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Re: [Xen-devel] [PATCH 1/2] xen/rwlocks: call preempt_disable() when taking a rwlock

On 13.03.20 11:02, Julien Grall wrote:
Hi Juergen,

On 13/03/2020 08:05, Juergen Gross wrote:
Similar to spinlocks preemption should be disabled while holding a

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
  xen/include/xen/rwlock.h | 18 +++++++++++++++++-
  1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
index 1c221dd0d9..4ee341a182 100644
--- a/xen/include/xen/rwlock.h
+++ b/xen/include/xen/rwlock.h
@@ -2,6 +2,7 @@
  #define __RWLOCK_H__
  #include <xen/percpu.h>
+#include <xen/preempt.h>
  #include <xen/smp.h>
  #include <xen/spinlock.h>
@@ -57,10 +58,12 @@ static inline int _read_trylock(rwlock_t *lock)
      cnts = atomic_read(&lock->cnts);
      if ( likely(_can_read_lock(cnts)) )

If you get preempted here, then it means the check below is likely going to fail. So I think it would be best to disable preemption before, to give more chance to succeed.

As preemption probability at this very point should be much lower than
that of held locks I think that is optimizing the wrong path. I'm not
opposed doing the modification you are requesting, but would like to
hear a second opinion on that topic, especially as I'd need to add
another preempt_enable() call when following your advice.


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