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Message-ID: <87czoweu2d.ffs@tglx> Date: Sat, 25 Sep 2021 14:08:42 +0200 From: Thomas Gleixner <tglx@...utronix.de> To: Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org Cc: Sohil Mehta <sohil.mehta@...el.com>, Tony Luck <tony.luck@...el.com>, Dave Hansen <dave.hansen@...el.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Jens Axboe <axboe@...nel.dk>, Christian Brauner <christian@...uner.io>, Peter Zijlstra <peterz@...radead.org>, Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>, Jonathan Corbet <corbet@....net>, Ashok Raj <ashok.raj@...el.com>, Jacob Pan <jacob.jun.pan@...ux.intel.com>, Gayatri Kammela <gayatri.kammela@...el.com>, Zeng Guang <guang.zeng@...el.com>, Dan Williams <dan.j.williams@...el.com>, Randy E Witt <randy.e.witt@...el.com>, Ravi V Shankar <ravi.v.shankar@...el.com>, Ramesh Thomas <ramesh.thomas@...el.com>, linux-api@...r.kernel.org, linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org Subject: Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall On Fri, Sep 24 2021 at 13:04, Thomas Gleixner wrote: > On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote: >> +int uintr_receiver_wait(void) >> +{ >> + struct uintr_upid_ctx *upid_ctx; >> + unsigned long flags; >> + >> + if (!is_uintr_receiver(current)) >> + return -EOPNOTSUPP; >> + >> + upid_ctx = current->thread.ui_recv->upid_ctx; >> + upid_ctx->upid->nc.nv = UINTR_KERNEL_VECTOR; >> + upid_ctx->waiting = true; >> + spin_lock_irqsave(&uintr_wait_lock, flags); >> + list_add(&upid_ctx->node, &uintr_wait_list); >> + spin_unlock_irqrestore(&uintr_wait_lock, flags); >> + >> + set_current_state(TASK_INTERRUPTIBLE); > > Because we have not enough properly implemented wait primitives you need > to open code one which is blantantly wrong vs. a concurrent wake up? > >> + schedule(); > > How is that correct vs. a spurious wakeup? What takes care that the > entry is removed from the list? > > Again. We have proper wait primitives. Aisde of that this is completely broken vs. CPU hotplug. CPUX switchto(tsk) tsk->upid.ndst = apicid(smp_processor_id(); ret_to_user() ... sys_uintr_wait() ... schedule() After that CPU X is unplugged which means the task won't be woken up by an user IPI which is issued after CPU X went down. Thanks, tglx
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