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binutils-gdb/gdb/infrun.h
Andrew Burgess 3df7843699 gdb: fix b/p conditions with infcalls in multi-threaded inferiors 
This commit fixes bug PR 28942, that is, creating a conditional
breakpoint in a multi-threaded inferior, where the breakpoint
condition includes an inferior function call.

Currently, when a user tries to create such a breakpoint, then GDB
will fail with:

  (gdb) break infcall-from-bp-cond-single.c:61 if (return_true ())
  Breakpoint 2 at 0x4011fa: file /tmp/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.threads/infcall-from-bp-cond-single.c, line 61.
  (gdb) continue
  Continuing.
  [New Thread 0x7ffff7c5d700 (LWP 2460150)]
  [New Thread 0x7ffff745c700 (LWP 2460151)]
  [New Thread 0x7ffff6c5b700 (LWP 2460152)]
  [New Thread 0x7ffff645a700 (LWP 2460153)]
  [New Thread 0x7ffff5c59700 (LWP 2460154)]
  Error in testing breakpoint condition:
  Couldn't get registers: No such process.
  An error occurred while in a function called from GDB.
  Evaluation of the expression containing the function
  (return_true) will be abandoned.
  When the function is done executing, GDB will silently stop.
  Selected thread is running.
  (gdb)

Or, in some cases, like this:

  (gdb) break infcall-from-bp-cond-simple.c:56 if (is_matching_tid (arg, 1))
  Breakpoint 2 at 0x401194: file /tmp/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.threads/infcall-from-bp-cond-simple.c, line 56.
  (gdb) continue
  Continuing.
  [New Thread 0x7ffff7c5d700 (LWP 2461106)]
  [New Thread 0x7ffff745c700 (LWP 2461107)]
  ../../src.release/gdb/nat/x86-linux-dregs.c:146: internal-error: x86_linux_update_debug_registers: Assertion `lwp_is_stopped (lwp)' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.

The precise error depends on the exact thread state; so there's race
conditions depending on which threads have fully started, and which
have not.  But the underlying problem is always the same; when GDB
tries to execute the inferior function call from within the breakpoint
condition, GDB will, incorrectly, try to resume threads that are
already running - GDB doesn't realise that some threads might already
be running.

The solution proposed in this patch requires an additional member
variable thread_info::in_cond_eval.  This flag is set to true (in
breakpoint.c) when GDB is evaluating a breakpoint condition.

In user_visible_resume_ptid (infrun.c), when the in_cond_eval flag is
true, then GDB will only try to resume the current thread, that is,
the thread for which the breakpoint condition is being evaluated.
This solves the problem of GDB trying to resume threads that are
already running.

The next problem is that inferior function calls are assumed to be
synchronous, that is, GDB doesn't expect to start an inferior function
call in thread #1, then receive a stop from thread #2 for some other,
unrelated reason.  To prevent GDB responding to an event from another
thread, we update fetch_inferior_event and do_target_wait in infrun.c,
so that, when an inferior function call (on behalf of a breakpoint
condition) is in progress, we only wait for events from the current
thread (the one evaluating the condition).

In do_target_wait I had to change the inferior_matches lambda
function, which is used to select which inferior to wait on.
Previously the logic was this:

   auto inferior_matches = [&wait_ptid] (inferior *inf)
     {
       return (inf->process_target () != nullptr
               && ptid_t (inf->pid).matches (wait_ptid));
     };

This compares the pid of the inferior against the complete ptid we
want to wait on.  Before this commit wait_ptid was only ever
minus_one_ptid (which is special, and means any process), and so every
inferior would match.

After this commit though wait_ptid might represent a specific thread
in a specific inferior.  If we compare the pid of the inferior to a
specific ptid then these will not match.  The fix is to compare
against the pid extracted from the wait_ptid, not against the complete
wait_ptid itself.

In fetch_inferior_event, after receiving the event, we only want to
stop all the other threads, and call inferior_event_handler with
INF_EXEC_COMPLETE, if we are not evaluating a conditional breakpoint.
If we are, then all the other threads should be left doing whatever
they were before.  The inferior_event_handler call will be performed
once the breakpoint condition has finished being evaluated, and GDB
decides to stop or not.

The final problem that needs solving relates to GDB's commit-resume
mechanism, which allows GDB to collect resume requests into a single
packet in order to reduce traffic to a remote target.

The problem is that the commit-resume mechanism will not send any
resume requests for an inferior if there are already events pending on
the GDB side.

Imagine an inferior with two threads.  Both threads hit a breakpoint,
maybe the same conditional breakpoint.  At this point there are two
pending events, one for each thread.

GDB selects one of the events and spots that this is a conditional
breakpoint, GDB evaluates the condition.

The condition includes an inferior function call, so GDB sets up for
the call and resumes the one thread, the resume request is added to
the commit-resume queue.

When the commit-resume queue is committed GDB sees that there is a
pending event from another thread, and so doesn't send any resume
requests to the actual target, GDB is assuming that when we wait we
will select the event from the other thread.

However, as this is an inferior function call for a condition
evaluation, we will not select the event from the other thread, we
only care about events from the thread that is evaluating the
condition - and the resume for this thread was never sent to the
target.

And so, GDB hangs, waiting for an event from a thread that was never
fully resumed.

To fix this issue I have added the concept of "forcing" the
commit-resume queue.  When enabling commit resume, if the force flag
is true, then any resumes will be committed to the target, even if
there are other threads with pending events.

A note on authorship: this patch was based on some work done by
Natalia Saiapova and Tankut Baris Aktemur from Intel[1].  I have made
some changes to their work in this version.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28942

[1] https://sourceware.org/pipermail/gdb-patches/2020-October/172454.html

Co-authored-by: Natalia Saiapova <natalia.saiapova@intel.com>
Co-authored-by: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
Tested-By: Luis Machado <luis.machado@arm.com>
Tested-By: Keith Seitz <keiths@redhat.com>
2024-03-25 17:25:06 +00:00

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/* Copyright (C) 1986-2024 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef INFRUN_H
#define INFRUN_H 1
#include "gdbthread.h"
#include "symtab.h"
#include "gdbsupport/byte-vector.h"
#include "gdbsupport/intrusive_list.h"
struct target_waitstatus;
class frame_info_ptr;
struct address_space;
struct return_value_info;
struct process_stratum_target;
struct thread_info;
/* True if we are debugging run control. */
extern bool debug_infrun;
/* Print an "infrun" debug statement. */
#define infrun_debug_printf(fmt, ...) \
debug_prefixed_printf_cond (debug_infrun, "infrun", fmt, ##__VA_ARGS__)
/* Print "infrun" start/end debug statements. */
#define INFRUN_SCOPED_DEBUG_START_END(fmt, ...) \
scoped_debug_start_end (debug_infrun, "infrun", fmt, ##__VA_ARGS__)
/* Print "infrun" enter/exit debug statements. */
#define INFRUN_SCOPED_DEBUG_ENTER_EXIT \
scoped_debug_enter_exit (debug_infrun, "infrun")
/* A infrun debug helper routine to print out all the threads in the set
THREADS (which should be a range type that returns thread_info*
objects).
The TITLE is a string that is printed before the list of threads.
Output is only produced when 'set debug infrun on'. */
template<typename ThreadRange>
static inline void
infrun_debug_show_threads (const char *title, ThreadRange threads)
{
if (debug_infrun)
{
INFRUN_SCOPED_DEBUG_ENTER_EXIT;
infrun_debug_printf ("%s:", title);
for (thread_info *thread : threads)
infrun_debug_printf (" thread %s, executing = %d, resumed = %d, "
"state = %s",
thread->ptid.to_string ().c_str (),
thread->executing (),
thread->resumed (),
thread_state_string (thread->state));
}
}
/* Nonzero if we want to give control to the user when we're notified
of shared library events by the dynamic linker. */
extern int stop_on_solib_events;
/* True if execution commands resume all threads of all processes by
default; otherwise, resume only threads of the current inferior
process. */
extern bool sched_multi;
/* When set, stop the 'step' command if we enter a function which has
no line number information. The normal behavior is that we step
over such function. */
extern bool step_stop_if_no_debug;
/* If set, the inferior should be controlled in non-stop mode. In
this mode, each thread is controlled independently. Execution
commands apply only to the selected thread by default, and stop
events stop only the thread that had the event -- the other threads
are kept running freely. */
extern bool non_stop;
/* When set (default), the target should attempt to disable the
operating system's address space randomization feature when
starting an inferior. */
extern bool disable_randomization;
/* Returns a unique identifier for the current stop. This can be used
to tell whether a command has proceeded the inferior past the
current location. */
extern ULONGEST get_stop_id (void);
/* Reverse execution. */
enum exec_direction_kind
{
EXEC_FORWARD,
EXEC_REVERSE
};
/* The current execution direction. */
extern enum exec_direction_kind execution_direction;
/* Call this to point 'previous_thread' at the thread returned by
inferior_thread, or at nullptr, if there's no selected thread. */
extern void update_previous_thread ();
/* Get a weak reference to 'previous_thread'. */
extern thread_info *get_previous_thread ();
extern void start_remote (int from_tty);
/* Clear out all variables saying what to do when inferior is
continued or stepped. First do this, then set the ones you want,
then call `proceed'. STEP indicates whether we're preparing for a
step/stepi command. */
extern void clear_proceed_status (int step);
extern void proceed (CORE_ADDR, enum gdb_signal);
/* Return a ptid representing the set of threads that we will proceed,
in the perspective of the user/frontend. We may actually resume
fewer threads at first, e.g., if a thread is stopped at a
breakpoint that needs stepping-off, but that should not be visible
to the user/frontend, and neither should the frontend/user be
allowed to proceed any of the threads that happen to be stopped for
internal run control handling, if a previous command wanted them
resumed. */
extern ptid_t user_visible_resume_ptid (int step);
/* Return the process_stratum target that we will proceed, in the
perspective of the user/frontend. If RESUME_PTID is
MINUS_ONE_PTID, then we'll resume all threads of all targets, so
the function returns NULL. Otherwise, we'll be resuming a process
or thread of the current process, so we return the current
inferior's process stratum target. */
extern process_stratum_target *user_visible_resume_target (ptid_t resume_ptid);
/* Return control to GDB when the inferior stops for real. Print
appropriate messages, remove breakpoints, give terminal our modes,
and run the stop hook. Returns true if the stop hook proceeded the
target, false otherwise. */
extern bool normal_stop ();
/* Return the cached copy of the last target/ptid/waitstatus returned
by target_wait(). The data is actually cached by handle_inferior_event(),
which gets called immediately after target_wait(). */
extern void get_last_target_status (process_stratum_target **target,
ptid_t *ptid,
struct target_waitstatus *status);
/* Set the cached copy of the last target/ptid/waitstatus. */
extern void set_last_target_status (process_stratum_target *target, ptid_t ptid,
const target_waitstatus &status);
/* Clear the cached copy of the last ptid/waitstatus returned by
target_wait(). */
extern void nullify_last_target_wait_ptid ();
/* Stop all threads. Only returns after everything is halted.
REASON is a string indicating the reason why we stop all threads, used in
debug messages.
If INF is non-nullptr, stop all threads of that inferior. Otherwise, stop
all threads of all inferiors. */
extern void stop_all_threads (const char *reason, inferior *inf = nullptr);
extern void prepare_for_detach (void);
extern void fetch_inferior_event ();
extern void init_wait_for_inferior (void);
extern void insert_step_resume_breakpoint_at_sal (struct gdbarch *,
struct symtab_and_line ,
struct frame_id);
/* Returns true if we're trying to step past the instruction at
ADDRESS in ASPACE. */
extern int stepping_past_instruction_at (struct address_space *aspace,
CORE_ADDR address);
/* Returns true if thread whose thread number is THREAD is stepping
over a breakpoint. */
extern int thread_is_stepping_over_breakpoint (int thread);
/* Returns true if we're trying to step past an instruction that
triggers a non-steppable watchpoint. */
extern int stepping_past_nonsteppable_watchpoint (void);
/* Record in TP the frame and location we're currently stepping through. */
extern void set_step_info (thread_info *tp,
const frame_info_ptr &frame,
struct symtab_and_line sal);
/* Notify interpreters and observers that the current inferior has stopped with
signal SIG. */
extern void notify_signal_received (gdb_signal sig);
/* Notify interpreters and observers that the current inferior has stopped
normally. */
extern void notify_normal_stop (bpstat *bs, int print_frame);
/* Notify interpreters and observers that the user focus has changed. */
extern void notify_user_selected_context_changed (user_selected_what selection);
/* Several print_*_reason helper functions to print why the inferior
has stopped to the passed in UIOUT. */
/* Signal received, print why the inferior has stopped. */
extern void print_signal_received_reason (struct ui_out *uiout,
enum gdb_signal siggnal);
/* The inferior was terminated by a signal, print why it stopped. */
extern void print_signal_exited_reason (struct ui_out *uiout,
enum gdb_signal siggnal);
/* The inferior program is finished, print why it stopped. */
extern void print_exited_reason (struct ui_out *uiout, int exitstatus);
/* Reverse execution: target ran out of history info, print why the
inferior has stopped. */
extern void print_no_history_reason (struct ui_out *uiout);
/* Print the result of a function at the end of a 'finish' command.
RV points at an object representing the captured return value/type
and its position in the value history. */
extern void print_return_value (struct ui_out *uiout,
struct return_value_info *rv);
/* Print current location without a level number, if we have changed
functions or hit a breakpoint. Print source line if we have one.
If the execution command captured a return value, print it. If
DISPLAYS is false, do not call 'do_displays'. */
extern void print_stop_event (struct ui_out *uiout, bool displays = true);
/* Pretty print the results of target_wait, for debugging purposes. */
extern void print_target_wait_results (ptid_t waiton_ptid, ptid_t result_ptid,
const struct target_waitstatus &ws);
extern int signal_stop_state (int);
extern int signal_print_state (int);
extern int signal_pass_state (int);
extern int signal_stop_update (int, int);
extern int signal_print_update (int, int);
extern int signal_pass_update (int, int);
extern void update_signals_program_target (void);
/* Clear the convenience variables associated with the exit of the
inferior. Currently, those variables are $_exitcode and
$_exitsignal. */
extern void clear_exit_convenience_vars (void);
extern void update_observer_mode (void);
extern void signal_catch_update (const unsigned int *);
/* In some circumstances we allow a command to specify a numeric
signal. The idea is to keep these circumstances limited so that
users (and scripts) develop portable habits. For comparison,
POSIX.2 `kill' requires that 1,2,3,6,9,14, and 15 work (and using a
numeric signal at all is obsolescent. We are slightly more lenient
and allow 1-15 which should match host signal numbers on most
systems. Use of symbolic signal names is strongly encouraged. */
enum gdb_signal gdb_signal_from_command (int num);
/* Enables/disables infrun's async event source in the event loop. */
extern void infrun_async (int enable);
/* Call infrun's event handler the next time through the event
loop. */
extern void mark_infrun_async_event_handler (void);
/* The global chain of threads that need to do a step-over operation
to get past e.g., a breakpoint. */
extern thread_step_over_list global_thread_step_over_list;
/* Remove breakpoints if possible (usually that means, if everything
is stopped). On failure, print a message. */
extern void maybe_remove_breakpoints (void);
/* If a UI was in sync execution mode, and now isn't, restore its
prompt (a synchronous execution command has finished, and we're
ready for input). */
extern void all_uis_check_sync_execution_done (void);
/* If a UI was in sync execution mode, and hasn't displayed the prompt
yet, re-disable its prompt (a synchronous execution command was
started or re-started). */
extern void all_uis_on_sync_execution_starting (void);
/* In all-stop, restart the target if it had to be stopped to
detach. */
extern void restart_after_all_stop_detach (process_stratum_target *proc_target);
/* RAII object to temporarily disable the requirement for target
stacks to commit their resumed threads.
On construction, set process_stratum_target::commit_resumed_state
to false for all process_stratum targets in all target
stacks.
On destruction (or if reset_and_commit() is called), set
process_stratum_target::commit_resumed_state to true for all
process_stratum targets in all target stacks, except those that:
- have no resumed threads
- have a resumed thread with a pending status
target_commit_resumed is not called in the destructor, because its
implementations could throw, and we don't to swallow that error in
a destructor. Instead, the caller should call the
reset_and_commit_resumed() method so that an eventual exception can
propagate. "reset" in the method name refers to the fact that this
method has the same effect as the destructor, in addition to
committing resumes.
The creation of nested scoped_disable_commit_resumed objects is
tracked, such that only the outermost instance actually does
something, for cases like this:
void
inner_func ()
{
scoped_disable_commit_resumed disable;
// do stuff
disable.reset_and_commit ();
}
void
outer_func ()
{
scoped_disable_commit_resumed disable;
for (... each thread ...)
inner_func ();
disable.reset_and_commit ();
}
In this case, we don't want the `disable` destructor in
`inner_func` to require targets to commit resumed threads, so that
the `reset_and_commit()` call in `inner_func` doesn't actually
resume threads. */
struct scoped_disable_commit_resumed
{
explicit scoped_disable_commit_resumed (const char *reason);
~scoped_disable_commit_resumed ();
DISABLE_COPY_AND_ASSIGN (scoped_disable_commit_resumed);
/* Undoes the disabling done by the ctor, and calls
maybe_call_commit_resumed_all_targets(). */
void reset_and_commit ();
private:
/* Undoes the disabling done by the ctor. */
void reset ();
/* Whether this object has been reset. */
bool m_reset = false;
const char *m_reason;
bool m_prev_enable_commit_resumed;
};
/* Call target_commit_resumed method on all target stacks whose
process_stratum target layer has COMMIT_RESUME_STATE set. */
extern void maybe_call_commit_resumed_all_targets ();
/* RAII object to temporarily enable the requirement for target stacks
to commit their resumed threads. This is the inverse of
scoped_disable_commit_resumed. The constructor calls the
maybe_call_commit_resumed_all_targets function itself, since it's
OK to throw from a constructor. */
struct scoped_enable_commit_resumed
{
explicit scoped_enable_commit_resumed (const char *reason,
bool force_p = false);
~scoped_enable_commit_resumed ();
DISABLE_COPY_AND_ASSIGN (scoped_enable_commit_resumed);
private:
const char *m_reason;
bool m_prev_enable_commit_resumed;
};
#endif /* INFRUN_H */
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