Date: Fri, 29 May 2015 15:00:48 +0200 From: Sebastian Huber <sebastian.huber@embedded-brains.de> To: freebsd-hackers@freebsd.org Cc: phk@phk.freebsd.dk Subject: Problem with timecounters and memory model Message-ID: <55686300.3080100@embedded-brains.de>
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Hello,
we ported the FreeBSD timecounters to the RTEMS real-time operating=20
system. On PowerPC with GCC 4.9 I noticed the following problem. We have
void
binuptime(struct bintime *bt)
{
struct timehands *th;
u_int gen;
do {
th =3D timehands;
gen =3D th->th_generation;
*bt =3D th->th_offset;
bintime_addx(bt, th->th_scale * tc_delta(th));
} while (gen =3D=3D 0 || gen !=3D th->th_generation);
}
This is only valid if th->th_offset is read after th->th_generation and=20
the last write to th->th_generation is observable to all consumers=20
before the next write to th->th_offset etc.
On PowerPC we get the following code via GCC 4.9:
binuptime:
stwu 1,-32(1) #,,
mflr 0 #,
stw 26,8(1) #,
stw 0,36(1) #,
stw 27,12(1) #,
stw 28,16(1) #,
stw 29,20(1) #,
stw 31,28(1) #,
stw 30,24(1) #,
mr 30,3 # bt, bt
.L41:
lwz 31,timehands@sdarel(13) # timehands, th
lwz 28,0(31) # th_3->th_counter, tc
lwz 8,40(31) # th_3->th_offset, th_3->th_offset
lwz 9,0(28) # tc_13->tc_get_timecount, tc_13->tc_get_timecount
mr 3,28 #, tc
lwz 10,44(31) # th_3->th_offset, th_3->th_offset
lwz 6,32(31) # th_3->th_offset, th_3->th_offset
mtctr 9 #, tc_13->tc_get_timecount
lwz 7,36(31) # th_3->th_offset, th_3->th_offset
<- we read th_generation after th_offset
lwz 27,64(31) # th_3->th_generation, gen
stw 8,8(30) # *bt_5(D), th_3->th_offset
stw 10,12(30) # *bt_5(D), th_3->th_offset
stw 6,0(30) # *bt_5(D), th_3->th_offset
stw 7,4(30) # *bt_5(D), th_3->th_offset
lwz 26,20(31) # th_3->th_scale, D.6097
lwz 29,16(31) # th_3->th_scale, D.6097
bctrl #
Adding a compiler memory barrier yields:
void
binuptime(struct bintime *bt)
{
struct timehands *th;
uint32_t gen;
do {
th =3D timehands;
gen =3D th->th_generation;
__asm__ volatile("" ::: "memory");
*bt =3D th->th_offset;
bintime_addx(bt, th->th_scale * tc_delta(th));
} while (gen =3D=3D 0 || gen !=3D th->th_generation);
}
binuptime:
stwu 1,-32(1) #,,
mflr 0 #,
stw 26,8(1) #,
stw 0,36(1) #,
stw 27,12(1) #,
stw 28,16(1) #,
stw 29,20(1) #,
stw 31,28(1) #,
stw 30,24(1) #,
mr 30,3 # bt, bt
.L41:
lwz 31,timehands@sdarel(13) # timehands, th
lwz 27,64(31) # th_3->th_generation, gen
<- we read th_generation before th_offset
lwz 28,0(31) # th_3->th_counter, tc
lwz 9,44(31) # th_3->th_offset, th_3->th_offset
lwz 8,36(31) # th_3->th_offset, th_3->th_offset
mr 3,28 #, tc
lwz 10,40(31) # th_3->th_offset, th_3->th_offset
lwz 7,32(31) # th_3->th_offset, th_3->th_offset
stw 9,12(30) # *bt_6(D), th_3->th_offset
lwz 9,0(28) # tc_14->tc_get_timecount, tc_14->tc_get_timecount
stw 8,4(30) # *bt_6(D), th_3->th_offset
mtctr 9 #, tc_14->tc_get_timecount
stw 10,8(30) # *bt_6(D), th_3->th_offset
stw 7,0(30) # *bt_6(D), th_3->th_offset
lwz 26,20(31) # th_3->th_scale, D.6097
lwz 29,16(31) # th_3->th_scale, D.6097
bctrl #
This version seems to work at least on uni-processor systems. Shouldn't=20
there be real memory barriers the synchronize the reads/writes to=20
th_generation for SMP machines?
--=20
Sebastian Huber, embedded brains GmbH
Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone : +49 89 189 47 41-16
Fax : +49 89 189 47 41-09
E-Mail : sebastian.huber@embedded-brains.de
PGP : Public key available on request.
Diese Nachricht ist keine gesch=C3=A4ftliche Mitteilung im Sinne des EHUG=
.
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