On Dec 24, 2019, at 7:12 PM, christophe leroy [off-list ref] wrote:



quoted

Le 24/12/2019 à 02:58, Andy Lutomirski a écrit :

quoted

On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy
[off-list ref] wrote:

READ_ONCE() forces the read of the 64 bit value of
vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec allthough
only the lower part is needed.

Seems reasonable and very unlikely to be harmful.  That being said,
this function really ought to be considered deprecated -- 32-bit
time_t is insufficient.
Do you get even better code if you move the read into the if statement?

Euh ...

How can you return t when time pointer is NULL if you read t only when time pointer is not NULL ?

Duh, never mind.

But this means your patch may be buggy: you need to make sure the compiler returns the *same* value it stores. Maybe you’re saved by the potential aliasing between the data page and the passed parameter and the value you read, but that’sa bad thing to rely on.

Try barrier() after the read.