Re: Current state of the Linux kernel on SPARC

[John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>]

Hello,

On Sun, 2025-08-24 at 23:09 +0200, John Paul Adrian Glaubitz wrote:
> First, let's start with the bugs. For a while it has been known that recent kernel
> versions can be very unstable on certain SPARC machines, this has been observed in
> particular with UltraSPARC III CPUs but also on certain newer CPUs such as SPARC T1.
> 
> After I started bisecting the issue, I ran into multiple false positives until I
> identified d53d2f78cead as the culprit which makes use of a new vmalloc flag called
> VM_FLUSH_RESET_PERMS.
> 
> However, this particular change is actually not broken but rather just uncovered the
> original bug. The introduction of VM_FLUSH_RESET_PERMS allowed the kernel to flush
> TLBs earlier after booting and more often. And since the original problem was suspected
> with the TLB flush management on SPARC, it was just natural that the change in d53d2f78cead
> turned out in the bisect.
> 
> Further investigation showed that the actual culprit are the CPU-specific implementations
> of copy_{from,to}_user which can be found in arch/sparc/lib. These are broken on different
> CPU types to a different degree which explains perfectly fine why recent kernels are more
> unstable on certain CPU types than on others.
> 
> Luckily, Michael Karcher has already made good progress in investigating and fixing these
> bugs so that, for example, a trial patch for the UltraSPARC III showed that a simple
> one-line change would fix all the stability issues currently seen on these CPU types.
> 
> It is expected that a series of patches will follow shortly that will address the bugs in
> the copy_{from,to}_user on all affected CPU types. In the mean time, it should be possible
> to switch the kernel to the generic code for copy_{from,to}_user that can be found in the
> same source directory to get a stable system on any CPU type.

A patch series to address these bugs was just posted:

https://lore.kernel.org/all/20250826160312.2070-1-kernel@mkarcher.dialup.fu-berlin.de/

Please test and report back!

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
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