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

[Thomas Hellstrom]

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().

Thanks,

Thomas