Oleg Nesterov
2010-10-12 18:55:48 UTC
I am trying to understand how ugdb can implement software watchpoints.
I am looking at what gdb does, and I am a bit confused.
Trivial test-case:
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
struct {
long v;
char pad[256];
} VAR;
void *tfunc(void *arg)
{
for (;;)
;
}
int main(void)
{
pthread_t thr;
printf("pid: %d\n", getpid());
pthread_create(&thr, NULL, tfunc, NULL);
for (;;)
VAR.v++;
return 0;
}
The sub-thread T does nothing but spins in the endless loop, the main
M thread changes VAR.
But, according to gdb, they both change VAR.
(gdb) attach PID
...
(gdb) watch VAR
(gdb) c -a
Continuing.
Watchpoint 1: VAR
Old value = {v = 394344995, pad = '\000' <repeats 255 times>}
New value = {v = 394344996, pad = '\000' <repeats 255 times>}
0x0000000000400634 in tfunc (arg=0x0) at /tmp/BWP.c:11
11 {
(gdb) Watchpoint 1: VAR
Old value = {v = 394344996, pad = '\000' <repeats 255 times>}
New value = {v = 394344997, pad = '\000' <repeats 255 times>}
0x0000000000400683 in main () at /tmp/BWP.c:26
26 VAR.v++;
gdb resumes (steps) both threads. If T reports %Stop while M changes
the memory, gdb notices the change and updates its copy of VAR. Then
it reports that VAR was changed to the user.
After that it inspects the stopped M and reads VAR again. Since the
copy was already updated it concludes it wasn't changed. It resumes
M again and only then notices another change.
Not that I really blame gdb, without hardware support it is not
possible to implement this 100% correctly. But I assume this is
not what we want?
IOW, I think that ugdb should do the following. If any thread changes
VAR, then all threads should stop and report T05watch to gdb.
Correct?
Another question. watch/Z2 is always per-process, there is no
"thread-local" watches, right?
And the last one. If gdb sends '$Z2' to gdbserver, the running
threads do not participate in monitoring, until gdb stops the
thread and resumes it again, correct?
Oleg.
I am looking at what gdb does, and I am a bit confused.
Trivial test-case:
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
struct {
long v;
char pad[256];
} VAR;
void *tfunc(void *arg)
{
for (;;)
;
}
int main(void)
{
pthread_t thr;
printf("pid: %d\n", getpid());
pthread_create(&thr, NULL, tfunc, NULL);
for (;;)
VAR.v++;
return 0;
}
The sub-thread T does nothing but spins in the endless loop, the main
M thread changes VAR.
But, according to gdb, they both change VAR.
(gdb) attach PID
...
(gdb) watch VAR
(gdb) c -a
Continuing.
Watchpoint 1: VAR
Old value = {v = 394344995, pad = '\000' <repeats 255 times>}
New value = {v = 394344996, pad = '\000' <repeats 255 times>}
0x0000000000400634 in tfunc (arg=0x0) at /tmp/BWP.c:11
11 {
(gdb) Watchpoint 1: VAR
Old value = {v = 394344996, pad = '\000' <repeats 255 times>}
New value = {v = 394344997, pad = '\000' <repeats 255 times>}
0x0000000000400683 in main () at /tmp/BWP.c:26
26 VAR.v++;
gdb resumes (steps) both threads. If T reports %Stop while M changes
the memory, gdb notices the change and updates its copy of VAR. Then
it reports that VAR was changed to the user.
After that it inspects the stopped M and reads VAR again. Since the
copy was already updated it concludes it wasn't changed. It resumes
M again and only then notices another change.
Not that I really blame gdb, without hardware support it is not
possible to implement this 100% correctly. But I assume this is
not what we want?
IOW, I think that ugdb should do the following. If any thread changes
VAR, then all threads should stop and report T05watch to gdb.
Correct?
Another question. watch/Z2 is always per-process, there is no
"thread-local" watches, right?
And the last one. If gdb sends '$Z2' to gdbserver, the running
threads do not participate in monitoring, until gdb stops the
thread and resumes it again, correct?
Oleg.