On Wed, Jan 19, 2022 at 03:28:32PM -0800, Peter Collingbourne wrote: > On Wed, Jan 19, 2022 at 2:03 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > On Tue, Jan 18, 2022 at 03:05:39PM -0800, Peter Collingbourne wrote: > > > After submitting a patch with a compare-exchange loop similar to this > > > one to set the KASAN tag in the page flags, Andrey Konovalov pointed > > > out that we should be using READ_ONCE() to read the page flags. Fix > > > it here. > > > > What does it actually fix? If it manages to split the read and read > > garbage the cmpxchg will fail and we go another round, no harm done. > > What I wasn't sure about was whether the compiler would be allowed to > break this code by hoisting the read of page->flags out of the loop > (because nothing in the loop actually writes to page->flags aside from > the compare-exchange, and if that succeeds we're *leaving* the loop). The cmpxchg is a barrier() and as such I don't think it's allowed to hoist anything out of the loop. Except perhaps since it's do-while, it could try and unroll the first iteration and wreck that something fierce. The bigger problem is I think that page_cpuid_last() usage which does a second load of page->flags, and given sufficient races that could actually load a different value and then things would be screwy. But that's not actually fixed. > > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > > > Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693 > > > > That's that doing here? > > I upload my changes to Gerrit and link to them here so that I (and > others) can see the progression of the patch via the web UI. What's the life-time guarantee for that URL existing? Because if it becomes part of the git commit, it had better stay around 'forever' etc..
