Re: [PATCH] sysfs: Add lockdep annotations for the sysfs active reference

From: Eric W. Biederman
Date: Sun Jan 17 2010 - 12:19:09 EST

Ming Lei <tom.leiming@xxxxxxxxx> writes:

> On Sat, 02 Jan 2010 13:37:12 -0800
> ebiederm@xxxxxxxxxxxx (Eric W. Biederman) wrote:
>> Holding locks over device_del -> kobject_del -> sysfs_deactivate can
>> cause deadlocks if those same locks are grabbed in sysfs show or store
>> methods.
>> The I model s_active count + completion as a sleeping read/write lock.
>> I describe to lockdep sysfs_get_active as a read_trylock,
>> sysfs_put_active as a read_unlock, and sysfs_deactivate as a
>> write_lock and write_unlock pair. This seems to capture the essence
>> for purposes of finding deadlocks, and in my testing gives finds real
>> issues and ignores non-issues.
>> This brings us back to holding locks over kobject_del is a problem
>> that ideally we should find a way of addressing, but at least lockdep
>> can tell us about the problems instead of requiring developers to
>> debug rare strange system deadlocks, that happen when sysfs files are
>> removed while being written to.
> The model has hit a possible deadlock in pcmcia, and the lockdep warning
> comes when I unplug my wlan card from pcmcia slot.
> Looks like socket->skt_mutex is held in remove path, and it is also
> grabbed in .stor method.

Looking a little closer this is simultaneously a legitimate problem
and also a false positive.

This is only legitimate if you add/remove a cardbus bridge, plugged into
another cardbus bridge, which I think is unlikely but physically possible.

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