Re: [PATCH v2 3/3] gdb/nat/linux: Fix attaching to process when it has zombie threads

[Pedro Alves <pedro@palves.net>]

On 2024-04-20 06:56, Thiago Jung Bauermann wrote:
> When GDB attaches to a multi-threaded process, it calls
> linux_proc_attach_tgid_threads () to go through all threads found in
> /proc/PID/task/ and call attach_proc_task_lwp_callback () on each of
> them.  If it does that twice without the callback reporting that a new
> thread was found, then it considers that all inferior threads have been
> found and returns.
> 
> The problem is that the callback considers any thread that it hasn't
> attached to yet as new.  This causes problems if the process has one or
> more zombie threads, because GDB can't attach to it and the loop will
> always "find" a new thread (the zombie one), and get stuck in an
> infinite loop.
> 
> This is easy to trigger (at least on aarch64-linux and powerpc64le-linux)
> with the gdb.threads/attach-many-short-lived-threads.exp testcase, because
> its test program constantly creates and finishes joinable threads so the
> chance of having zombie threads is high.
> 
> This problem causes the following failures:
> 
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: attach (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: no new threads (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: set breakpoint always-inserted on (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break break_fn (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break at break_fn: 1 (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break at break_fn: 2 (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: break at break_fn: 3 (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: reset timer in the inferior (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: print seconds_left (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: detach (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: set breakpoint always-inserted off (timeout)
> FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 8: delete all breakpoints, watchpoints, tracepoints, and catchpoints in delete_breakpoints (timeout)
> ERROR: breakpoints not deleted
> 
> The iteration number is random, and all tests in the subsequent iterations
> fail too, because GDB is stuck in the attach command at the beginning of
> the iteration.
> 
> The solution is to make linux_proc_attach_tgid_threads () remember when it
> has already processed a given LWP and skip it in the subsequent iterations.
> 
> PR testsuite/31312
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31312
> 
> Reviewed-By: Luis Machado <luis.machado@arm.com>

Approved-By: Pedro Alves <pedro@palves.net>

BTW, after seeing the other patches after patch #1, I do agree with giving names
to the stat fields.