Re: [PATCH -rt] race condition in fs/compat.c with compat_sys_ioctl

[Ingo Molnar]

* Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> It's still there in 2.6.15-rc1-git3 (the sem is the down_read of 
> ioctl32_sem in fs/compat.c).
> 
> No, the problem was unique to the rt patch.  In -rt the default 
> down_read is just like a down (since it is very hard to do PI on 
> readers and writer locks).  So the solution in -rt was to convert this 
> back to a normal RW sem.

to be precise: rwsems in PREEMPT_RT are "only one reader, but reader 
might self-recurse". I.e. the sequence of:

	down_read();
	...
		down_read();
		...
		up_read();
	...
	up_read();

is still supported.

this very simple rw-semaphore type covers the overwhelming majority of 
uses: out of hundreds of rwsems in the kernel, ioctl32_sem is the first 
one that had to be converted to a 'compat' rwsem.

	Ingo
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