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

[Jinjie Ruan <ruanjinjie@huawei.com>]

On 7/21/2026 4:51 PM, Linus Walleij wrote:
> On Tue, Jul 21, 2026 at 10:19 AM Jinjie Ruan <ruanjinjie@huawei.com> 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.

Yes, there is a little unclear.

The expression "!IS_ENABLED(CONFIG_DEBUG_RSEQ)" evaluates to true only
when CONFIG_DEBUG_RSEQ is not set; with the guarantee that it is
disabled, the condition is always satisfied.

> 
>> 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@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Ada Couprie Diaz <ada.coupriediaz@arm.com>
>> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> 
> 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@kernel.org>
> 
> Yours,
> Linus Walleij