[REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere

[Mathias Stearn]

On Thu, Apr 23, 2026 at 7:48 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
> I see some patches flying around, but am not clear on whether there
> was an actual patch that make this work again?

Thomas's patch from upthread appears in initial testing to address the
arm64 preemption breakage. Thanks! I'm currently building with the
following patch on top of that and will test it once it is ready.

---

diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h
index a36b472627de..e26bf249bbd8 100644
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -300,12 +300,15 @@ rseq_update_user_cs(struct task_struct *t,
struct pt_regs *regs, unsigned long c

         /* Invalidate the critical section */
         unsafe_put_user(0ULL, &t->rseq.usrptr->rseq_cs, efault);
+        /* TCMalloc kludge - it relies on cpu_id_start being overwritten */
+        unsafe_put_user((u32)task_cpu(t),
&t->rseq.usrptr->cpu_id_start, efault);
         /* Update the instruction pointer */
         instruction_pointer_set(regs, (unsigned long)abort_ip);
         rseq_stat_inc(rseq_stats.fixup);
         break;
     clear:
         unsafe_put_user(0ULL, &t->rseq.usrptr->rseq_cs, efault);
+        unsafe_put_user((u32)task_cpu(t),
&t->rseq.usrptr->cpu_id_start, efault);
         rseq_stat_inc(rseq_stats.clear);
         abort_ip = 0ULL;
     }