[patch 1/4] Fix hw watchpoints #2: [x86*] repeated rwatch output

[Jan Kratochvil <jan dot kratochvil at redhat dot com>]

Hi,

while this patch has been discussed as approved by Joel Brobecker its check-in
would cause a regression without the patch 3/4 on:
	gdb.base/watch_thread_num.exp
	gdb.threads/watchthreads.exp

------------------------------------------------------------------------------

Intel Developer's Manual Volume 3B: System Programming Guide:
	"debug handlers should clear the [DR6] register before returning to
	the interrupted task"

This currently does not happen for x86* targets on the watchpoint trigger so:
* "watch" (read/write): the referenced value is read on each SIGTRAP to
  silently find out it did not change.
* "rwatch" (read access): The hit is falsely duplicated on each SIGTRAP
  unrelated of its reason.

This problem does not affect all the targets - ppc gets the trigger reported
through `struct siginfo' from linux_nat_get_siginfo which needs no reset.

On x86* there could also be some warnings/errors if the DR_STATUS bit triggers
with no reason but the bits are no silently cleared.


Thanks,
Jan


gdb/
2009-11-16  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Fix repeated rwatch output.
	* amd64-linux-nat.c (amd64_linux_dr_set, amd64_linux_dr_set_control)
	(amd64_linux_dr_set_addr, amd64_linux_dr_reset_addr)
	(amd64_linux_dr_get_status): New comments.
	(amd64_linux_dr_unset_status): New function.
	(_initialize_amd64_linux_nat): Install it.
	* i386-linux-nat.c (i386_linux_dr_get, i386_linux_dr_set)
	(i386_linux_dr_set_control, i386_linux_dr_set_addr)
	(i386_linux_dr_reset_addr, i386_linux_dr_get_status): New comments.
	(i386_linux_dr_unset_status): New function.
	(_initialize_i386_linux_nat): Install it.
	* i386-nat.c (I386_DR_WATCH_MASK): New macro.
	(I386_DR_WATCH_HIT): Use I386_DR_WATCH_MASK.
	(i386_insert_aligned_watchpoint): Call i386_dr_low.unset_status.
	* i386-nat.h (struct i386_dr_low_type): Extend comments for
	set_control, set_addr, reset_addr and get_status.  New unset_status.
    	* breakpoint.c (update_watchpoint): Extend the comment.

gdb/testsuite/
2009-11-16  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/watchpoint-hw-hit-once.exp,
	gdb.base/watchpoint-hw-hit-once.c: New.

--- a/gdb/amd64-linux-nat.c
+++ b/gdb/amd64-linux-nat.c
@@ -270,6 +270,8 @@ amd64_linux_dr_get (ptid_t ptid, int regnum)
   return value;
 }
 
+/* Set debug register REGNUM to VALUE in only the one LWP of PTID.  */
+
 static void
 amd64_linux_dr_set (ptid_t ptid, int regnum, unsigned long value)
 {
@@ -286,6 +288,8 @@ amd64_linux_dr_set (ptid_t ptid, int regnum, unsigned long value)
     perror_with_name (_("Couldn't write debug register"));
 }
 
+/* Set DR_CONTROL to ADDR in all LWPs of LWP_LIST.  */
+
 static void
 amd64_linux_dr_set_control (unsigned long control)
 {
@@ -297,6 +301,8 @@ amd64_linux_dr_set_control (unsigned long control)
     amd64_linux_dr_set (ptid, DR_CONTROL, control);
 }
 
+/* Set address REGNUM (zero based) to ADDR in all LWPs of LWP_LIST.  */
+
 static void
 amd64_linux_dr_set_addr (int regnum, CORE_ADDR addr)
 {
@@ -310,18 +316,41 @@ amd64_linux_dr_set_addr (int regnum, CORE_ADDR addr)
     amd64_linux_dr_set (ptid, DR_FIRSTADDR + regnum, addr);
 }
 
+/* Set address REGNUM (zero based) to zero in all LWPs of LWP_LIST.  */
+
 static void
 amd64_linux_dr_reset_addr (int regnum)
 {
   amd64_linux_dr_set_addr (regnum, 0);
 }
 
+/* Get DR_STATUS from only the one LWP of INFERIOR_PTID.  */
+
 static unsigned long
 amd64_linux_dr_get_status (void)
 {
   return amd64_linux_dr_get (inferior_ptid, DR_STATUS);
 }
 
+/* Unset MASK bits in DR_STATUS in all LWPs of LWP_LIST.  */
+
+static void
+amd64_linux_dr_unset_status (unsigned long mask)
+{
+  struct lwp_info *lp;
+  ptid_t ptid;
+
+  ALL_LWPS (lp, ptid)
+    {
+      unsigned long value;
+      
+      value = amd64_linux_dr_get (ptid, DR_STATUS);
+      value &= ~mask;
+      amd64_linux_dr_set (ptid, DR_STATUS, value);
+    }
+}
+
+
 static void
 amd64_linux_new_thread (ptid_t ptid)
 {
@@ -672,6 +701,7 @@ _initialize_amd64_linux_nat (void)
   i386_dr_low.set_addr = amd64_linux_dr_set_addr;
   i386_dr_low.reset_addr = amd64_linux_dr_reset_addr;
   i386_dr_low.get_status = amd64_linux_dr_get_status;
+  i386_dr_low.unset_status = amd64_linux_dr_unset_status;
   i386_set_debug_register_length (8);
 
   /* Override the GNU/Linux inferior startup hook.  */
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -993,7 +993,46 @@ fetch_watchpoint_value (struct expression *exp, struct value **valp,
    - Update the list of values that must be watched in B->loc.
 
    If the watchpoint disposition is disp_del_at_next_stop, then do nothing.
-   If this is local watchpoint that is out of scope, delete it.  */
+   If this is local watchpoint that is out of scope, delete it.
+
+   Even with `set breakpoint always-inserted on' the watchpoints are removed
+   + inserted on each stop here.  Normal breakpoints must never be removed
+   because they might be missed by a running thread when debugging in non-stop
+   mode.  On the other hand, hardware watchpoints (is_hardware_watchpoint;
+   processed here) are specific to each LWP since they are stored in each LWP's
+   hardware debug registers.  Therefore, such LWP must be stopped first in
+   order to be able to modify its hardware watchpoints.
+
+   Hardware watchpoints must be reset exactly once after being presented to the
+   user.  It cannot be done sooner, because it would reset the data used to
+   present the watchpoint hit to the user.  And it must not be done later
+   because it could display the same single watchpoint hit during multiple GDB
+   stops.  Note that the latter is relevant only to the hardware watchpoint
+   types bp_read_watchpoint and bp_access_watchpoint.  False hit by
+   bp_hardware_watchpoint is not user-visible - its hit is suppressed if the
+   memory content has not changed.
+
+   The following constraints influence the location where we can reset hardware
+   watchpoints:
+
+   * target_stopped_by_watchpoint and target_stopped_data_address are called
+     several times when GDB stops.
+
+   [linux]
+   * Multiple hardware watchpoints can be hit at the same time, causing GDB to
+     stop.  GDB only presents one hardware watchpoint hit at a time as the
+     reason for stopping, and all the other hits are presented later, one after
+     the other, each time the user requests the execution to be resumed.
+     Execution is not resumed for the threads still having pending hit event
+     stored in LWP_INFO->STATUS.  While the watchpoint is already removed from
+     the inferior on the first stop the thread hit event is kept being reported
+     from its cached value by linux_nat_stopped_data_address until the real
+     thread resume happens after the watchpoint gets presented and thus its
+     LWP_INFO->STATUS gets reset.
+
+   Therefore the hardware watchpoint hit can get safely reset on the watchpoint
+   removal from inferior.  */
+
 static void
 update_watchpoint (struct breakpoint *b, int reparse)
 {
--- a/gdb/i386-linux-nat.c
+++ b/gdb/i386-linux-nat.c
@@ -586,6 +586,8 @@ i386_linux_store_inferior_registers (struct target_ops *ops,
 
 static unsigned long i386_linux_dr[DR_CONTROL + 1];
 
+/* Get debug register REGNUM value from only the one LWP of PTID.  */
+
 static unsigned long
 i386_linux_dr_get (ptid_t ptid, int regnum)
 {
@@ -614,6 +616,8 @@ i386_linux_dr_get (ptid_t ptid, int regnum)
   return value;
 }
 
+/* Set debug register REGNUM to VALUE in only the one LWP of PTID.  */
+
 static void
 i386_linux_dr_set (ptid_t ptid, int regnum, unsigned long value)
 {
@@ -630,6 +634,8 @@ i386_linux_dr_set (ptid_t ptid, int regnum, unsigned long value)
     perror_with_name (_("Couldn't write debug register"));
 }
 
+/* Set DR_CONTROL to ADDR in all LWPs of LWP_LIST.  */
+
 static void
 i386_linux_dr_set_control (unsigned long control)
 {
@@ -641,6 +647,8 @@ i386_linux_dr_set_control (unsigned long control)
     i386_linux_dr_set (ptid, DR_CONTROL, control);
 }
 
+/* Set address REGNUM (zero based) to ADDR in all LWPs of LWP_LIST.  */
+
 static void
 i386_linux_dr_set_addr (int regnum, CORE_ADDR addr)
 {
@@ -654,18 +662,40 @@ i386_linux_dr_set_addr (int regnum, CORE_ADDR addr)
     i386_linux_dr_set (ptid, DR_FIRSTADDR + regnum, addr);
 }
 
+/* Set address REGNUM (zero based) to zero in all LWPs of LWP_LIST.  */
+
 static void
 i386_linux_dr_reset_addr (int regnum)
 {
   i386_linux_dr_set_addr (regnum, 0);
 }
 
+/* Get DR_STATUS from only the one LWP of INFERIOR_PTID.  */
+
 static unsigned long
 i386_linux_dr_get_status (void)
 {
   return i386_linux_dr_get (inferior_ptid, DR_STATUS);
 }
 
+/* Unset MASK bits in DR_STATUS in all LWPs of LWP_LIST.  */
+
+static void
+i386_linux_dr_unset_status (unsigned long mask)
+{
+  struct lwp_info *lp;
+  ptid_t ptid;
+
+  ALL_LWPS (lp, ptid)
+    {
+      unsigned long value;
+      
+      value = i386_linux_dr_get (ptid, DR_STATUS);
+      value &= ~mask;
+      i386_linux_dr_set (ptid, DR_STATUS, value);
+    }
+}
+
 static void
 i386_linux_new_thread (ptid_t ptid)
 {
@@ -837,6 +867,7 @@ _initialize_i386_linux_nat (void)
   i386_dr_low.set_addr = i386_linux_dr_set_addr;
   i386_dr_low.reset_addr = i386_linux_dr_reset_addr;
   i386_dr_low.get_status = i386_linux_dr_get_status;
+  i386_dr_low.unset_status = i386_linux_dr_unset_status;
   i386_set_debug_register_length (4);
 
   /* Override the default ptrace resume method.  */
--- a/gdb/i386-nat.c
+++ b/gdb/i386-nat.c
@@ -137,8 +137,11 @@ struct i386_dr_low_type i386_dr_low;
 #define I386_DR_GET_RW_LEN(i) \
   ((dr_control_mirror >> (DR_CONTROL_SHIFT + DR_CONTROL_SIZE * (i))) & 0x0f)
 
+/* Mask that this I'th watchpoint has triggered.  */
+#define I386_DR_WATCH_MASK(i)	(1 << (i))
+
 /* Did the watchpoint whose address is in the I'th register break?  */
-#define I386_DR_WATCH_HIT(i)	(dr_status_mirror & (1 << (i)))
+#define I386_DR_WATCH_HIT(i)	(dr_status_mirror & I386_DR_WATCH_MASK (i))
 
 /* A macro to loop over all debug registers.  */
 #define ALL_DEBUG_REGISTERS(i)	for (i = 0; i < DR_NADDR; i++)
@@ -358,6 +361,10 @@ i386_insert_aligned_watchpoint (CORE_ADDR addr, unsigned len_rw_bits)
   i386_dr_low.set_addr (i, addr);
   i386_dr_low.set_control (dr_control_mirror);
 
+  /* Only a sanity check for leftover bits (set possibly only by inferior).  */
+  if (i386_dr_low.unset_status)
+    i386_dr_low.unset_status (I386_DR_WATCH_MASK (i));
+
   return 0;
 }
 
--- a/gdb/i386-nat.h
+++ b/gdb/i386-nat.h
@@ -49,16 +49,19 @@ extern void i386_use_watchpoints (struct target_ops *);
    functions are:
 
       set_control              -- set the debug control (DR7)
-				  register to a given value
+				  register to a given value for all LWPs
 
       set_addr                 -- put an address into one debug
-				  register
+				  register for all LWPs
 
       reset_addr               -- reset the address stored in
-				  one debug register
+				  one debug register for all LWPs
 
       get_status               -- return the value of the debug
-				  status (DR6) register.
+				  status (DR6) register for current LWP
+
+      unset_status             -- unset the specified bits of the debug
+				  status (DR6) register for all LWPs
 
    Additionally, the native file should set the debug_register_length
    field to 4 or 8 depending on the number of bytes used for
@@ -70,6 +73,7 @@ struct i386_dr_low_type
     void (*set_addr) (int, CORE_ADDR);
     void (*reset_addr) (int);
     unsigned long (*get_status) (void);
+    void (*unset_status) (unsigned long);
     int debug_register_length;
   };
 
--- /dev/null
+++ b/gdb/testsuite/gdb.base/watchpoint-hw-hit-once.c
@@ -0,0 +1,34 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2009 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+static int watchee;
+
+int
+main (void)
+{
+  volatile int dummy;
+
+  /* Stub lines are present as no breakpoints/watchpoint gets hit if current PC
+     already stays on the line PC while entering "step"/"continue".  */
+
+  dummy = 0;	/* Stub to catch WATCHEE access after runto_main.  */
+  dummy = watchee;
+  dummy = 1;	/* Stub to catch break-at-exit after WATCHEE has been hit.  */
+  dummy = 2;	/* break-at-exit */
+
+  return 0;
+}
--- /dev/null
+++ b/gdb/testsuite/gdb.base/watchpoint-hw-hit-once.exp
@@ -0,0 +1,43 @@
+# Copyright 2009 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Arch not supporting hw watchpoints does not imply no_hardware_watchpoints set.
+if {(![istarget "i?86-*-*"] && ![istarget "x86_64-*-*"]
+     && ![istarget "ia64-*-*"] && ![istarget "s390*-*-*"])
+    || [target_info exists gdb,no_hardware_watchpoints]} then {
+    verbose "Skipping watchpoint-hw-hit-once test."
+    return
+}
+
+set test watchpoint-hw-hit-once
+set srcfile ${test}.c
+if { [prepare_for_testing ${test}.exp ${test} ${srcfile}] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_test "rwatch watchee"
+
+gdb_breakpoint [gdb_get_line_number "break-at-exit"]
+
+gdb_test "continue" "Continuing.\r\nHardware read watchpoint \[0-9\]+: watchee\r\n\r\nValue = 0\r\n.*"
+
+# Here should be no repeated notification of the read watchpoint.
+gdb_test "continue" \
+	 "Continuing\\.\[ \r\n\]+Breakpoint \[0-9\]+, .*break-at-exit.*" \
+	 "continue to break-at-exit"