serverid(s)_exist, critical path work.

simo idra at
Thu Feb 7 10:57:09 MST 2013

On Thu, 2013-02-07 at 17:06 +0100, Volker Lendecke wrote:
> On Thu, Feb 07, 2013 at 10:40:48AM -0500, simo wrote:
> > Well if you want to avoid the syscall you need to always consider that
> > rogue writes may destroy memory locations. It's either/or, so I guess
> > what you need to choose is whether you can accept the risk and do
> > everything in user space using shared memory or similar techniques or
> > whether the risk is too high and you need the kernel to act as arbiter.
> > 
> > I wonder if there is a way to set canaries from user space to prevent
> > processes to accidentally clobber memory areas with large memcpy or
> > similar ... maybe a clever setting of mmap regions ...
> > 
> > I've been using memory barrier and synchronization primitives to
> > implement single writer multiple readers w/o locking, and robust mutexes
> > basically do the same I think.
> > 
> > Maybe there is a way to combine robust mutexes for performance with a
> > fallback to fcntl locks for recovering from very bad situations ?
> Falling back to fcntl under high pressure in my tests made
> the situation much worse due to the single system-wide
> spinlock gating every single fcntl lock operation. I made
> the reference to epoll because I could imagine this is a
> mechanism not dying under 10.000 simultaneous events.
> Do the mutexes, fall back to a locking daemon nicely
> queueing stuff in user space, communicated to via epoll. We
> just have to fill in the details ... :-)
> We need to coordinate share modes and leases of 30.000
> clients connected to a cluster all opening the share root
> directory.

Yeah epoll as a fallback would probably be nice, but I would still try
hard to avoid syscall if possible at all and use mutexes in normal


Simo Sorce
Samba Team GPL Compliance Officer <simo at>
Principal Software Engineer at Red Hat, Inc. <simo at>

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