src/gdb ChangeLog remote.c

[palves at sourceware dot org]

CVSROOT:	/cvs/src
Module name:	src
Changes by:	palves@sourceware.org	2013-05-29 11:57:48

Modified files:
	gdb            : ChangeLog remote.c 

Log message:
	[remote] Insert breakpoints in the right process.
	
	I noticed that gdb.multi/multi-arch.exp wasn't passing with
	extended-remote GDBserver with my pending multi-process+multi-arch
	series anymore on current mainline, while it used to pass before:
	
	(gdb) run
	Starting program: /home/pedro/gdb/mygit/build/gdb/testsuite/gdb.multi/ma-hangout
	Process /home/pedro/gdb/mygit/build/gdb/testsuite/gdb.multi/ma-hangout created; pid = 32067
	Warning:
	Cannot insert breakpoint 2.
	Error accessing memory address 0x4005c2: Unknown error -1.
	Cannot insert breakpoint -1.
	Temporarily disabling shared library breakpoints:
	breakpoint #-1
	
	(gdb) FAIL: gdb.multi/multi-arch.exp: starting inferior 2
	
	Investigating manually, I found an easy way to reproduce.  You just
	need breakpoints on distinct inferiors, and a way to have GDB install
	them in one go:
	
	(gdb) set breakpoint always-inserted on
	(gdb) info breakpoints
	Num     Type           Disp Enb Address            What
	2       breakpoint     del  n   <MULTIPLE>
	2.1                         y     0x00000000004005c2 in main at ../../../src/gdb/testsuite/gdb.multi/hello.c:40 inf 1
	2.2                         y     0x08048475         in main at ../../../src/gdb/testsuite/gdb.multi/hangout.c:22 inf 2
	(gdb) enable 2
	Warning:
	Cannot insert breakpoint 2.
	Error accessing memory address 0x4005c2: Unknown error -1.
	
	And turning on remote debugging, we see:
	
	(gdb) set debug remote 1
	(gdb) disable 2
	(gdb) enable 2
	Sending packet: $Z0,4005c2,1#71...Packet received: E01
	Sending packet: $Z0,8048475,1#87...Packet received: OK
	Warning:
	Cannot insert breakpoint 2.
	Error accessing memory address 0x4005c2: Unknown error -1.
	
	Notice that each of those Z0 breakpoints should be set in different
	processes.  However, no Hg packet to select a process has been sent in
	between, so GDBserver tries to plant both on the same process that
	happens to be current.  The first Z0 then not so surprisingly fails.
	IOW, the blame is on GDB, for telling GDBserver to plant both
	breakpoints in the same process.
	
	remote.c has a lazy scheme where it keeps a local cache of the
	remote's selected general thread, and delays updating it on the remote
	side until necessary (memory/register reads/writes, etc.).  This is
	done to reduce RSP traffic.  The bug is that the Zx breakpoint
	insert/remove methods weren't committing the selected thread/process
	back to the remote side:
	
	Breakpoint 3, remote_insert_breakpoint (gdbarch=0x1383ae0, bp_tgt=0x140c2b0) at ../../src/gdb/remote.c:8148
	8148      if (remote_protocol_packets[PACKET_Z0].support != PACKET_DISABLE)
	(top-gdb) p inferior_ptid
	$3 = {pid = 3670, lwp = 0, tid = 3670}
	(top-gdb) p general_thread
	$4 = {pid = 3671, lwp = 0, tid = 3671}
	
	IOW, a call to set_general_process is missing.
	
	I did some auditing over remote.c, and added calls to all places I
	found missing it.
	
	This only used to work by chance before.  breakpoint.c switches to a
	thread of the target process before installing a breakpoint location.
	That calls switch_to_thread.  Before:
	
	2012-07-27  Yao Qi  <yao@codesourcery.com>
	
	* thread.c (switch_to_thread): Don't call registers_changed.
	
	that caused the register caches to all be flushed and refetched before
	installing the breakpoint location.  Given fetching registers commits
	the remote general thread (with Hg), masking out the latent bug.
	
	Tested on x86_64 Fedora 17 with GDBserver.
	
	gdb/
	2013-05-29  Pedro Alves  <palves@redhat.com>
	
	* remote.c (remote_insert_breakpoint, remote_remove_breakpoint)
	(remote_insert_watchpoint, remote_remove_watchpoint)
	(remote_insert_hw_breakpoint, remote_remove_hw_breakpoint)
	(remote_verify_memory, compare_sections_command)
	(remote_search_memory): Set the general process/thread on the
	remote side.

Patches:
http://sourceware.org/cgi-bin/cvsweb.cgi/src/gdb/ChangeLog.diff?cvsroot=src&r1=1.15629&r2=1.15630
http://sourceware.org/cgi-bin/cvsweb.cgi/src/gdb/remote.c.diff?cvsroot=src&r1=1.549&r2=1.550