Puranjay Mohan <puran...@kernel.org> writes:
Somehow the cover letter for this patch was missed, adding it here:
To test the functionality of these special instructions, a tool called
blitmus[0] was used to convert the following baseline litmus test[1] to bpf
programs:
C MP+poonceonces
(*
* Result: Sometimes
*
* Can the counter-intuitive message-passing outcome be prevented with
* no ordering at all?
*)
{}
P0(int *buf, int *flag)
{
WRITE_ONCE(*buf, 1);
WRITE_ONCE(*flag, 1);
}
P1(int *buf, int *flag)
{
int r0;
int r1;
r0 = READ_ONCE(*flag);
r1 = READ_ONCE(*buf);
}
exists (1:r0=1 /\ 1:r1=0) (* Bad outcome. *)
Running the generated bpf program shows that the bad outcome is possible on
powerpc:
[fedora@linux-kernel blitmus]$ sudo ./mp_poonceonces
Starting litmus test with configuration:
Test: MP+poonceonces
Iterations: 4100
Test MP+poonceonces Allowed
Histogram (4 states)
21548375 :>1:r0=0; 1:r1=0;
301187 :>1:r0=0; 1:r1=1;
337147 *>1:r0=1; 1:r1=0;
18813291 :>1:r0=1; 1:r1=1;
Ok
Witnesses
Positive: 337147, Negative: 40662853
Condition exists (1:r0=1 /\ 1:r1=0) is validated
Observation MP+poonceonces Sometimes 337147 40662853
Time MP+poonceonces 13.48
Thu Jul 17 18:12:51 UTC
Now the second write and the first read is converted to store_release and
load_acquire and it gives us the following litmus test[2]
C MP+pooncerelease+poacquireonce
(*
* Result: Never
*
* This litmus test demonstrates that smp_store_release() and
* smp_load_acquire() provide sufficient ordering for the message-passing
* pattern.
*)
{}
P0(int *buf, int *flag)
{
WRITE_ONCE(*buf, 1);
smp_store_release(flag, 1);
}
P1(int *buf, int *flag)
{
int r0;
int r1;
r0 = smp_load_acquire(flag);
r1 = READ_ONCE(*buf);
}
exists (1:r0=1 /\ 1:r1=0) (* Bad outcome. *)
Running the generated bpf program shows that the bad outcome is *not* possible
on powerpc with the implementation in this patch:
[fedora@linux-kernel blitmus]$ sudo ./mp_pooncerelease_poacquireonce
Starting litmus test with configuration:
Test: MP+pooncerelease+poacquireonce
Iterations: 4100
Test MP+pooncerelease+poacquireonce Allowed
Histogram (3 states)
21036021 :>1:r0=0; 1:r1=0;
14488694 :>1:r0=0; 1:r1=1;
5475285 :>1:r0=1; 1:r1=1;
No
Witnesses
Positive: 0, Negative: 41000000
Condition exists (1:r0=1 /\ 1:r1=0) is NOT validated
Observation MP+pooncerelease+poacquireonce Never 0 41000000
Time MP+pooncerelease+poacquireonce 13.74
Thu Jul 17 18:13:40 UTC
[0] https://github.com/puranjaymohan/blitmus
[1]
https://github.com/puranjaymohan/blitmus/blob/main/litmus_tests/MP%2Bpoonceonces.litmus
[2]
https://github.com/puranjaymohan/blitmus/blob/main/litmus_tests/MP%2Bpooncerelease%2Bpoacquireonce.litmus
