Re: [PATCH 5/7] FUSE: implement ioctl support

[H. Peter Anvin]

Miklos Szeredi wrote:
> On Thu, 28 Aug 2008, H. Peter Anvin wrote:
> > Tejun Heo wrote:
> > > Ah.... funky.  If this retry thing is too repulsive, I guess the best
> > > alternative would be directly accessing caller's memory as Miklos suggested.
> > Be careful -- there are some serious dragons there in the presence of 
> > multiple threads.
>
> OK, it should map /proc/pid/task/tid/mem.  Or rather
> /proc/tid/task/tid/mem, as the pid (tgid) of the caller is not
> currently passed to the filesystem.

Uhm, no.  You can still have it change underneath you as long as you 
have any thread of execution with access to the same memory.

This is *hard* to get right, and we screw this up in the kernel with 
painful regularity.  The throught of having user-space processes, which 
don't have access to the kernel locking primitives and functions like 
copy_from_user() dealing with this stuff scares me crazy.

That is why I'm suggesting using an in-kernel linearizer.

	-hpa
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