Re: [PATCH] sched_clock: Disable seqlock lockdep usage in sched_clock

From: John Stultz
Date: Thu Jan 02 2014 - 17:21:55 EST

On 01/02/2014 02:15 PM, Linus Torvalds wrote:
> On Thu, Jan 2, 2014 at 1:54 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>> Unforunately the seqlock lockdep enablmenet can't be used
>> in sched_clock, since the lockdep infrastructure eventually
>> calls into sched_clock, which causes a deadlock.
>> Thus, this patch adds _no_lockdep() seqlock methods for the
>> writer side, and changes all generic sched_clock usage to use
>> the _no_lockdep methods.
> Ugh.
> On the x86 vclock_gettime() side, we only do this for the reader. Why
> did you make the generic version do it for the writer too, adding the
> necessity for those new operations? It's only the reader side that
> doesn't want it.

So the problem is that the update side calls the lockdep code which
calls sched_clock, which then deadlocks because the seqcount is odd
(held by the updater).

Thus we have to drop the lockdep usage in the updater as well.

On x86 vclock_gettime, we're in userspace, and that's why we can't call
the lockdep code. The update for that code however happens in kernel
space, so it doesn't have the same problem.

> Talking about the new operations, that "*_no_lockdep()" naming annoys
> me. It doesn't match the spinlock naming, which is to just use
> "raw_*()" instead. Wouldn't it be nice to make the naming be
> consistent too? Especially when it's paired with raw_local_irq_save()
> that shares that "raw_" model for non-checking stuff.

Sure, I can change the naming. New patch to follow in a bit.


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