Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

Le 11/01/2020 à 12:07, Thomas Gleixner a écrit : Christophe Leroy writes: With READ_ONCE() the 64 bits are being read: Without the READ_ONCE() only 32 bits are read. That's the most optimal. Without READ_ONCE() but with a barrier() after the read, we should get the same result but GCC (GC

Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

Christophe Leroy writes: > > With READ_ONCE() the 64 bits are being read: > > Without the READ_ONCE() only 32 bits are read. That's the most optimal. > > Without READ_ONCE() but with a barrier() after the read, we should get > the same result but GCC (GCC 8.1) does less good: > > Assuming both pa

Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

On 01/10/2020 09:12 PM, Thomas Gleixner wrote: Christophe Leroy writes: diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c index 17b4cff6e5f0..5a17a9d2e6cd 100644 --- a/lib/vdso/gettimeofday.c +++ b/lib/vdso/gettimeofday.c @@ -144,7 +144,7 @@ __cvdso_gettimeofday(const struct vd

Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

Christophe Leroy writes: > > diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c > index 17b4cff6e5f0..5a17a9d2e6cd 100644 > --- a/lib/vdso/gettimeofday.c > +++ b/lib/vdso/gettimeofday.c > @@ -144,7 +144,7 @@ __cvdso_gettimeofday(const struct vdso_data *vd, struct > __kernel_old_timeva

Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

> On Dec 24, 2019, at 7:12 PM, christophe leroy wrote: > >  > >> Le 24/12/2019 à 02:58, Andy Lutomirski a écrit : >>> On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy >>> wrote: >>> >>> READ_ONCE() forces the read of the 64 bit value of >>> vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec al

Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

Le 24/12/2019 à 02:58, Andy Lutomirski a écrit : On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy 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 har

Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy 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 ough

[RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

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. This results in a suboptimal code: 0af4 <__c_kernel_time>: af4: 2c 03 00 00 cmpwi r3,0 af8: 81 44 00 20 lwz r10,32(r4) afc: 81 64