Re: [Qemu-devel] [PATCH 0/2] Introduce QemuRWLock

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, 03 Oct 2011 14:16:09 +0200, Jan Kiszka <jan.kiszka@web.de> wrote:
Non-text part: multipart/signed
> On 2011-10-03 13:23, Harsh Prateek Bora wrote:
> > SynthFS uses rwlocks, which raise the need of a generic QemuRWLock APIs.
> > This patchset introduces the same making necessary changes to relevant code.
> 
> Is the impact of using a plain mutex measurable with 9pfs? Usually it
> takes very heavy write sections or highly concurrent read sections to
> actually make a difference. Looking at the cited code, I would dare to
> rule out the former (even more if the malloc was moved out of the
> critical section). But I cannot assess the latter.
> 
> If it does matter, I would vote for introducing RCU directly.

I haven't done any measurements. The lock is taken in write mode
when creating new file system object and is taken in read mode during
lookup(walk) and readdir. Considering we allow creation of objects only
during init, it mostly will be taken in read mode. Currently there is no
deletion of object. We didn't want those parallel reads to be
mutually exclusive.

For RCU are you suggesting to work with userspace RCU implementation at 
http://lttng.org/urcu

-aneesh