[src/trunk]: src/sys/kern Revert "entropy(9): Nix rnd_trylock_sources."

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/ee7502a01d4c
branches:  trunk
changeset: 364390:ee7502a01d4c
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Mar 20 14:05:41 2022 +0000

description:
Revert "entropy(9): Nix rnd_trylock_sources."

Not a premature optimization after all -- this is necessary because
entropy_request can run in softint context, where the cv_wait_sig in
rnd_lock_sources is forbidden.  Need to do this another way.

diffstat:

 sys/kern/kern_entropy.c |  37 +++++++++++++++++++++++++++++--------
 1 files changed, 29 insertions(+), 8 deletions(-)

diffs (68 lines):

diff -r 44725368cc64 -r ee7502a01d4c sys/kern/kern_entropy.c
--- a/sys/kern/kern_entropy.c   Sun Mar 20 13:44:18 2022 +0000
+++ b/sys/kern/kern_entropy.c   Sun Mar 20 14:05:41 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_entropy.c,v 1.47 2022/03/20 13:44:18 riastradh Exp $      */
+/*     $NetBSD: kern_entropy.c,v 1.48 2022/03/20 14:05:41 riastradh Exp $      */
 
 /*-
  * Copyright (c) 2019 The NetBSD Foundation, Inc.
@@ -75,7 +75,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_entropy.c,v 1.47 2022/03/20 13:44:18 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_entropy.c,v 1.48 2022/03/20 14:05:41 riastradh Exp $");
 
 #include <sys/param.h>
 #include <sys/types.h>
@@ -1702,10 +1702,30 @@
 }
 
 /*
+ * rnd_trylock_sources()
+ *
+ *     Try to lock the list of sources, but if it's already locked,
+ *     fail.  Caller must hold the global entropy lock.  If
+ *     successful, no rndsource will go away until rnd_unlock_sources
+ *     even while the caller releases the global entropy lock.
+ */
+static bool
+rnd_trylock_sources(void)
+{
+
+       KASSERT(E->stage == ENTROPY_COLD || mutex_owned(&E->lock));
+
+       if (E->sourcelock)
+               return false;
+       E->sourcelock = curlwp;
+       return true;
+}
+
+/*
  * rnd_unlock_sources()
  *
- *     Unlock the list of sources after rnd_lock_sources.  Caller must
- *     hold the global entropy lock.
+ *     Unlock the list of sources after rnd_lock_sources or
+ *     rnd_trylock_sources.  Caller must hold the global entropy lock.
  */
 static void
 rnd_unlock_sources(void)
@@ -1748,11 +1768,12 @@
        KASSERT(E->stage == ENTROPY_COLD || mutex_owned(&E->lock));
 
        /*
-        * Lock the list of entropy sources to block rnd_detach_source
-        * until we're done, and to serialize calls to the entropy
-        * callbacks as guaranteed to drivers.
+        * If there is a request in progress, let it proceed.
+        * Otherwise, note that a request is in progress to avoid
+        * reentry and to block rnd_detach_source until we're done.
         */
-       rnd_lock_sources();
+       if (!rnd_trylock_sources())
+               return;
        entropy_request_evcnt.ev_count++;
 
        /* Clamp to the maximum reasonable request.  */