linux-kernel - Re: [PATCH 11/35] media: s2255: Use refcount_t instead of atomic_t for num

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Message-ID: <cf050593-44c2-4b4b-8198-215455c630f9@moroto.mountain>
Date: Tue, 23 Apr 2024 16:22:12 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
Cc: Ricardo Ribalda <ribalda@...omium.org>,
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Subject: Re: [PATCH 11/35] media: s2255: Use refcount_t instead of atomic_t
 for num_channels

On Tue, Apr 23, 2024 at 01:43:49PM +0100, Bryan O'Donoghue wrote:
> On 15/04/2024 20:34, Ricardo Ribalda wrote:
> > Use an API that resembles more the actual use of num_channels.
> > 
> > Found by cocci:
> > drivers/media/usb/s2255/s2255drv.c:2362:5-24: WARNING: atomic_dec_and_test variation before object free at line 2363.
> > drivers/media/usb/s2255/s2255drv.c:1557:5-24: WARNING: atomic_dec_and_test variation before object free at line 1558.
> 
> Hmm, that commit log needs more detail.
> 
> "Convert from atomic_t to refcount_t because refcount_t has memory ordering
> guarantees which atomic does not, hence the WARNING for the free after the
> atomic dec."
> 

The memory ordering rules are the same.  They're basically identical.
The difference is that if you decrement a refcount_t past zero it will
trigger an error and refuse but atomic_t will merrily keep decrementing
forever.  With refcount_t you can still have a use after free bug but
afterward the double free will trigger a warning.

There are time where people use atomic_t to get unique number and don't
care about wrapping so that's fine.  But if it's reference counting then
use refcount_t.

There wouldn't be a Fixes tag in this case, because hopefully it's just
hardenning and not a bugfix.

regards,
dan carpenter