linux-kernel - Re: Hi, I have one question about rt

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Date:	Thu, 10 May 2007 09:00:28 -0400
From:	Steven Rostedt <srostedt@...hat.com>
To:	Li Yu <raise.sail@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Hi, I have one question about rt_mutex.

Li Yu wrote:
>> Now since mutexes can be defined by user-land applications, we don't
> want a DOS
>> type of application that nests large amounts of mutexes to create a large
>> PI chain, and have the code holding spin locks while looking at a large
>> amount of data. So to prevent this, the implementation not only implements
>> a maximum lock depth, but also only holds at most two different locks at a
>> time, as it walks the PI chain. More about this below.
> 
> After read the implementation of rt_mutex_adjust_prio_chain(), I found
> the we really require maximin lock depth (1024 default), but I can not
> see the check for more same locks duplication. Does this doc is
> inconsistent with code?

Nope, the code and the doc are still the same.

The thing that was most difficult in writing that document, was a way to
talk about the user locks (futex - fast user mutex) and the kernel locks
(spin_locks) without confusing the two.  The max depth is in reference
to the user futex, but the comment about the "at most two different
locks" is referencing the kernel's spin_locks.

I don't remember talking about looking for "lock duplication", which I'm
thinking you are referring to circular dead locks. I didn't cover that
in the document and I believe I even mentioned that I would not cover
the debug aspect of the code which would handle catching circular deadlocks.

But back to the "no more than two kernel locks held". This is very
important. Some PI implementations requires all locks in the PI chain to
have their internal locks held (as in spin_locks).  But letting user
space determine the number of spin locks held can cause large latencies
for the rest of the system.  So we designed a method to only need to
hold two internal spin_locks in the PI chain at a time.  The kernel
doesn't care if the user application is abusing itself (holding too many
of it's own user locks).  But the kernel does care if a user application
can affect other non related applications.

As Esben already mentioned, the PI chain even lets the locking user
mutex schedule without holding any kernel locks.  This is very key. It
keeps the latency down on setting up a PI chain which can be very expensive.

Note: Esben helped a lot in the development of the final design of
rtmutex.c.

-- Steve

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