[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2] xen/x86: lock cacheline for add_sized()
On 07.08.2019 09:32, Juergen Gross wrote: add_sized() should use an atomic update of the memory word, as it is used by spin_unlock(). Ticket locks are using a read-modify-write operation on parts of the lockword for unlocking, while trying to lock is done by an atomic update of the complete lockword. This requires the unlock operation to be atomic, too, as otherwise the unlock might not write back the correct data. I have to take back my reply to v1, and hence I'm afraid that if the change is really needed the description is still insufficient. Let's look at both sides: Acquire is a LOCKed read-modify-write of the full word, with the additional property that the value written back to the low half is unchanged from the value read. Release is read low half increment low half write low half Since the low half doesn't change during any acquire (including attempts, i.e. try-lock), it doesn't matter if it races with the above sequence. It can freely happen between any two of the three steps. Therefore what I'm really after is (a) clarification whether the issue you mean to fix was observed in practice and (b) a concrete scenario where things would go wrong. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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