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pkt_sched: Make sure RTNL is held in qdisc_root_lock(). 

It is the only legal environment in which this can be
used.

Add some commentary explaining the situation.

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller 2008-08-02 23:27:37 -07:00
parent 35ed4e7598
commit 7e43f1128d
1 changed files with 12 additions and 0 deletions
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12
include/net/sch_generic.h
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@ -193,10 +193,22 @@ static inline struct Qdisc *qdisc_root(struct Qdisc *qdisc)
return qdisc->dev_queue->qdisc;
}
/* The qdisc root lock is a mechanism by which to top level
* of a qdisc tree can be locked from any qdisc node in the
* forest. This allows changing the configuration of some
* aspect of the qdisc tree while blocking out asynchronous
* qdisc access in the packet processing paths.
*
* It is only legal to do this when the root will not change
* on us. Otherwise we'll potentially lock the wrong qdisc
* root. This is enforced by holding the RTNL semaphore, which
* all users of this lock accessor must do.
*/
static inline spinlock_t *qdisc_root_lock(struct Qdisc *qdisc)
{
struct Qdisc *root = qdisc_root(qdisc);
ASSERT_RTNL();
return qdisc_lock(root);
}