From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
The "random" pkey allocation code currently does the good old:
srand((unsigned int)time(NULL));
*But*, it unfortunately does this on every random pkey allocation.
There may be thousands of these a second. time() has a one second
resolution. So, each time alloc_random_pkey() is called, the PRNG is
*RESET* to time(). This is nasty. Normally, if you do:
srand(<ANYTHING>);
foo = rand();
bar = rand();
You'll be quite guaranteed that 'foo' and 'bar' are different.
But, if you do:
srand(1);
foo = rand();
srand(1);
bar = rand();
You are quite guaranteed that 'foo' and 'bar' are the *SAME*.
The recent "fix" effectively forced the test case to use the
same "random" pkey for the whole test, unless the test run
crossed a second boundary.
Only run srand() once at program startup.
This explains some very odd and persistent test failures I've been
seeing.
Fixes: 6e373263ce07 ("selftests/vm/pkeys: fix alloc_random_pkey() to make it really random")
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ram Pai <linuxram@xxxxxxxxxx>
Cc: Sandipan Das <sandipan@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Florian Weimer <fweimer@xxxxxxxxxx>
Cc: "Desnes A. Nunes do Rosario" <desnesn@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Michal Suchanek <msuchanek@xxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
---
b/tools/testing/selftests/vm/protection_keys.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff -puN tools/testing/selftests/vm/protection_keys.c~selftests_vm_pkeys_Fix_alloc_random_pkey_to_make_it_really_really_random-1 tools/testing/selftests/vm/protection_keys.c
--- a/tools/testing/selftests/vm/protection_keys.c~selftests_vm_pkeys_Fix_alloc_random_pkey_to_make_it_really_really_random-1 2021-06-11 09:41:31.385468066 -0700
+++ b/tools/testing/selftests/vm/protection_keys.c 2021-06-11 09:41:31.389468066 -0700
@@ -561,7 +561,6 @@ int alloc_random_pkey(void)
int nr_alloced = 0;
int random_index;
memset(alloced_pkeys, 0, sizeof(alloced_pkeys));
- srand((unsigned int)time(NULL));
/* allocate every possible key and make a note of which ones we got */
max_nr_pkey_allocs = NR_PKEYS;
@@ -1552,6 +1551,8 @@ int main(void)
int nr_iterations = 22;
int pkeys_supported = is_pkeys_supported();
+ srand((unsigned int)time(NULL));
+
setup_handlers();
printf("has pkeys: %d\n", pkeys_supported);
_
