Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

From: Thomas Hellstrom
Date: Thu Jun 14 2018 - 09:43:27 EST

On 06/14/2018 03:29 PM, Matthew Wilcox wrote:
On Thu, Jun 14, 2018 at 01:54:15PM +0200, Thomas Hellstrom wrote:
On 06/14/2018 01:36 PM, Peter Zijlstra wrote:
Currently you don't allow mixing WD and WW contexts (which is not
immediately obvious from the above code), and the above hard relies on
that. Are there sensible use cases for mixing them? IOW will your
current restriction stand without hassle?
Contexts _must_ agree on the algorithm used to resolve deadlocks. With
Wait-Die, for example, older transactions will wait if a lock is held by a
younger transaction and with Wound-Wait, younger transactions will wait if a
lock is held by an older transaction so there is no way of mixing them.
Maybe the compiler should be enforcing that; ie make it a different type?

It's intended to be enforced by storing the algorithm choice in the WW_MUTEX_CLASS which must be common for an acquire context and the ww_mutexes it acquires. However, I don't think there is a check that that holds. I guess we could add it as a DEBUG_MUTEX test in ww_mutex_lock().