re: f2fs: factor out discard_cmd usage from general rb_tree use

[Colin King (gmail)]

Hi,

static analysis with clang scan build has detected a potential issue 
introduced by the following commit:

commit 7e9775a516ff6c1e73ee2b42ec563cafee38f42f
Author: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
Date:   Fri Mar 10 11:12:35 2023 -0800

f2fs: factor out discard_cmd usage from general rb_tree use


The warning is as follows:

fs/f2fs/segment.c:1425:4: warning: Value stored to 'tdc' is never read 
[deadcode.DeadStores]

The while loop in function __update_discard_tree_range is as follows (+ 
my annotations):


        while (1) {
                struct rb_node *node;
                struct discard_cmd *tdc = NULL;

### tdc is set to NULL

                if (prev_dc) {
                        di.lstart = prev_dc->di.lstart + prev_dc->di.len;
                        if (di.lstart < lstart)
                                di.lstart = lstart;
                        if (di.lstart >= end)
                                break;

                        if (!next_dc || next_dc->di.lstart > end)
                                di.len = end - di.lstart;
                        else
                                di.len = next_dc->di.lstart - di.lstart;
                        di.start = start + di.lstart - lstart;
                }

                if (!di.len)
                        goto next;

                if (prev_dc && prev_dc->state == D_PREP &&
                        prev_dc->bdev == bdev &&
                        __is_discard_back_mergeable(&di, &prev_dc->di,

max_discard_blocks)) {
                        prev_dc->di.len += di.len;
                        dcc->undiscard_blks += di.len;
                        __relocate_discard_cmd(dcc, prev_dc);
                        di = prev_dc->di;
                        tdc = prev_dc;

### tdc is set to prev_dc, however, it is not not read any more with th 
introduction of the "goto next"" statement introduced in the commit 
mentioned earlier

                        goto next;
                }

                if (next_dc && next_dc->state == D_PREP &&
                        next_dc->bdev == bdev &&
                        __is_discard_front_mergeable(&di, &next_dc->di,

max_discard_blocks)) {
                        next_dc->di.lstart = di.lstart;
                        next_dc->di.len += di.len;
                        next_dc->di.start = di.start;
                        dcc->undiscard_blks += di.len;
                        __relocate_discard_cmd(dcc, next_dc);

### tdc is always NULL, there is no path to this code where tdc is ever 
set to a non-NULL value.

                        if (tdc)
                                __remove_discard_cmd(sbi, tdc);
                        goto next;
                }

                __insert_discard_cmd(sbi, bdev, di.lstart, di.start, 
di.len);
 next:
                prev_dc = next_dc;
                if (!prev_dc)
                        break;

                node = rb_next(&prev_dc->rb_node);
                next_dc = rb_entry_safe(node, struct discard_cmd, rb_node);
        }