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IB/qib: Fix QP locate/remove race 

remove_qp() can execute concurrently with a qib_lookup_qpn() on
another CPU, which in of itself, is ok, given the RCU locking.

The issue is that remove_qp() NULLs out the qp->next field so that a
qib_lookup_qpn() might fail to find a qp if it occurs after the one
that is being deleted.  This is a momentary issue and subsequent
qib_lookup_qpn() calls would find the qp's since the search restarts
from the bucket head.  At scale, the issue might causes dropped
packets and unnecessary retransmissions.

The fix just deletes the qp->next NULL assignment to prevent the
remove_qp() from hiding qp's from qib_lookup_qpn().

Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
This commit is contained in:
Mike Marciniszyn 2013-02-07 20:47:51 +00:00 committed by Roland Dreier
parent 836dc9e3fb
commit bcc9b67a5b
1 changed files with 3 additions and 2 deletions
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5
drivers/infiniband/hw/qib/qib_qp.c
View File

@ -268,8 +268,9 @@ static void remove_qp(struct qib_ibdev *dev, struct qib_qp *qp)
qpp = &q->next)
if (q == qp) {
atomic_dec(&qp->refcount);
*qpp = qp->next;
rcu_assign_pointer(qp->next, NULL);
rcu_assign_pointer(*qpp,
rcu_dereference_protected(qp->next,
lockdep_is_held(&dev->qpt_lock)));
break;
}
}