netdev - Re: [RFC] tulip: Support for byte queue limits

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	17 Jul 2013 00:09:59 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	bhutchings@...arflare.com, linux@...izon.com
Cc:	eric.dumazet@...il.com, grantgrundler@...il.com,
	grundler@...isc-linux.org, netdev@...r.kernel.org
Subject: Re: [RFC] tulip: Support for byte queue limits

>>  	wmb();
>>  
>> -	tp->cur_tx++;
>> -
>>  	/* Trigger an immediate transmit demand. */
>>  	iowrite32(0, tp->base_addr + CSR1);
>>  
>> +	tp->cur_tx++;
>> +	netdev_sent_queue(dev, skb->len);
>>  	spin_unlock_irqrestore(&tp->lock, flags);

> This is not good practice, because once you start DMA you have
> effectively passed ownership of the skb to the TX completion handler.

Thank you for this advice.  Just to be clear, is the only issue reading
skb->len from a potentially deallocated skb?  Or is it also going go
give the byte queue system fits if the transmit complete handler calls
netdev_completed_queue before the transmitter calls netdev_sent_queue?

I'd hope it can underflow safely, and only look at the net value after
the transmit handler returns.

> Presumably the TX completion handler will hold this spinlock and
> therefore cannot free the skb before you use skb->len above.  So this
> will be safe now.  But one day someone may want to get rid of this lock,
> so this is a trap waiting to spring.

Sounds like a fun project.  But I have to dig into the BQL code; if it's
getting and dropping locks inside netdev_*_queue, the win is limited.

(A lock-free version would have separate "sent" and "completed" counters,
and compute the difference when a snapshot is required.)
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html