Disable interrupts while in kdb_trap() to handle cases where the caller

doesn't do it.  This fixes all known causes of "Context switches not
allowed in the debugger" in mi_switch().  The main cause was trap_fatal()
calling kdb_trap() with interrupts enabled.  Switching to ithreads for
interrupt handling then made fatal traps more fatal and harder to debug.
The problem was limited in -current because most interrupt handlers are
blocked by Giant, but it occurred almost deterministically for me because
my clock interrupt handlers are non-fast and not blocked by Giant.

1 files changed, 7 insertions, 0 deletions

diff --git a/sys/i386/i386/db_interface.c b/sys/i386/i386/db_interface.c
index f1ba48c28dae..e2b936bb1f0d 100644
--- a/sys/i386/i386/db_interface.c
+++ b/sys/i386/i386/db_interface.c
@@ -77,6 +77,7 @@ rss(void)
 int
 kdb_trap(int type, int code, struct i386_saved_state *regs)
 {
+	u_int ef;
 	volatile int ddb_mode = !(boothowto & RB_GDB);
 
 	/*
@@ -96,6 +97,9 @@ kdb_trap(int type, int code, struct i386_saved_state *regs)
 	    return (0);
 	}
 
+	ef = read_eflags();
+	disable_intr();
+
 	switch (type) {
 	    case T_BPTFLT:	/* breakpoint */
 	    case T_TRCTRAP:	/* debug exception */
@@ -216,6 +220,9 @@ kdb_trap(int type, int code, struct i386_saved_state *regs)
 	regs->tf_fs     = ddb_regs.tf_fs & 0xffff;
 	regs->tf_cs     = ddb_regs.tf_cs & 0xffff;
 	regs->tf_ds     = ddb_regs.tf_ds & 0xffff;
+
+	write_eflags(ef);
+
 	return (1);
 }