RFA: gdb linux nptl patch 1

[J. Johnston]

The following is the first of a set of proposed patches for gdb to support
the new nptl threading model for linux.

In the old linuxthreads model, the lwp was equal to the pid.  In the new
nptl model, the lwp is unique for each thread and the pid value is common
among sibling threads.  A side-effect of this difference is that with linuxthreads,
you could see individual threads externally on the ps list.  With the new
nptl kernel, you will not be able to see child thread lwps on the list.

Another key difference is how a tid is represented.  In the linuxthreads model,
a tid was an index into a table and was restricted to 16-bits.  In the new nptl
model, there is no such restriction on the number of threads and the tid is in
fact an address (i.e. not 16 bits).  In the current linux-proc.c logic, when
performing a gcore command, pr_status note information is created by taking the pid and
concatenating this to the tid to form a 32-bit construct.  Obviously, in the new
nptl model this won't work as the tid itself is either 32-bits or 64-bits.
It should also be mentioned that this logic does not match what the kernel
spits out in a corefile.  For linuxthreads, the kernel is just putting the
lwp = pid in the pr_status notes.

Logic that works for either model and matches what the kernel is doing is to
use the lwp and only the lwp in the pr_status notes.  This creates a slight
problem because linux-proc.c uses the thread_info list to run through all the
threads.  Unfortunately, the thread_info list contains ptid_t structs that
contain the pid and the tid, but no lwp.  In the old linuxthreads model, this
isn't a problem because the pid is in fact equal to the lwp.  In the nptl
model, we can't simply convert a pid/tid combination into its associated lwp
because this logic is hidden in libthread_db.  At present, the libthread_db
library is only loaded by the thread-db layer.  While there are routines to
convert a tid to lwp and vice-versa, these routines are not externally exposed
in the target vector.  The only externalized method that could do such a job
is to_pid_to_str() which converts to character.  Usage of this interface would
require parsing the output for the characters "LWP".

This patch proposes adding a secondary list of thread_info structs that keeps
track of the lwps.  This list is kept distinct from the regular thread list.
Since manipulating either list involves common operations, some base operations
are added for generic thread_info list manipulation.  The lin-lwp.c layer is
changed to add and delete lwp thread_info structs to the lwp list when lwps
are added or deleted.  The linux-proc.c code is changed to iterate through the
lwp thread_info list rather than the thread list when creating the pr_status
notes.

I had brought up this idea on the gdb mailing list and there was some discussion
about how the thread_info struct should be renamed to something more generic.
That issue was not resolved, but I feel that it can be performed distinct from this
patch as it will require a global change to many pieces of code.  To keep some
new names at a reasonable length, I chose to use the name tinfo_list rather than
thread_info_list.  This would obviously also be renamed.

Ok to commit?  I have reindented and checked in the three C files involved prior to
building this patch.

-- Jeff J.

2003-03-28  Jeff Johnston  <jjohnstn@redhat.com>

	* gdbthread.h (tinfo_list): New struct to represent list of
	thread_info structs.
	(get_thread_tinfo_list, get_lwp_tinfo_list): New prototypes.
	(add_thread_info,  delete_thread_info): Ditto.
	(thread_info_id_to_pid, pid_to_thread_info_id): Ditto.
	(in_tinfo_list, valid_thread_info_id, find_thread_info_pid): Ditto.
	(iterate_over_tinfo_list): Ditto.	
	* thread.c (init_thread_list): Clear lwp list as well as thread list.
	(get_thread_tinfo_list, get_lwp_tinfo_list): New functions
	to get either the thread or lwp lists respectively.
	(add_thread_info,  delete_thread_info): New generic thread_info functions.
	(thread_info_id_to_pid, pid_to_thread_info_id): Ditto.
	(in_tinfo_list, valid_thread_info_id, find_thread_info_pid): Ditto.
	(iterate_over_tinfo_list): Ditto.
	* linux-proc.c (linux_do_thread_registers): Store lwp for prstatus note
	rather than a merge of pid and tid.
	(linux_make_note_section): Iterate over list of lwps rather than threads
	to create note data.
	* lin-lwp.c (add_lwp): Call add_thread_info() to add new lwp ptid to lwp list.
	(delete_lwp): Call delete_thread_info() to delete lwp ptid from lwp list.


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