Re: [PATCHv4 01/17] zram: switch to non-atomic entry locking

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (25/01/31 14:55), Andrew Morton wrote:
> > +static void zram_slot_write_lock(struct zram *zram, u32 index)
> > +{
> > +	atomic_t *lock = &zram->table[index].lock;
> > +	int old = atomic_read(lock);
> > +
> > +	do {
> > +		if (old != ZRAM_ENTRY_UNLOCKED) {
> > +			cond_resched();
> > +			old = atomic_read(lock);
> > +			continue;
> > +		}
> > +	} while (!atomic_try_cmpxchg(lock, &old, ZRAM_ENTRY_WRLOCKED));
> > +}
> 
> I expect that if the calling userspace process has realtime policy (eg
> SCHED_FIFO) then the cond_resched() won't schedule SCHED_NORMAL tasks
> and this becomes a busy loop.  And if the machine is single-CPU, the
> loop is infinite.

So for that scenario to happen zram needs to see two writes() to the same
index (page) simultaneously?  Or read() and write() on the same index (page)
concurrently?