linux-kernel - [PATCH RESEND] tracing: make trace_marker{,

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Date:   Tue,  7 Dec 2021 14:25:58 +0000
From:   John Keeping <john@...anate.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     John Keeping <john@...anate.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH RESEND] tracing: make trace_marker{,_raw} stream-like

The tracing marker files are write-only streams with no meaningful
concept of file position.  Using stream_open() to mark them as
stream-link indicates this and has the added advantage that a single
file descriptor can now be used from multiple threads without contention
thanks to clearing FMODE_ATOMIC_POS.

Note that this has the potential to break existing userspace by since
both lseek(2) and pwrite(2) will now return ESPIPE when previously lseek
would have updated the stored offset and pwrite would have appended to
the trace.  A survey of libtracefs and several other projects found to
use trace_marker(_raw) [1][2][3] suggests that everyone limits
themselves to calling write(2) and close(2) on these file descriptors so
there is a good chance this will go unnoticed and the benefits of
reduced overhead and lock contention seem worth the risk.

[1] https://github.com/google/perfetto
[2] https://github.com/intel/media-driver/
[3] https://w1.fi/cgit/hostap/

Signed-off-by: John Keeping <john@...anate.com>
---
Resending as requested at [1].

[1] https://lore.kernel.org/all/20211207090347.15822d87@gandalf.local.home/

 kernel/trace/trace.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 88de94da596b1..7c91103ac1a04 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4834,6 +4834,12 @@ int tracing_open_generic_tr(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+static int tracing_mark_open(struct inode *inode, struct file *filp)
+{
+	stream_open(inode, filp);
+	return tracing_open_generic_tr(inode, filp);
+}
+
 static int tracing_release(struct inode *inode, struct file *file)
 {
 	struct trace_array *tr = inode->i_private;
@@ -7110,9 +7116,6 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
 	if (tt)
 		event_triggers_post_call(tr->trace_marker_file, tt);
 
-	if (written > 0)
-		*fpos += written;
-
 	return written;
 }
 
@@ -7171,9 +7174,6 @@ tracing_mark_raw_write(struct file *filp, const char __user *ubuf,
 
 	__buffer_unlock_commit(buffer, event);
 
-	if (written > 0)
-		*fpos += written;
-
 	return written;
 }
 
@@ -7573,16 +7573,14 @@ static const struct file_operations tracing_free_buffer_fops = {
 };
 
 static const struct file_operations tracing_mark_fops = {
-	.open		= tracing_open_generic_tr,
+	.open		= tracing_mark_open,
 	.write		= tracing_mark_write,
-	.llseek		= generic_file_llseek,
 	.release	= tracing_release_generic_tr,
 };
 
 static const struct file_operations tracing_mark_raw_fops = {
-	.open		= tracing_open_generic_tr,
+	.open		= tracing_mark_open,
 	.write		= tracing_mark_raw_write,
-	.llseek		= generic_file_llseek,
 	.release	= tracing_release_generic_tr,
 };
 
-- 
2.34.1