[Xen-devel] rwlock and per-cpu rwlock recursive?

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On Arm, we made the (wrong?) assumption that the rwlock was recursive. 
We have a couple of places where the read lock can be nested (mostly the 
memaccess code).
I found out today that it can actually deadlock in the following case:
        1) CPU A -> Ask for the read lock
                => Succeed
        2) CPU B -> Ask for the write lock
                => Already taken by CPU A so go to the slowpath
                => Take the internal lock (lock->lock)
                => Wait until the lock is released.
        3) CPU A -> Ask for the read lock recursively
                => A writer is waiting so go to the slowpath
                => Try to take the internal lock (lock->lock)
                        => Blocking because CPU B already has it

So we end up in a deadlock case. Can someone confirm whether whether 
rwlock was meant to be recursive?
Similarly, I was thinking to move the p2m code to the per-cpu rwlock as 
x86 does. From what I gathered by reading the x86 code, this lock could 
be taken recursively. Am I right?
Lastly, the x86 p2m code seemed to use rwlock before hand. How did the 
p2m code was handling recursive locking?
Cheers,

--
Julien Grall

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