2020-04-14 02:42:59 +08:00
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/* Async events for the GDB event loop.
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2023-01-01 20:49:04 +08:00
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Copyright (C) 1999-2023 Free Software Foundation, Inc.
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2020-04-14 02:42:59 +08:00
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef ASYNC_EVENT_H
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#define ASYNC_EVENT_H
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2020-04-14 02:42:59 +08:00
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#include "gdbsupport/event-loop.h"
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2020-04-14 02:42:59 +08:00
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struct async_signal_handler;
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struct async_event_handler;
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typedef void (sig_handler_func) (gdb_client_data);
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gdb: make async event handlers clear themselves
The `ready` flag of async event handlers is cleared by the async event
handler system right before invoking the associated callback, in
check_async_event_handlers.
This is not ideal with how the infrun subsystem consumes events: all
targets' async event handler callbacks essentially just invoke
`inferior_event_handler`, which eventually calls `fetch_inferior_event`
and `do_target_wait`. `do_target_wait` picks an inferior at random,
and thus a target at random (it could be the target whose `ready` flag
was cleared, or not), and pulls one event from it.
So it's possible that:
- the async event handler for a target A is called
- we end up consuming an event for target B
- all threads of target B are stopped, target_async(0) is called on it,
so its async event handler is cleared (e.g.
record_btrace_target::async)
As a result, target A still has events to report while its async event
handler is left unmarked, so these events are not consumed. To counter
this, at the end of their async event handler callbacks, targets check
if they still have something to report and re-mark their async event
handler (e.g. remote_async_inferior_event_handler).
The linux_nat target does not suffer from this because it doesn't use an
async event handler at the moment. It only uses a pipe registered with
the event loop. It is written to in the SIGCHLD handler (and in other
spots that want to get target wait method called) and read from in
the target's wait method. So if linux_nat happened to be target A in
the example above, the pipe would just stay readable, and the event loop
would wake up again, until linux_nat's wait method is finally called and
consumes the contents of the pipe.
I think it would be nicer if targets using async_event_handler worked in
a similar way, where the flag would stay set until the target's wait
method is actually called. As a first step towards that, this patch
moves the responsibility of clearing the ready flags of async event
handlers to the invoked callback.
All async event handler callbacks are modified to clear their ready flag
before doing anything else. So in practice, nothing changes with this
patch. It's only the responsibility of clearing the flag that is
shifted toward the callee.
gdb/ChangeLog:
* async-event.h (async_event_handler_func): Add documentation.
* async-event.c (check_async_event_handlers): Don't clear
async_event_handler ready flag.
* infrun.c (infrun_async_inferior_event_handler): Clear ready
flag.
* record-btrace.c (record_btrace_handle_async_inferior_event):
Likewise.
* record-full.c (record_full_async_inferior_event_handler):
Likewise.
* remote-notif.c (remote_async_get_pending_events_handler):
Likewise.
* remote.c (remote_async_inferior_event_handler): Likewise.
Change-Id: I179ef8e99580eae642d332846fd13664dbddc0c1
2021-02-05 02:13:30 +08:00
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/* Type of async event handler callbacks.
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DATA is the client data originally passed to create_async_event_handler.
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The callback is called when the async event handler is marked. The callback
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is responsible for clearing the async event handler if it no longer needs
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to be called. */
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2020-04-14 02:42:59 +08:00
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typedef void (async_event_handler_func) (gdb_client_data);
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extern struct async_signal_handler *
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2020-10-03 02:44:38 +08:00
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create_async_signal_handler (sig_handler_func *proc,
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gdb_client_data client_data,
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const char *name);
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2020-04-14 02:42:59 +08:00
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extern void delete_async_signal_handler (struct async_signal_handler **);
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/* Call the handler from HANDLER the next time through the event
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loop. */
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extern void mark_async_signal_handler (struct async_signal_handler *handler);
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/* Returns true if HANDLER is marked ready. */
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extern int
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async_signal_handler_is_marked (struct async_signal_handler *handler);
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/* Mark HANDLER as NOT ready. */
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extern void clear_async_signal_handler (struct async_signal_handler *handler);
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/* Create and register an asynchronous event source in the event loop,
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and set PROC as its callback. CLIENT_DATA is passed as argument to
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PROC upon its invocation. Returns a pointer to an opaque structure
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used to mark as ready and to later delete this event source from
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2020-10-03 02:44:38 +08:00
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the event loop.
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NAME is a user-friendly name for the handler, used in debug statements. The
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name is not copied: its lifetime should be at least as long as that of the
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handler. */
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2020-04-14 02:42:59 +08:00
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extern struct async_event_handler *
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create_async_event_handler (async_event_handler_func *proc,
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2020-10-03 02:44:38 +08:00
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gdb_client_data client_data,
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const char *name);
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2020-04-14 02:42:59 +08:00
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/* Remove the event source pointed by HANDLER_PTR created by
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CREATE_ASYNC_EVENT_HANDLER from the event loop, and release it. */
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extern void
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delete_async_event_handler (struct async_event_handler **handler_ptr);
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/* Call the handler from HANDLER the next time through the event
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loop. */
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extern void mark_async_event_handler (struct async_event_handler *handler);
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gdb: defer commit resume until all available events are consumed
Rationale
---------
Let's say you have multiple threads hitting a conditional breakpoint
at the same time, and all of these are going to evaluate to false.
All these threads will need to be resumed.
Currently, GDB fetches one target event (one SIGTRAP representing the
breakpoint hit) and decides that the thread should be resumed. It
calls resume and commit_resume immediately. It then fetches the
second target event, and does the same, until it went through all
threads.
The result is therefore something like:
- consume event for thread A
- resume thread A
- commit resume (affects thread A)
- consume event for thread B
- resume thread B
- commit resume (affects thread B)
- consume event for thread C
- resume thread C
- commit resume (affects thread C)
For targets where it's beneficial to group resumptions requests (most
likely those that implement target_ops::commit_resume), it would be
much better to have:
- consume event for thread A
- resume thread A
- consume event for thread B
- resume thread B
- consume event for thread C
- resume thread C
- commit resume (affects threads A, B and C)
Implementation details
----------------------
To achieve this, this patch adds another check in
maybe_set_commit_resumed_all_targets to avoid setting the
commit-resumed flag of targets that readily have events to provide to
infrun.
To determine if a target has events readily available to report, this
patch adds an `has_pending_events` target_ops method. The method
returns a simple bool to say whether or not it has pending events to
report.
Testing
=======
To test this, I start GDBserver with a program that spawns multiple
threads:
$ ../gdbserver/gdbserver --once :1234 ~/src/many-threads-stepping-over-breakpoints/many-threads-stepping-over-breakpoints
I then connect with GDB and install a conditional breakpoint that always
evaluates to false (and force the evaluation to be done by GDB):
$ ./gdb -nx --data-directory=data-directory \
/home/simark/src/many-threads-stepping-over-breakpoints/many-threads-stepping-over-breakpoints \
-ex "set breakpoint condition-evaluation host" \
-ex "set pag off" \
-ex "set confirm off" \
-ex "maint set target-non-stop on" \
-ex "tar rem :1234" \
-ex "tb main" \
-ex "b 13 if 0" \
-ex c \
-ex "set debug infrun" \
-ex "set debug remote 1" \
-ex "set debug displaced"
I then do "continue" and look at the log.
The remote target receives a bunch of stop notifications for all
threads that have hit the breakpoint. infrun consumes and processes
one event, decides it should not cause a stop, prepares a displaced
step, after which we should see:
[infrun] maybe_set_commit_resumed_all_process_targets: not requesting commit-resumed for target remote, target has pending events
Same for a second thread (since we have 2 displaced step buffers).
For the following threads, their displaced step is deferred since
there are no more buffers available.
After consuming the last event the remote target has to offer, we get:
[infrun] maybe_set_commit_resumed_all_process_targets: enabling commit-resumed for target remote
[infrun] maybe_call_commit_resumed_all_process_targets: calling commit_resumed for target remote
[remote] Sending packet: $vCont;s:p14d16b.14d1b1;s:p14d16b.14d1b2#55
[remote] Packet received: OK
Without the patch, there would have been one vCont;s just after each
prepared displaced step.
gdb/ChangeLog:
yyyy-mm-dd Simon Marchi <simon.marchi@efficios.com>
Pedro Alves <pedro@palves.net>
* async-event.c (async_event_handler_marked): New.
* async-event.h (async_event_handler_marked): Declare.
* infrun.c (maybe_set_commit_resumed_all_targets): Switch to
inferior before calling target method. Don't commit-resumed if
target_has_pending_events is true.
* remote.c (remote_target::has_pending_events): New.
* target-delegates.c: Regenerate.
* target.c (target_has_pending_events): New.
* target.h (target_ops::has_pending_events): New target method.
(target_has_pending_events): New.
Change-Id: I18112ba19a1ff4986530c660f530d847bb4a1f1d
2020-07-07 03:53:28 +08:00
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/* Return true if HANDLER is marked. */
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extern bool async_event_handler_marked (async_event_handler *handler);
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2020-04-14 02:42:59 +08:00
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/* Mark the handler (ASYNC_HANDLER_PTR) as NOT ready. */
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extern void clear_async_event_handler (struct async_event_handler *handler);
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extern void initialize_async_signal_handlers (void);
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#endif /* ASYNC_EVENT_H */
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