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Date: Thu, 28 Aug 2008 13:21:24 -0700 From: "H. Peter Anvin" <hpa@...nel.org> To: Miklos Szeredi <miklos@...redi.hu> CC: tj@...nel.org, fuse-devel@...ts.sourceforge.net, greg@...ah.com, linux-kernel@...r.kernel.org Subject: Re: [PATCH 5/7] FUSE: implement ioctl support 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 -- 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/
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