 
 #endif /* __ALPHA_SWITCH_TO_H */
-- 
2.43.0