Re: Several tst-robust* tests time out with recent Linux kernel

On Wed, 2023-11-15 at 09:51 +0100, Peter Zijlstra wrote:

On Wed, Nov 15, 2023 at 01:11:20AM +0000, Edgecombe, Rick P wrote:

quoted

On Tue, 2023-11-14 at 21:14 +0100, Peter Zijlstra wrote:

quoted

Urgh, thanks!

Confirmed, the below cures things. Although I should probably
make
that
FLAGS_SIZE_32 | FLAGS_SHARED against Linus' tree.

Let me go do a proper patch.

I saw these fail on the glibc shadow stack branch today, and I also
saw
this one failing:
FAIL: nptl/tst-robustpi8

tip/locking/urgent (branch with the fix on) gets me:

root@ivb-ep:/usr/local/src/glibc# ./build/nptl/tst-robustpi8 
running child
verifying locks
running child
verifying locks
running child
verifying locks
running child
verifying locks
running child
verifying locks
root@ivb-ep:/usr/local/src/glibc#

Which, to my untrained eye, looks like a pass to me.

It bisects to this for me:
fbeb558b0dd0 ("futex/pi: Fix recursive rt_mutex waiter state")

Reading the patch, I'm not immediately clear what is going on but a few
comments stood out: "There be dragons here" "What could possibly go
wrong..." "This is a somewhat dangerous proposition".

Seems a likelihood of some race, but it reproduces reliably on my
machine. Haven't dug into debugging it yet. Any pointers?