statedump: introduce file_table_address

statedump: introduce file_table_address

Currently the LTTng-modules statedump simply iterates over all processes
in the system and assumes all threads share the same file descriptor
table, which is only true if threads were created with clone
CLONE_FILES.

Directly invoking clone without the CLONE_FILES creates threads which
belong to the same process, but have their own file descriptor table.

Therefore, model-wise, we cannot assume that all threads in a process
have the same fd table content.

Add a new "file_table_address" field to the lttng_statedump_process_state
event, which dumps the address of the thread's struct files_struct
pointer. This pointer is guaranteed to never be re-used while we hold
the RCU read-side lock (so for the entire iteration over
processes/threads).

For the lttng_statedump_file_descriptor event, remove the "pid" field
(which is semantically inaccurate) and add a "file_table_address" field,
which contains the struct files_struct address of the file table
containing the file descriptor.

An optimization is performed to eliminate most duplcated file table
content by skipping file table dump if the same file table address is
encountered consecutively while iterating over a process' threads.

This introduces a semantic change to the statedump fields, and will
therefore be introduced in lttng-modules 2.12 onwards, not backported as
a fix.

Fixes: #1245
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>