Re: [Patch v2] sysfs: add lockdep class support to s_active

[Cong Wang]

Eric W. Biederman wrote:
> Xiaotian Feng <xtfeng@xxxxxxxxx> writes:
>
> > On Fri, Feb 5, 2010 at 2:42 PM, Amerigo Wang <amwang@xxxxxxxxxx> wrote:
> > > Recently we met a lockdep warning from sysfs during s2ram or cpu hotplug.
> > > As reported by several people, it is something like:
> > >
> > > [ 6967.926563] ACPI: Preparing to enter system sleep state S3
> > > [ 6967.956156] Disabling non-boot CPUs ...
> > > [ 6967.970401]
> > > [ 6967.970408] =============================================
> > > [ 6967.970419] [ INFO: possible recursive locking detected ]
> > > [ 6967.970431] 2.6.33-rc2-git6 #27
> > > [ 6967.970439] ---------------------------------------------
> > > [ 6967.970450] pm-suspend/22147 is trying to acquire lock:
> > > [ 6967.970460]  (s_active){++++.+}, at: [<c10d2941>]
> > > sysfs_hash_and_remove+0x3d/0x4f
> > > [ 6967.970493]
> > > [ 6967.970497] but task is already holding lock:
> > > [ 6967.970506]  (s_active){++++.+}, at: [<c10d4110>]
> > > sysfs_get_active_two+0x16/0x36
> > > [...]
> > >
> > > Eric already provides a patch for this[1], but it still can't fix the
> > > problem. Based on his work and Peter's suggestion, I write this patch,
> > > hopefully we can fix the warning completely.
> > >
> > > This patch put sysfs s_active into two classes, one is for PM, the other
> > > is for the rest, so lockdep will distinguish them.
> > I think this patch does not hit the root cause, we have a similiar
> > warning which is not related with PM.
>
> The root cause is that our locking is crazy complicated.  No lockdep
> changes are going to fix that.
>
> What we can do and what the patch does is teach lockdep to treat some
> of the sysfs files as a different group (subclass) from other sysfs
> files.  Which keeps us from overgeneralizing too much and having
> a better signal to noise ratio.
>
> As for the block device problem goes, I can't easily say that
> the block layer is correct.  I expect it is because changing
> the scheduler is unlikely to delete block devices.  If the block layer
> has bugs then adding another subclass as Amerigo suggests should simply
> make lockdep warnings harder to trigger and more accurate so that
> sounds like a path worth walking.
>
> In general I recommend that pieces of code that need to do a lot of
> work in a sysfs attribute consider using a work queue or a kernel
> thread, as that can be easier to analyze.

Cc'ing Jens Axboe.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/