Modify stack(9) stack_print() and stack_sbuf_print() routines to use new

linker interfaces for looking up function names and offsets from
instruction pointers.  Create two variants of each call: one that is
"DDB-safe" and avoids locking in the linker, and one that is safe for
use in live kernels, by virtue of observing locking, and in particular
safe when kernel modules are being loaded and unloaded simultaneous to
their use.  This will allow them to be used outside of debugging
contexts.

Modify two of three current stack(9) consumers to use the DDB-safe
interfaces, as they run in low-level debugging contexts, such as inside
lockmgr(9) and the kernel memory allocator.

Update man page.

1 files changed, 15 insertions, 2 deletions

diff --git a/share/man/man9/stack.9 b/share/man/man9/stack.9
index dabef1aa321b..17cf50946d79 100644
--- a/share/man/man9/stack.9
+++ b/share/man/man9/stack.9
@@ -54,6 +54,8 @@ In kernel configuration files:
 .Ft void
 .Fn stack_sbuf_print "struct sbuf sb*" "struct stack *st"
 .Ft void
+.Fn stack_sbuf_print_ddb "struct sbuf sb*" "struct stack *st"
+.Ft void
 .Fn stack_save "struct stack *st"
 .Sh DESCRIPTION
 The
@@ -96,7 +98,17 @@ as described in
 .Xr sbuf 9 .
 This function may sleep if an auto-extending
 .Dv struct sbuf
-is used.
+is used, and because the kernel linker (used to look up symbol names) uses
+.Xr sx 9
+locks.
+.Pp
+In locking-sensitive environments, such as
+.Xr DDB 4 ,
+the unsynchronized
+.Fn stack_sbuf_print_ddb
+variant may be invoked; this makes use of kernel linker data structures to
+look up symbol names without following locking protocols, so is appropriate
+for use in the debugger but not while the system is live.
 .Pp
 The utility functions
 .Nm stack_zero ,
@@ -107,7 +119,8 @@ may be used to manipulate stack data structures directly.
 .Sh SEE ALSO
 .Xr DDB 4 ,
 .Xr printf 9 ,
-.Xr sbuf 9
+.Xr sbuf 9 ,
+.Xr sx 9
 .Sh AUTHORS
 .An -nosplit
 The