From: Mathieu Desnoyers <mathieu.desnoyers@efficios•com>
To: "André Almeida" <andrealmeid@igalia•com>,
"Catalin Marinas" <catalin.marinas@arm•com>,
"Will Deacon" <will@kernel•org>,
"Thomas Gleixner" <tglx@kernel•org>,
"Mark Rutland" <mark.rutland@arm•com>,
"Sebastian Andrzej Siewior" <bigeasy@linutronix•de>,
"Carlos O'Donell" <carlos@redhat•com>,
"Peter Zijlstra" <peterz@infradead•org>,
"Florian Weimer" <fweimer@redhat•com>,
"Rich Felker" <dalias@aerifal•cx>,
"Torvald Riegel" <triegel@redhat•com>,
"Darren Hart" <dvhart@infradead•org>,
"Ingo Molnar" <mingo@kernel•org>,
"Davidlohr Bueso" <dave@stgolabs•net>,
"Arnd Bergmann" <arnd@arndb•de>,
"Liam R . Howlett" <Liam.Howlett@oracle•com>,
"Uros Bizjak" <ubizjak@gmail•com>,
"Thomas Weißschuh" <linux@weissschuh•net>
Cc: linux-arm-kernel@lists•infradead.org,
linux-kernel@vger•kernel.org, linux-arch@vger•kernel.org,
kernel-dev@igalia•com
Subject: Re: [PATCH v3 2/5] arm64: vdso: Implement __vdso_futex_robust_try_unlock()
Date: Fri, 29 May 2026 13:47:28 -0400 [thread overview]
Message-ID: <844b56fe-b5d4-42b2-ae8e-ce07980d0f50@efficios.com> (raw)
In-Reply-To: <20260529-tonyk-robust_arm-v3-2-a6f02684d4fe@igalia.com>
On 2026-05-29 12:33, André Almeida wrote:
> Based on the x86 implementation, implement the vDSO function for unlocking
> a robust futex correctly.
>
> Commit xxxxxxxxxxxx ("x86/vdso: Implement __vdso_futex_robust_try_unlock()") has
> the full explanation about why this mechanism is needed.
>
> The unlock assembly sequence for arm64 is:
>
> __vdso_futex_robust_list64_try_unlock:
> retry:
> ldxr w8, [x0] // Load the value from *futex
> cmp w1, w8 // Compare with TID
> b.ne __vdso_futex_list64_try_unlock_cs_end
> stlxr w9, wzr, [x0] // Try to zero *futex
So it looks like stlxr can be successful, and the process is killed
right here. This is not within the start/end critical section, so the
fixup is missed ? Or am I missing something ?
> cbnz w9, retry
> __vdso_futex_list64_try_unlock_cs_start:
> str xzr, [x2] // After zeroing *futex, zero *op_pending
> __vdso_futex_list64_try_unlock_cs_end>:
>
> The decision regarding if the pointer should be cleared or not lies on checking
> the condition flag zero:
>
> return (regs->user_regs.pstate & PSR_Z_BIT) ?
> (void __user *) regs->user_regs.regs[2] : NULL;
>
> If it's not zero, that means that the comparassion worked and the kernel should
comparison
> clear op_pending (if userspace didn't managed to) stored at x2.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
next prev parent reply other threads:[~2026-05-29 17:47 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-29 16:33 [PATCH v3 0/5] arm64: vdso: Implement __vdso_futex_robust_try_unlock() André Almeida
2026-05-29 16:33 ` [PATCH v3 1/5] arm64: vdso: Prepare for robust futex unlock support André Almeida
2026-05-29 17:16 ` Thomas Weißschuh
2026-06-01 16:17 ` André Almeida
2026-05-29 16:33 ` [PATCH v3 2/5] arm64: vdso: Implement __vdso_futex_robust_try_unlock() André Almeida
2026-05-29 17:18 ` Thomas Weißschuh
2026-05-29 17:47 ` Mathieu Desnoyers [this message]
2026-06-01 16:22 ` André Almeida
2026-05-29 16:33 ` [PATCH v3 3/5] selftests: futex: Add support for aarch64 in robust_list_critical André Almeida
2026-05-29 16:33 ` [PATCH v3 4/5] arm64: vdso32: Bring vdso32-offsets.h back André Almeida
2026-05-29 16:33 ` [PATCH v3 5/5] arm64: vdso32: Implement __vdso_futex_robust_try_unlock() André Almeida
2026-05-29 19:15 ` Mathieu Desnoyers
2026-06-02 8:30 ` Thomas Weißschuh
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