[PATCH] make idr_remove_all() do removal -before- free_layer()

[Paul E. McKenney]

The following patch fixes a problem in the IDR system, where an
idr_remove_all() hands a data element to call_rcu() (via free_layer())
before making that data element inaccessible to new readers.  This is
very bad, and results in readers still having a reference to this data
element at the end of the grace period.  Tests on large machines that
concurrently map and unmap user-space memory within the same multithreaded
process result in crashes within about five minutes.  Applying this
patch increases the kernel's longevity to the three-to-eight-hour range.

There appear to be other similar problems in idr_get_empty_slot() and
sub_remove(), but I fixed the easy one in idr_remove_all() first.  It is
therefore no surprise that failures still occur.

(Yes, and I did look at the relevant patch last year without spotting
this one.  Goes to show the value of testing as well as code review,
I guess...)

Nadia, Manfred, any thoughts?

Located-by: Milton Miller II <miltonm@xxxxxxxxxxxxxx>
Tested-by: Milton Miller II <miltonm@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---

diff --git a/lib/idr.c b/lib/idr.c
index c11c576..dab4bca 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -449,6 +449,7 @@ void idr_remove_all(struct idr *idp)
 
 	n = idp->layers * IDR_BITS;
 	p = idp->top;
+	rcu_assign_pointer(idp->top, NULL);
 	max = 1 << n;
 
 	id = 0;
@@ -467,7 +468,6 @@ void idr_remove_all(struct idr *idp)
 			p = *--paa;
 		}
 	}
-	rcu_assign_pointer(idp->top, NULL);
 	idp->layers = 0;
 }
 EXPORT_SYMBOL(idr_remove_all);

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