Re: [tracepoint] cargo-culting considered harmful...

Al Viro
2013-01-25T15:32:05Z

On Fri, Jan 25, 2013 at 09:49:53AM -0500, Mathieu Desnoyers wrote:
> static
> void lttng_enumerate_task_fd(struct lttng_session *session,
> struct task_struct *p, char *tmp)
> {
> struct fdtable *fdt;
> struct file *filp;
> unsigned int i;
> const unsigned char *path;
>
> task_lock(p);
> if (!p->files)
> goto unlock_task;
> spin_lock(&p->files->file_lock);
> fdt = files_fdtable(p->files);
> for (i = 0; i < fdt->max_fds; i++) {
> filp = fcheck_files(p->files, i);
> if (!filp)
> continue;
> path = d_path(&filp->f_path, tmp, PAGE_SIZE);
> /* Make sure we give at least some info */
> trace_lttng_statedump_file_descriptor(session, p, i,
> IS_ERR(path) ?
> filp->f_dentry->d_name.name :
> path);
> }
> spin_unlock(&p->files->file_lock);
> unlock_task:
> task_unlock(p);
> }

*cringe*

a) yes, it needs d_lock for that ->d_name access
b) iterate_fd() is there for purpose; use it, instead of open-coding the
damn loop. Something like

struct ctx {
char *page;
struct lttng_session *session,
struct task_struct *p;
};

static int dump_one(void *p, struct file *file, unsigned fd)
{
struct ctx *ctx = p;
const char *s = d_path(&file->f_path, ctx->page, PAGE_SIZE);
struct dentry *dentry;
if (!IS_ERR(s)) {
trace_lttng_statedump_file_descriptor(ctx->session, ctx->p, fd, s);
return 0;
}
/* Make sure we give at least some info */
dentry = file->f_path.dentry;
spin_lock(&dentry->d_lock);
trace_lttng_statedump_file_descriptor(ctx->session, ctx->p, fd,
dentry->d_name);
spin_unlock(&dentry->d_lock);
return 0;
}

...
task_lock(p);
iterate_fd(p->files, 0, dump_one, &(struct ctx){tmp, session, p});
task_unlock(p);

assuming it wouldn't be better to pass tmp/session/p as the single pointer
to struct in the first place - I don't know enough about the callers of
that sucker to tell. And yes, iterate_fd() will DTRT if given NULL as the
first argument. The second argument is "which descriptor should I start
from?", callback is called for everything present in the table starting from
that place until it returns non-zero or the end of table is reached...

PS: people really ought to be forced to read their code aloud over the phone -
that would rapidly improve the choice of identifiers ;-)