Re: context switch within RCU read-side critical section in next-20260518+ with PREEMPT_RT

[Mateusz Guzik]

On Thu, May 21, 2026 at 12:52:44AM +0200, Bert Karwatzki wrote:
> Since version next-20260518 (with PREEMPT_RT) I noticed that my debian stable/trixie system
> would sometimes hang when booting displaying the following error message. After about ~1min
> booting continues to a rescue shell where I could save the dmesg output (The output shown 
> here is not from next-20260519 but from a step in the bisection).
> 
[..]
> So I reverted the this commit in next-20260519 (to get a clean revert I needed to
> revert commit
> 36b3306779ea ("fs: cache the string generated by reading /proc/filesystems") first.
> 
> $ git log --oneline
> c7321982a5d0 (HEAD -> rcu_critical_readside_bug) Revert "fs: RCU-ify filesystems list"
> 16ff8d6e7c28 Revert "fs: cache the string generated by reading /proc/filesystems"
> 6a50ba100ace (tag: next-20260519, origin/master, origin/HEAD, master) Add linux-next specific files for 20260519
> 
> With these reverts next-20260519 boots 30 times in a row without error, so
> it appears that commit dc651e25a6d2 ("fs: RCU-ify filesystems list") causing the
> error.
> 

I think the patch below will do the trick.

If someone wonders how come the missing unlocks: the original patch had
them in place, but when I was rebasing on top of the RCU-ifing commit I
figured I'm going to do guard/scoped_guard in there as well. Later it
started failing as the compiler did not like goto retry out of a scoped
guard area and the unlocks did not come back.

tl;dr there is definitely my bug here and it is most likely *the* bug

Christian, can you fold this in please.

diff --git a/fs/filesystems.c b/fs/filesystems.c
index 771fc31a69b8..8f17c0abbc95 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -289,6 +289,7 @@ static __cold noinline int regen_filesystems_string(void)
         * Did someone beat us to it?
         */
        if (old && old->gen == file_systems_gen) {
+               spin_unlock(&file_systems_lock);
                kfree(new);
                return 0;
        }
@@ -297,6 +298,7 @@ static __cold noinline int regen_filesystems_string(void)
         * Did the list change in the meantime?
         */
        if (gen != file_systems_gen) {
+               spin_unlock(&file_systems_lock);
                kfree(new);
                goto retry;
        }