[PATCH for-4.19 3/9] xen/cpu: ensure get_cpu_maps() returns false if CPU operations are underway

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Due to the current rwlock logic, if the CPU calling get_cpu_maps() does so from
a cpu_hotplug_{begin,done}() region the function will still return success,
because a CPU taking the rwlock in read mode after having taken it in write
mode is allowed.  Such behavior however defeats the purpose of get_cpu_maps(),
as it should always return false when called with a CPU hot{,un}plug operation
is in progress.  Otherwise the logic in send_IPI_mask() for example is wrong,
as it could decide to use the shorthand even when a CPU operation is in
progress.

Adjust the logic in get_cpu_maps() to return false when the CPUs lock is
already hold in write mode by the current CPU, as read_trylock() would
otherwise return true.

Fixes: 868a01021c6f ('rwlock: allow recursive read locking when already locked 
in write mode')
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/common/cpu.c         | 3 ++-
 xen/include/xen/rwlock.h | 2 ++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/common/cpu.c b/xen/common/cpu.c
index 6173220e771b..d76f80fe2e99 100644
--- a/xen/common/cpu.c
+++ b/xen/common/cpu.c
@@ -49,7 +49,8 @@ static DEFINE_RWLOCK(cpu_add_remove_lock);
 
 bool get_cpu_maps(void)
 {
-    return read_trylock(&cpu_add_remove_lock);
+    return !rw_is_write_locked_by_me(&cpu_add_remove_lock) &&
+           read_trylock(&cpu_add_remove_lock);
 }
 
 void put_cpu_maps(void)
diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
index a2e98cad343e..4e7802821859 100644
--- a/xen/include/xen/rwlock.h
+++ b/xen/include/xen/rwlock.h
@@ -316,6 +316,8 @@ static always_inline void write_lock_irq(rwlock_t *l)
 
 #define rw_is_locked(l)               _rw_is_locked(l)
 #define rw_is_write_locked(l)         _rw_is_write_locked(l)
+#define rw_is_write_locked_by_me(l) \
+    lock_evaluate_nospec(_is_write_locked_by_me(atomic_read(&(l)->cnts)))
 
 
 typedef struct percpu_rwlock percpu_rwlock_t;
-- 
2.44.0