Re: Request for backport fd6bc19d7676 to 4.14 and 4.19 branch

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Aug 16, 2021 at 07:19:34PM +0000, David Chen wrote:
> Hi Greg,
> 
> We recently hit a hung task timeout issue in synchronize_rcu_expedited on 4.14 branch.
> The issue seems to be identical to the one described in `fd6bc19d7676 rcu: Fix missed wakeup of exp_wq waiters`
> Can we backport it to 4.14 and 4.19 branch?
> The patch doesn't apply cleanly, but it should be trivial to resolve, just do this
> 
> -		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]);
> +		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
> 
> I don't know if we should do it for 4.9, because the handling of sequence number is a bit different.

Please provide a working backport, me hand-editing patches does not
scale, and this way you get the proper credit for backporting it (after
testing it).

You have tested, this, right?

thanks,

greg k-h