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

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Date:   Fri, 17 Nov 2023 01:22:56 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "xry111@...111.site" <xry111@...111.site>,
        "andrealmeid@...lia.com" <andrealmeid@...lia.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Several tst-robust* tests time out with recent Linux kernel

A bit more info...

The error returned to userspace is originating from:
https://github.com/torvalds/linux/blob/master/kernel/futex/pi.c#L295

'uval' is often zero in that error case, but sometimes just a
mismatching value like: uval=0x567, task_pid_vnr()=0x564

Depending on the number of CPUs the VM is running on it reproduces or
not. When it does reproduce, the newly added path here is taken:
https://github.com/torvalds/linux/blob/master/kernel/futex/pi.c#L1185
The path is taken a lot during the test, sometimes >400 times before
the above linked error is generated during the syscall. When it doesn't
reproduce, I never saw that new path taken.

More print statements make the reproduction less reliable, so it does
seem to have a race in the mix at least somewhat. Otherwise, I haven't
tried to understand what is going on here with all this highwire
locking.

Hope it helps.