Re: [PATCH v17 11/15] arm64: syscall: Simplify el0_svc_common() syscall exit path

[Linus Walleij]

On Tue, Jul 21, 2026 at 10:19 AM Jinjie Ruan <ruanjinjie@xxxxxxxxxx> wrote:

> Remove the redundant nested conditional check within the system call
> exit path of el0_svc_common() to streamline the exit sequence.
>
> When entering this fast-path block, CONFIG_DEBUG_RSEQ is guaranteed to be
> disabled so the call of rseq_syscall() is a no-op.

Please explain why this is guaranteed.

> Under this constraint,
> the code logic inside the block becomes completely identical to what
> the arm64_syscall_exit_to_user_mode_work() helper already does. Replace
> that nested logic with a direct invocation of the helper, eliminating
> redundant code.
>
> Before:
>
> | if (... && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
> |       flags = read_thread_flags();
> |       if (unlikely(flags & _TIF_SYSCALL_EXIT_WORK) || flags & _TIF_SINGLESTEP)
> |               arm64_syscall_exit_to_user_mode_work(regs);
> |       return;
> | }
> | trace_exit:
> |       arm64_syscall_exit_to_user_mode_work(regs);
>
> After simplify:
>
> | if (... && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
> |       arm64_syscall_exit_to_user_mode_work(regs);
> |       return;
> | }
> | trace_exit:
> |       arm64_syscall_exit_to_user_mode_work(regs);
>
> Furthermore, Since both the conditional fast-path and the fallback
> slow-path now uniformly invoke arm64_syscall_exit_to_user_mode_work(),
> this explicit conditional branch is entirely redundant regardless of
> whether the evaluation is true or false. Removing it collapses
> the duplicated logic into a single, unconditional path.
>
> No functional changes.
>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Ada Couprie Diaz <ada.coupriediaz@xxxxxxx>
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>

After looking at it for a while I conclude you are right, provided
the reasoning about !CONFIG_DEBUG_RSEQ holds. So with
that explained:
Reviewed-by: Linus Walleij <linusw@xxxxxxxxxx>

Yours,
Linus Walleij