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

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Message-ID: <dj2k36obadhbxlvecsdhdyf7edkmtgocf55jkbiq5gbnpvkygt@mcwolkrhicvq>
Date: Mon, 3 Feb 2025 12:26:12 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 01/17] zram: switch to non-atomic entry locking

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?