On Mon, Sep 24, 2012 at 03:02:35PM +0000, Luck, Tony wrote:
> > And my plan was to get rid of the fact that backends touch pstore->buf
> > directly. Backends would always receive anonymous 'buf' pointer (we
> > already have write_buf callback that does exactly this), and thus it
>
> It feels like we are just shuffling the lock problem from one place
> to another. In the panic case we have to use a pre-allocated buffer
> (hoping that we can allocate one seems to be a foolish plan).
Yes, we definitely need some buffer pre-allocated for panics, so I have no
plans to get rid of the 'buf' -- both 'buf' and 'buf_lock' are going to
stay. But it will not be multi-purpose anymore (which I see as an
improvement).
The thing is, what I dislike is the whole console_write routine:
static void pstore_console_write(struct console *con, const char *s, unsigned c)
{
const char *e = s + c;
while (s < e) {
unsigned long flags;
if (c > psinfo->bufsize)
c = psinfo->bufsize;
spin_lock_irqsave(&psinfo->buf_lock, flags);
memcpy(psinfo->buf, s, c);
psinfo->write(PSTORE_TYPE_CONSOLE, 0, NULL, 0, c, psinfo);
spin_unlock_irqrestore(&psinfo->buf_lock, flags);
s += c;
c = e - s;
}
}
It's bad not because of the locks, but because we unnecessary copy things
around -- and that's the only reason why we need the lock in the first
place.
With write_buf, the above would turn into just:
static void pstore_console_write(struct console *con, const char *s, unsigned c)
{
psinfo->write_buf(PSTORE_TYPE_CONSOLE, 0, NULL, 0, s, c, psinfo);
}
Of course, this assumes that write_buf does its own hw/driver-specific
protection, but only if necessary: for ram backend no locks would be
necessary at all.
And as it appears, both erst and efivars drivers do their own locks in the
->write callback anyways. (Btw, efi pstore backend just grabs its lock w/o
trying it first, is it in trouble?)
But for panic case, we will use buf and buf_lock:
pstore_dump()
{
lock(buf_lock);
...
psinfo->write_buf(PSTORE_TYPE_DMESG, ..., psinfo->buf, ...);
...
unlock(buf_lock);
}
So, we will use them, but only when necessary (for dumping). We can think
of them as dump_buf and dump_buf_lock, because that's the only when this
stuff will be needed, actually.
Thanks,
Anton.
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