[OpenSBI] rfence related question

[Atish Patra]

On Thu, Oct 14, 2021 at 2:41 AM Leo Liang <ycliang at andestech.com> wrote:
>
> Hi,
>
> I am studying the rfence part of OpenSBI recently.
>
> From what I understand the mechanism of the IPI framework is
> to do "update" to update the IPI information for remote harts,
> and then send IPI through "ipi_dev" device to remote harts sequentially.
>
> After the first IPI is sent (let's say from hart 0 to hart 1),
> hart 0 waits in the "sync" for the remote hart(hart 1) to do "process".
> Hart 0 waits until the per hart tlb_sync flag is set from the remote hart(hart1) after "process" is done,
> and then keep on sending the next IPI to hart 2, and then hart 3 ... etc.
>
> If I understand it correctly, then would the following scenario have the possibility to occur?
>
> In Linux, Hart 0 executes remote I cache flush instruction and sends IPI to hart 1;
> At the meantime, hart 2 executes the same instruction and tries to send IPI to hart 0.
>
> Both hart 0 and hart 2 step into m-mode, their mstatus.mie are disabled,
> so, when hart 0 tries to send IPI to hart 2 and wait for the sync, deadlock happens.
> Hart 0 is waiting for hart 2 and hart 2 is also waiting for hart 0 indefinitely because the interrupts being disabled.
>
> Is this case likely to happen?
>

Nope because each hart uses a busy loop and consume the pending IPIs

https://github.com/riscv-software-src/opensbi/blob/master/lib/sbi/sbi_tlb.c#L268

> Best regards,
> Leo
>
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-- 
Regards,
Atish