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

[Mateusz Guzik]

On Thu, May 21, 2026 at 10:53:03AM +0200, Mateusz Guzik wrote:
> 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;
>         }
> 
> 

Even better, I got the above fixup + some polish listed below: 
- removed an extra space in newlen calculation
- the WARN_ON_ONCE case needs to free 'new', not 'old'
- there is no READ_ONCE anymore in filesystems_proc_show()

goes into the "fs: cache the string generated by reading /proc/filesystems"
commit.

diff --git a/fs/filesystems.c b/fs/filesystems.c
index 771fc31a69b8..712316a1e3e0 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -269,7 +269,7 @@ static __cold noinline int regen_filesystems_string(void)
 	hlist_for_each_entry_rcu(p, &file_systems, list) {
 		if (!(p->fs_flags & FS_REQUIRES_DEV))
 			newlen += strlen("nodev");
-		newlen += strlen("\t") + strlen(p->name) +  strlen("\n");
+		newlen += strlen("\t") + strlen(p->name) + strlen("\n");
 	}
 	spin_unlock(&file_systems_lock);
 
@@ -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;
 	}
@@ -321,13 +323,12 @@ static __cold noinline int regen_filesystems_string(void)
 		 * generation above and messes it up.
 		 */
 		spin_unlock(&file_systems_lock);
-		if (old)
-			kfree_rcu(old, rcu);
+		kfree(new);
 		return -EINVAL;
 	}
 
 	/*
-	 * Paired with consume fence in READ_ONCE() in filesystems_proc_show()
+	 * Paired with consume fence in rcu_dereference() in filesystems_proc_show()
 	 */
 	smp_store_release(&file_systems_string, new);
 	spin_unlock(&file_systems_lock);