Consistently enforce the restriction against calling malloc/free when in a

critical section.

uma_zalloc_arg()/uma_zalloc_free() may acquire a sleepable lock on the
zone. The malloc() family of functions may call uma_zalloc_arg() or
uma_zalloc_free().

The malloc(9) man page currently claims that free() will never sleep.
It also implies that the malloc() family of functions will not sleep
when called with M_NOWAIT. However, it is more correct to say that
these functions will not sleep indefinitely. Indeed, they may acquire
a sleepable lock. However, a developer may overlook this restriction
because the WITNESS check that catches attempts to call the malloc()
family of functions within a critical section is inconsistenly
applied.

This change clarifies the language of the malloc(9) man page to clarify
the restriction against calling the malloc() family of functions
while in a critical section or holding a spin lock. It also adds
KASSERTs at appropriate points to make the enforcement of this
restriction more consistent.

PR:		204633
Differential Revision:	https://reviews.freebsd.org/D4197
Reviewed by:	markj
Approved by:	gnn (mentor)
Sponsored by:	Juniper Networks

1 files changed, 7 insertions, 0 deletions

diff --git a/sys/vm/uma_core.c b/sys/vm/uma_core.c
index 8169256..1f57dff 100644
--- a/sys/vm/uma_core.c
+++ b/sys/vm/uma_core.c
@@ -2149,6 +2149,10 @@ uma_zalloc_arg(uma_zone_t zone, void *udata, int flags)
 		WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
 		    "uma_zalloc_arg: zone \"%s\"", zone->uz_name);
 	}
+
+	KASSERT(curthread->td_critnest == 0,
+	    ("uma_zalloc_arg: called with spinlock or critical section held"));
+
 #ifdef DEBUG_MEMGUARD
 	if (memguard_cmp_zone(zone)) {
 		item = memguard_alloc(zone->uz_size, flags);
@@ -2686,6 +2690,9 @@ uma_zfree_arg(uma_zone_t zone, void *item, void *udata)
 	CTR2(KTR_UMA, "uma_zfree_arg thread %x zone %s", curthread,
 	    zone->uz_name);
 
+	KASSERT(curthread->td_critnest == 0,
+	    ("uma_zfree_arg: called with spinlock or critical section held"));
+
         /* uma_zfree(..., NULL) does nothing, to match free(9). */
         if (item == NULL)
                 return;