[PATCH 2/2] srcuclassic: allow using same SRCU in process and interrupt context

[Paolo Bonzini]

Linu Cherian reported a WARN in cleanup_srcu_struct when shutting
down a guest that has iperf running on a VFIO assigned device.

This happens because irqfd_wakeup calls srcu_read_lock(&kvm->irq_srcu)
in interrupt context, while a worker thread does the same inside
kvm_set_irq.  If the interrupt happens while the worker thread is
executing __srcu_read_lock, lock_count can fall behind.

The docs say you are not supposed to call srcu_read_lock() and
srcu_read_unlock() from irq context, but KVM interrupt injection happens
from (host) interrupt context and it would be nice if SRCU supported the
use case.  KVM is using SRCU here not really for the "sleepable" part,
but rather due to its faster detection of grace periods, therefore it
is not possible to switch back to RCU, effectively reverting commit
719d93cd5f5c ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING", 2014-01-16).

However, the docs are painting a worse situation than it actually is.
You can have an SRCU instance only has users in irq context, and you
can mix process and irq context as long as process context users
disable interrupts.  In addition, __srcu_read_unlock() actually uses
this_cpu_dec, so that only srcu_read_lock() is unsafe.

When srcuclassic's __srcu_read_unlock() was changed to use this_cpu_dec(),
in commit 5a41344a3d83 ("srcu: Simplify __srcu_read_unlock() via 
this_cpu_dec()", 2012-11-29), __srcu_read_lock() did two increments.
Therefore it kept __this_cpu_inc, with preempt_disable/enable in the
caller.  Nowadays however it only does one increment, so on most
architectures it is more efficient for __srcu_read_lock to use
this_cpu_inc, too.

There would be a slowdown if 1) fast this_cpu_inc is not available and
cannot be implemented (this usually means that atomic_inc has implicit
memory barriers), and 2) local_irq_save/restore is slower than disabling
preemption.  The main architecture with these constraints is s390, which
however is already paying the price in __srcu_read_unlock and has not
complained.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 719d93cd5f5c ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING")
Reported-by: Linu Cherian <linuc.decode@xxxxxxxxx>
Suggested-by: Linu Cherian <linuc.decode@xxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 include/linux/srcu.h | 2 --
 kernel/rcu/srcu.c    | 5 ++---
 2 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 167ad8831aaf..4c1d5f7e62c4 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -172,9 +172,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
 {
 	int retval;
 
-	preempt_disable();
 	retval = __srcu_read_lock(sp);
-	preempt_enable();
 	rcu_lock_acquire(&(sp)->dep_map);
 	return retval;
 }
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index 584d8a983883..dea03614263f 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -263,7 +263,7 @@ void cleanup_srcu_struct(struct srcu_struct *sp)
 
 /*
  * Counts the new reader in the appropriate per-CPU element of the
- * srcu_struct.  Must be called from process context.
+ * srcu_struct.
  * Returns an index that must be passed to the matching srcu_read_unlock().
  */
 int __srcu_read_lock(struct srcu_struct *sp)
@@ -271,7 +271,7 @@ int __srcu_read_lock(struct srcu_struct *sp)
 	int idx;
 
 	idx = READ_ONCE(sp->completed) & 0x1;
-	__this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
+	this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
 	return idx;
 }
@@ -281,7 +281,6 @@ int __srcu_read_lock(struct srcu_struct *sp)
  * Removes the count for the old reader from the appropriate per-CPU
  * element of the srcu_struct.  Note that this may well be a different
  * CPU than that which was incremented by the corresponding srcu_read_lock().
- * Must be called from process context.
  */
 void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 {
-- 
1.8.3.1