Messages in this thread | | | Subject | Re: [RFC][PATCH] Fix a race between rwsem and the scheduler | From | Benjamin Herrenschmidt <> | Date | Wed, 31 Aug 2016 20:55:44 +1000 |
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On Wed, 2016-08-31 at 09:20 +0200, Peter Zijlstra wrote: > On Wed, Aug 31, 2016 at 07:25:01AM +1000, Benjamin Herrenschmidt wrote: > > > > On Tue, 2016-08-30 at 15:04 +0200, Oleg Nesterov wrote: > > > > > > > > > Confused... how this connects to UNLOCK+LOCK on rq->lock? A LOAD can > > > leak into the critical section. > > > > > > But context switch should imply mb() we can rely on? > > > > Between setting of ->on_rq and returning to the task so it can > > change its state back to [UN]INTERRUPTIBLE, there will be at least one > > write barrier (spin unlock of the rq), > > spin-unlock is _not_ a write barrier, its a RELEASE barrier, and is not > sufficient for this.
Ah yes well it's an lwsync so it's a wmb for us :-) .
> > possibly even a full barrier > > (context switch). The write barrier is enough so I didn't dig to make > > sure we always context switch in the scenario we're looking at but I > > think we do. > > There is enough, you just need to pair the RELEASE with an ACQUIRE to > get a full load-store barrier.
Right so I *think* there will be at least the release of the rq_lock by the IPI followed by schedule itself taking and releasing it again, but I can't vouch for it. As I said, I didn't dig deeper on that side of things as for us a spin_unlock is a write barrier and for the write side that concerns me here it's sufficient ;-) It's the read side that has a problem.
That said you may want to investigate more to make sure there is no way out of schedule where that spin_unlock is the only thing between setting on_rq and coming out (which leads to setting the task state).
I suspect there will be at least one more re-aquisition & release of the rq lock but I may be wrong.
Cheers, Ben.
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