[PATCH -mm v2] nilfs2: Use __field_struct() for a bitwise field

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

From: Bart Van Assche <bvanassche@acm.org>

As one can see in include/trace/stages/stage4_event_fields.h, the
implementation of __field() uses the is_signed_type() macro. As one can see
in commit dcf8e5633e2e ("tracing: Define the is_signed_type() macro once"),
there has been an attempt to not make is_signed_type() trigger sparse
warnings for bitwise types.

Despite that change, sparse complains when passing a bitwise type to
is_signed_type(). The reason is that in its definition below, an inequality
comparison will be made against bitwise types, which are random collections
of bits (the casts to bitwise types themselves are semantically valid and
not problematic):

 #define is_signed_type(type) (((type)(-1)) < (__force type)1)

So, as a workaround, follow the example of <trace/events/initcall.h> and
suppress the following sparse warnings by changing __field() into
__field_struct() that doesn't use is_signed_type():

 fs/nilfs2/segment.c: note: in included file (through
   include/trace/trace_events.h, include/trace/define_trace.h,
   include/trace/events/nilfs2.h):
 ./include/trace/events/nilfs2.h:191:1: warning: cast to restricted
   blk_opf_t
 ./include/trace/events/nilfs2.h:191:1: warning: restricted blk_opf_t
   degrades to integer
 ./include/trace/events/nilfs2.h:191:1: warning: restricted blk_opf_t
   degrades to integer

[konishi.ryusuke: described the reason for the warnings based on Linus'
 explanation]
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401092241.I4mm9OWl-lkp@intel.com/
Reported-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Closes: https://lore.kernel.org/all/20240430080019.4242-2-konishi.ryusuke@gmail.com/
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
 include/trace/events/nilfs2.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/nilfs2.h b/include/trace/events/nilfs2.h
index 8efc6236f57c..8880c11733dd 100644
--- a/include/trace/events/nilfs2.h
+++ b/include/trace/events/nilfs2.h
@@ -200,7 +200,11 @@ TRACE_EVENT(nilfs2_mdt_submit_block,
 		    __field(struct inode *, inode)
 		    __field(unsigned long, ino)
 		    __field(unsigned long, blkoff)
-		    __field(enum req_op, mode)
+		    /*
+		     * Use field_struct() to avoid is_signed_type() on the
+		     * bitwise type enum req_op.
+		     */
+		    __field_struct(enum req_op, mode)
 	    ),
 
 	    TP_fast_assign(
-- 
2.34.1