[PATCH v3 1/7] 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, which goes to schedule_tail() and then to user space
rather than returning to the code following alpha_switch_to(). A new
kernel thread reaches schedule_tail() the same way, through
ret_from_kernel_thread().

asn_lock is left set on that CPU, so the forked task runs user space
with it set and interrupts enabled. A TLB shootdown IPI arriving in
that window takes the deferred path, and the need_new_asn handshake
meant to cover that never runs.

finish_task_switch() calls finish_arch_post_lock_switch() with
preemption disabled, on the CPU that ran switch_mm(), so hooking
check_mmu_context() there completes the bookkeeping for both. The
existing call from switch_to() then becomes redundant, since
finish_task_switch() runs immediately afterwards and does the same
work, so drop it.

kthread_use_mm() and sched_force_init_mm() reach the same hook outside
the scheduler's preemption-disabled switch tail, where the CPU may have
changed since switch_mm(). check_mmu_context() acts on per-CPU state,
so testing preemptible() expresses the required condition directly
rather than naming particular callers. alpha selects ARCH_NO_PREEMPT,
so unless something else turns on PREEMPT_COUNT the test is a
compile-time 0 and the hook runs everywhere, including at the end of
kthread_use_mm(); it only takes effect in PREEMPT_COUNT builds.

Moving the call changes when it runs. switch_to() ran it with the rq
lock held and interrupts off, while finish_arch_post_lock_switch() runs
after finish_lock_switch() has dropped that lock and re-enabled
interrupts, with preemption still disabled. A shootdown IPI taken in
that window either finds asn_lock still set and defers, or finds it
already cleared and flushes directly and by then PAL_swpctx has
installed the incoming context, so the direct flush acts on the right
one. need_new_asn is only ever set while asn_lock is 1.

It also changes one case for the worse, which the series that follows
does not fix. Before this patch nothing cleared asn_lock during a
kthread_use_mm() borrow, since check_mmu_context() ran only from
switch_to(), so a shootdown IPI for the borrowed mm found asn_locked()
true and took the conservative flush_tlb_other() path, retiring
mm->context[cpu]. With the hook in place asn_lock is cleared when
kthread_use_mm() returns, so such an IPI instead issues a targeted tbi()
against a context that ev5_switch_mm() prepared but never installed, and
leaves the slot valid. Loading the context on a direct mm switch closes
that window; where both changes are applied, that one belongs first.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Magnus Lindholm <linmag7@xxxxxxxxx>
---
 arch/alpha/include/asm/mmu_context.h | 8 ++++++++
 arch/alpha/include/asm/switch_to.h   | 1 -
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/include/asm/mmu_context.h b/arch/alpha/include/asm/mmu_context.h
index eee8fe836a59..825d3b9605c9 100644
--- a/arch/alpha/include/asm/mmu_context.h
+++ b/arch/alpha/include/asm/mmu_context.h
@@ -181,6 +181,14 @@ do {								\
 #define check_mmu_context()  do { } while(0)
 #endif
 
+/* Per-CPU state: only safe while still on the switching CPU.  */
+#define finish_arch_post_lock_switch finish_arch_post_lock_switch
+static inline void finish_arch_post_lock_switch(void)
+{
+	if (!preemptible())
+		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