linux-kernel - Re: [PATCH 2/2] [RFC] reservation: add suppport for read-only access using rcu

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]

Message-ID: <53467FA3.5040805@vmware.com>
Date:	Thu, 10 Apr 2014 13:25:23 +0200
From:	Thomas Hellstrom <thellstrom@...are.com>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	ccross@...gle.com, linux-media@...r.kernel.org
Subject: Re: [PATCH 2/2] [RFC] reservation: add suppport for read-only access
 using rcu

On 04/10/2014 01:08 PM, Thomas Hellstrom wrote:
> On 04/10/2014 12:07 PM, Maarten Lankhorst wrote:
>> Hey,
>>
>> op 10-04-14 10:46, Thomas Hellstrom schreef:
>>> Hi!
>>>
>>> Ugh. This became more complicated than I thought, but I'm OK with moving
>>> TTM over to fence while we sort out
>>> how / if we're going to use this.
>>>
>>> While reviewing, it struck me that this is kind of error-prone, and hard
>>> to follow since we're operating on a structure that may be
>>> continually updated under us, needing a lot of RCU-specific macros and
>>> barriers.
>> Yeah, but with the exception of dma_buf_poll I don't think there is
>> anything else
>> outside drivers/base/reservation.c has to deal with rcu.
>>
>>> Also the rcu wait appears to not complete until there are no busy fences
>>> left (new ones can be added while we wait) rather than
>>> waiting on a snapshot of busy fences.
>> This has been by design, because 'wait for bo idle' type of functions
>> only care
>> if the bo is completely idle or not.
> No, not when using RCU, because the bo may be busy again before the
> function returns :)
> Complete idleness can only be guaranteed if holding the reservation, or
> otherwise making sure
> that no new rendering is submitted to the buffer, so it's an overkill to
> wait for complete idleness here.
>
Although, if we fail to get a refcount for a fence, and it's still busy
we need to do  a seq retry,
because the fence might have been replaced by another fence from the
same context, without being idle. That check is not present in the
snapshot code I sent.

/Thomas
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/