linux.git/kernel/trace/trace_kdb.c, branch v6.18.21

ring-buffer: Remove ring_buffer_read_prepare_sync()

2025-07-23T00:01:41+00:00

When the ring buffer was first introduced, reading the non-consuming
"trace" file required disabling the writing of the ring buffer. To make
sure the writing was fully disabled before iterating the buffer with a
non-consuming read, it would set the disable flag of the buffer and then
call an RCU synchronization to make sure all the buffers were
synchronized.

The function ring_buffer_read_start() originally  would initialize the
iterator and call an RCU synchronization, but this was for each individual
per CPU buffer where this would get called many times on a machine with
many CPUs before the trace file could be read. The commit 72c9ddfd4c5bf
("ring-buffer: Make non-consuming read less expensive with lots of cpus.")
separated ring_buffer_read_start into ring_buffer_read_prepare(),
ring_buffer_read_sync() and then ring_buffer_read_start() to allow each of
the per CPU buffers to be prepared, call the read_buffer_read_sync() once,
and then the ring_buffer_read_start() for each of the CPUs which made
things much faster.

The commit 1039221cc278 ("ring-buffer: Do not disable recording when there
is an iterator") removed the requirement of disabling the recording of the
ring buffer in order to iterate it, but it did not remove the
synchronization that was happening that was required to wait for all the
buffers to have no more writers. It's now OK for the buffers to have
writers and no synchronization is needed.

Remove the synchronization and put back the interface for the ring buffer
iterator back before commit 72c9ddfd4c5bf was applied.

Cc: Mathieu Desnoyers 
Link: https://lore.kernel.org/20250630180440.3eabb514@batman.local.home
Reported-by: David Howells 
Fixes: 1039221cc278 ("ring-buffer: Do not disable recording when there is an iterator")
Tested-by: David Howells 
Reviewed-by: Masami Hiramatsu (Google) 
Signed-off-by: Steven Rostedt (Google)

tracing: kdb: Use tracer_tracing_on/off() instead of setting per CPU disabled

2025-05-09T19:18:47+00:00

The per CPU "disabled" value was the original way to disable tracing when
the tracing subsystem was first created. Today, the ring buffer
infrastructure has its own way to disable tracing. In fact, things have
changed so much since 2008 that many things ignore the disable flag.

The kdb_ftdump() function iterates over all the current tracing CPUs and
increments the "disabled" counter before doing the dump, and decrements it
afterward.

As the disabled flag can be ignored, doing this today is not reliable.
Instead, simply call tracer_tracing_off() and then tracer_tracing_on() to
disable and then enabled the entire ring buffer in one go!

Cc: Jason Wessel 
Cc: Masami Hiramatsu 
Cc: Mark Rutland 
Cc: Mathieu Desnoyers 
Cc: Andrew Morton 
Cc: Daniel Thompson 
Reviewed-by: Douglas Anderson 
Link: https://lore.kernel.org/20250505212235.549033722@goodmis.org
Signed-off-by: Steven Rostedt (Google)

trace: kdb: Replace simple_strtoul with kstrtoul in kdb_ftdump

2024-11-02T08:33:13+00:00

The function simple_strtoul performs no error checking in scenarios
where the input value overflows the intended output variable.
This results in this function successfully returning, even when the
output does not match the input string (aka the function returns
successfully even when the result is wrong).

Or as it was mentioned [1], "...simple_strtol(), simple_strtoll(),
simple_strtoul(), and simple_strtoull() functions explicitly ignore
overflows, which may lead to unexpected results in callers."
Hence, the use of those functions is discouraged.

This patch replaces all uses of the simple_strtoul with the safer
alternatives kstrtoint and kstrtol.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#simple-strtol-simple-strtoll-simple-strtoul-simple-strtoull

Signed-off-by: Yuran Pereira 
Reviewed-by: Douglas Anderson 
Acked-by: Masami Hiramatsu (Google) 
Acked-by: Steven Rostedt (Google) 
[nir: style fixes]
Signed-off-by: Nir Lichtman 
Link: https://lore.kernel.org/r/20241028192100.GB918454@lichtman.org
Signed-off-by: Daniel Thompson

kdb: Rename members of struct kdbtab_t

2021-07-27T16:05:06+00:00

Remove redundant prefix "cmd_" from name of members in struct kdbtab_t
for better readibility.

Suggested-by: Doug Anderson 
Signed-off-by: Sumit Garg 
Reviewed-by: Douglas Anderson 
Link: https://lore.kernel.org/r/20210712134620.276667-5-sumit.garg@linaro.org
Signed-off-by: Daniel Thompson

kdb: Get rid of redundant kdb_register_flags()

2021-07-27T16:03:16+00:00

Commit e4f291b3f7bb ("kdb: Simplify kdb commands registration")
allowed registration of pre-allocated kdb commands with pointer to
struct kdbtab_t. Lets switch other users as well to register pre-
allocated kdb commands via:
- Changing prototype for kdb_register() to pass a pointer to struct
  kdbtab_t instead.
- Embed kdbtab_t structure in kdb_macro_t rather than individual params.

With these changes kdb_register_flags() becomes redundant and hence
removed. Also, since we have switched all users to register
pre-allocated commands, "is_dynamic" flag in struct kdbtab_t becomes
redundant and hence removed as well.

Suggested-by: Daniel Thompson 
Signed-off-by: Sumit Garg 
Acked-by: Steven Rostedt (VMware) 
Reviewed-by: Douglas Anderson 
Link: https://lore.kernel.org/r/20210712134620.276667-3-sumit.garg@linaro.org
Signed-off-by: Daniel Thompson

tracing: Rename trace_buffer to array_buffer

2020-01-13T18:19:38+00:00

As we are working to remove the generic "ring_buffer" name that is used by
both tracing and perf, the ring_buffer name for tracing will be renamed to
trace_buffer, and perf's ring buffer will be renamed to perf_buffer.

As there already exists a trace_buffer that is used by the trace_arrays, it
needs to be first renamed to array_buffer.

Link: https://lore.kernel.org/r/20191213153553.GE20583@krava

Signed-off-by: Steven Rostedt (VMware)

tracing: Silence GCC 9 array bounds warning

2019-05-26T03:04:30+00:00

Starting with GCC 9, -Warray-bounds detects cases when memset is called
starting on a member of a struct but the size to be cleared ends up
writing over further members.

Such a call happens in the trace code to clear, at once, all members
after and including `seq` on struct trace_iterator:

    In function 'memset',
        inlined from 'ftrace_dump' at kernel/trace/trace.c:8914:3:
    ./include/linux/string.h:344:9: warning: '__builtin_memset' offset
    [8505, 8560] from the object at 'iter' is out of the bounds of
    referenced subobject 'seq' with type 'struct trace_seq' at offset
    4368 [-Warray-bounds]
      344 |  return __builtin_memset(p, c, size);
          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

In order to avoid GCC complaining about it, we compute the address
ourselves by adding the offsetof distance instead of referring
directly to the member.

Since there are two places doing this clear (trace.c and trace_kdb.c),
take the chance to move the workaround into a single place in
the internal header.

Link: http://lkml.kernel.org/r/20190523124535.GA12931@gmail.com

Signed-off-by: Miguel Ojeda 
[ Removed unnecessary parenthesis around "iter" ]
Signed-off-by: Steven Rostedt (VMware)

tracing: kdb: Allow ftdump to skip all but the last few entries

2019-05-03T01:32:55+00:00

The 'ftdump' command in kdb is currently a bit of a last resort, at
least if you have lots of traces turned on.  It's going to print a
whole boatload of data out your serial port which is probably running
at 115200.  This could easily take many, many minutes.

Usually you're most interested in what's at the _end_ of the ftrace
buffer, AKA what happened most recently.  That means you've got to
wait the full time for the dump.  The 'ftdump' command does attempt to
help you a little bit by allowing you to skip a fixed number of
entries.  Unfortunately it provides no way for you to know how many
entries you should skip.

Let's do similar to python and allow you to use a negative number to
indicate that you want to skip all entries except the last few.  This
allows you to quickly see what you want.

Note that we also change the printout in ftdump to print the
(positive) number of entries actually skipped since that could be
helpful to know when you've specified a negative skip count.

Link: http://lkml.kernel.org/r/20190319171206.97107-3-dianders@chromium.org

Signed-off-by: Douglas Anderson 
Signed-off-by: Steven Rostedt (VMware)

tracing: kdb: The skip_lines parameter should have been skip_entries

2019-05-03T01:31:19+00:00

The things skipped by kdb's "ftdump" command when you pass it a
parameter has always been entries, not lines.  The difference usually
doesn't matter but when the trace buffer has multi-line entries (like
a stack dump) it can matter.

Let's fix this both in the help text for ftdump and also in the local
variable names.

Link: http://lkml.kernel.org/r/20190319171206.97107-1-dianders@chromium.org

Acked-by: Daniel Thompson 
Signed-off-by: Douglas Anderson 
Signed-off-by: Steven Rostedt (VMware)

tracing: kdb: Fix ftdump to not sleep

2019-03-13T13:46:10+00:00

As reported back in 2016-11 [1], the "ftdump" kdb command triggers a
BUG for "sleeping function called from invalid context".

kdb's "ftdump" command wants to call ring_buffer_read_prepare() in
atomic context.  A very simple solution for this is to add allocation
flags to ring_buffer_read_prepare() so kdb can call it without
triggering the allocation error.  This patch does that.

Note that in the original email thread about this, it was suggested
that perhaps the solution for kdb was to either preallocate the buffer
ahead of time or create our own iterator.  I'm hoping that this
alternative of adding allocation flags to ring_buffer_read_prepare()
can be considered since it means I don't need to duplicate more of the
core trace code into "trace_kdb.c" (for either creating my own
iterator or re-preparing a ring allocator whose memory was already
allocated).

NOTE: another option for kdb is to actually figure out how to make it
reuse the existing ftrace_dump() function and totally eliminate the
duplication.  This sounds very appealing and actually works (the "sr
z" command can be seen to properly dump the ftrace buffer).  The
downside here is that ftrace_dump() fully consumes the trace buffer.
Unless that is changed I'd rather not use it because it means "ftdump
| grep xyz" won't be very useful to search the ftrace buffer since it
will throw away the whole trace on the first grep.  A future patch to
dump only the last few lines of the buffer will also be hard to
implement.

[1] https://lkml.kernel.org/r/20161117191605.GA21459@google.com

Link: http://lkml.kernel.org/r/20190308193205.213659-1-dianders@chromium.org

Reported-by: Brian Norris 
Signed-off-by: Douglas Anderson 
Signed-off-by: Steven Rostedt (VMware)