Issue using GDB for non-stop multi-core debugging.

[Rejeesh S. Babu]

Hi All,

 

We are using GDB 7.5 (via MI protocol) to debug a remote multi-core target in non-stop mode. We use custom GDB Server and GDB thread packets are interpreted as hardware core for multi-core debugging.

 

Everything is working fine except that step over through a piece of code which is simultaneously accessed by both cores is failing.

 

The failing scenario can be explained as below:

 

1.       Core 0 is halted and core 1 is running before single stepping

2.       When user single step core 0, GDB puts a BP in next location and run the target

3.       Since core 1 is also executing this piece of code, core 1 takes this BP (GDB expect core 0 to take BP)

4.       Our custom GDB Server sends the stop notification packet of core 1

5.       Irrespective of the fact that notification packet of core 1 was send, GDB tries to read register/memory values of core0 (which is running)

6.       GDB Server respond with error packet and GDB stops functioning

 

Packets exchanged between GDB and GDB Server for single step :

 

Sending packet: $m80009000,4#5e...Ack

Packet received: 0800e003

Sending packet: $m80009000,4#5e...Ack

Packet received: 0800e003

Sending packet: $qTStatus#49...Ack

Packet received:

Sending packet: $m80009000,4#5e...Ack

Packet received: 0800e003

Sending packet: $m80009000,4#5e...Ack

Packet received: 0800e003

Sending packet: $Z0,8000b024,4#d6...Ack

Packet received: OK

Packet Z0 (software-breakpoint) is supported

Sending packet: $vCont;c:1#13...Ack //GDB run core 1

Packet received: OK

  Notification received: Stop:T05thread:02; //Core 2 is halted

stop notification captured

Sending packet: $vStopped#55...Ack

Packet received: OK

Sending packet: $Hg2#e1...Ack

Packet received: OK

Sending packet: $g#67...Ack

Packet received: 00000000008b0010e08f0080020000000800c7cf424cfb231a0000001a00000000000000000000001a000000000062cfb81962cfa412b97f00ca9a3b00004800020000006476bc800000bc80accdbb800000bc808722c73b4337c3336322cf1f00000000d0300577081fc7cf081fc7cf0000c7cf081fc7cf032387bd24b00080018b00107cf82198d64f000094c40e770000800024b00080

Sending packet: $Hg1#e0...Ack //Though Core 2 was halted, GDB is requesting for Core 1 details

Packet received: E 00

Sending packet: $g#67...Ack

Packet received: 00000000008b0010e08f0080020000000800c7cf424cfb231a0000001a00000000000000000000001a000000000062cfb81962cfa412b97f00ca9a3b00004800020000006476bc800000bc80accdbb800000bc808722c73b4337c3336322cf1f00000000d0300577081fc7cf081fc7cf0000c7cf081fc7cf032387bd24b00080018b00107cf82198d64f000094c40e770000800024b00080

Target is executing.

 

 

Is there a way to overcome this limitation? Is there any commands in GDB to handle such scenarios? Or is there any GDB patch? Or is it because something is missing in our custom GDB Server?

 

Any hint to help us progress is highly appreciated.

 

Thanking in anticipation,

Rejeesh

 

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