Fix rwlock stall with PREFER_WRITER_NONRECURSIVE_NP (bug 23861)
Commit Message
[BZ #23861]
* nptl/pthread_rwlock_common.c (__pthread_rwlock_rdlock_full):
Update expected value for __readers while waiting on
PTHREAD_RWLOCK_RWAITING.
* nptl/tst-rwlock-pwn.c: New file.
* nptl/Makefile (tests): Add tst-rwlock-pwn.
---
nptl/Makefile | 3 +-
nptl/pthread_rwlock_common.c | 6 +--
nptl/tst-rwlock-pwn.c | 78 ++++++++++++++++++++++++++++++++++++
3 files changed, 83 insertions(+), 4 deletions(-)
create mode 100644 nptl/tst-rwlock-pwn.c
Comments
On 11/8/18 9:54 AM, Andreas Schwab wrote:
> [BZ #23861]
> * nptl/pthread_rwlock_common.c (__pthread_rwlock_rdlock_full):
> Update expected value for __readers while waiting on
> PTHREAD_RWLOCK_RWAITING.
> * nptl/tst-rwlock-pwn.c: New file.
> * nptl/Makefile (tests): Add tst-rwlock-pwn.
Is this at all related to this bug?
https://sourceware.org/bugzilla/show_bug.cgi?id=23844
I know this bug is not using PREFER_WRITER_NONRECURSIVE_NP, but I was curious
if you looked at this bug also when reviewing this code?
> ---
> nptl/Makefile | 3 +-
> nptl/pthread_rwlock_common.c | 6 +--
> nptl/tst-rwlock-pwn.c | 78 ++++++++++++++++++++++++++++++++++++
> 3 files changed, 83 insertions(+), 4 deletions(-)
> create mode 100644 nptl/tst-rwlock-pwn.c
>
> diff --git a/nptl/Makefile b/nptl/Makefile
> index 49b6faa330..c164b929b7 100644
> --- a/nptl/Makefile
> +++ b/nptl/Makefile
> @@ -318,7 +318,8 @@ tests = tst-attr1 tst-attr2 tst-attr3 tst-default-attr \
> tst-minstack-throw \
> tst-cnd-basic tst-mtx-trylock tst-cnd-broadcast \
> tst-cnd-timedwait tst-thrd-detach tst-mtx-basic tst-thrd-sleep \
> - tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock
> + tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock \
> + tst-rwlock-pwn
Please don't call this pwn :-)
Please use a more descriptive name like "tst-rwlock-stall"
>
> tests-internal := tst-rwlock19 tst-rwlock20 \
> tst-sem11 tst-sem12 tst-sem13 \
> diff --git a/nptl/pthread_rwlock_common.c b/nptl/pthread_rwlock_common.c
> index a290d08332..e95cbe4033 100644
> --- a/nptl/pthread_rwlock_common.c
> +++ b/nptl/pthread_rwlock_common.c
> @@ -314,12 +314,12 @@ __pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
> harmless because the flag is just about the state of
> __readers, and all threads set the flag under the same
> conditions. */
> - while ((atomic_load_relaxed (&rwlock->__data.__readers)
> - & PTHREAD_RWLOCK_RWAITING) != 0)
> + while (((r = atomic_load_relaxed (&rwlock->__data.__readers))
> + & PTHREAD_RWLOCK_RWAITING) != 0)
> {
> int private = __pthread_rwlock_get_private (rwlock);
> int err = futex_abstimed_wait (&rwlock->__data.__readers,
> - r, abstime, private);
> + r, abstime, private);
Why is this change correct?
> /* We ignore EAGAIN and EINTR. On time-outs, we can just
> return because we don't need to clean up anything. */
> if (err == ETIMEDOUT)
> diff --git a/nptl/tst-rwlock-pwn.c b/nptl/tst-rwlock-pwn.c
> new file mode 100644
> index 0000000000..a2be59576a
> --- /dev/null
> +++ b/nptl/tst-rwlock-pwn.c
> @@ -0,0 +1,78 @@
> +/* Test rwlock with PREFER_WRITER_NONRECURSIVE_NP.
OK.
> + Copyright (C) 2018 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <http://www.gnu.org/licenses/>. */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <pthread.h>
> +#include <support/xthread.h>
> +
> +#define LOOPS 10
Why 10?
> +#define NTHREADS 3
Why 3?
Suggest:
/* We choose 10 iterations and 3 threads because this happens
to be able to trigger the stall today on modern hardware. */
> +
> +volatile int do_exit;
> +pthread_rwlockattr_t mylock_attr;
> +pthread_rwlock_t mylock;
> +
> +void *
> +wrloop (void *a)
Misleading function name, it executes a wrlock or rdlock
not just 'writes in loop'.
Suggest 'wr_rd_loop' or just 'lock_loop'.
> +{
> + while (!do_exit)
> + {
> + if (random () & 1)
Please make the test deterministic.
Does it still stall if you just increment a counter
and then alternate between the two operations?
> + {
> + xpthread_rwlock_wrlock (&mylock);
> + xpthread_rwlock_unlock (&mylock);
> + }
> + else
> + {
> + xpthread_rwlock_rdlock (&mylock);
> + xpthread_rwlock_unlock (&mylock);
> + }
> + }
> + return NULL;
> +}
> +
> +int
> +do_test (void)
> +{
> + xpthread_rwlockattr_init (&mylock_attr);
> + xpthread_rwlockattr_setkind_np (&mylock_attr,
> + PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
> + xpthread_rwlock_init (&mylock, &mylock_attr);
OK.
> +
> + for (int n = 0; n < LOOPS; n++)
> + {
> + pthread_t tids[NTHREADS];
> + do_exit = 0;
> + for (int i = 0; i < NTHREADS; i++)
> + tids[i] = xpthread_create (NULL, wrloop, NULL);
> + sleep (1);
Why sleep?
> + printf ("Exiting..."); fflush (stdout);
Two lines please.
> + do_exit = 1;
> + for (int i = 0; i < NTHREADS; i++)
> + xpthread_join (tids[i]);
> + printf ("done.\n");
> + }
> + pthread_rwlock_destroy (&mylock);
> + pthread_rwlockattr_destroy (&mylock_attr);
> + return 0;
> +}
> +
> +#define TIMEOUT 3 * LOOPS
Please add a constant startup time.
Suggest:
#define TIMEOUT (DEFAULT_TIMEOUT + 3 * LOOPS)
> +#include <support/test-driver.c>
>
On Nov 08 2018, Carlos O'Donell <carlos@redhat.com> wrote:
> On 11/8/18 9:54 AM, Andreas Schwab wrote:
>> [BZ #23861]
>> * nptl/pthread_rwlock_common.c (__pthread_rwlock_rdlock_full):
>> Update expected value for __readers while waiting on
>> PTHREAD_RWLOCK_RWAITING.
>> * nptl/tst-rwlock-pwn.c: New file.
>> * nptl/Makefile (tests): Add tst-rwlock-pwn.
>
> Is this at all related to this bug?
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=23844
This part of the code is exclusively used for
PREFER_WRITER_NONRECURSIVE_NP rwlocks.
> Please don't call this pwn :-)
Why?
> Please use a more descriptive name like "tst-rwlock-stall"
It is testing PREFER_WRITER_NONRECURSIVE_NP, so pwn is very descriptive.
>> int err = futex_abstimed_wait (&rwlock->__data.__readers,
>> - r, abstime, private);
>> + r, abstime, private);
>
> Why is this change correct?
Emacs told me so, and Emacs is always right.
>> +#define LOOPS 10
>
> Why 10?
Just to make the bug reproducable enough.
>
>> +#define NTHREADS 3
>
> Why 3?
From the original test case.
> Does it still stall if you just increment a counter
> and then alternate between the two operations?
No.
>> + for (int n = 0; n < LOOPS; n++)
>> + {
>> + pthread_t tids[NTHREADS];
>> + do_exit = 0;
>> + for (int i = 0; i < NTHREADS; i++)
>> + tids[i] = xpthread_create (NULL, wrloop, NULL);
>> + sleep (1);
>
> Why sleep?
To let the threads running for some time.
Andreas.
On Thu, 2018-11-08 at 14:36 -0500, Carlos O'Donell wrote:
> On 11/8/18 9:54 AM, Andreas Schwab wrote:
> > [BZ #23861]
> > * nptl/pthread_rwlock_common.c (__pthread_rwlock_rdlock_full):
> > Update expected value for __readers while waiting on
> > PTHREAD_RWLOCK_RWAITING.
> > * nptl/tst-rwlock-pwn.c: New file.
> > * nptl/Makefile (tests): Add tst-rwlock-pwn.
>
> Is this at all related to this bug?
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=23844
No that's a different bug. At least the bug in the code is different, not
sure whether it could result in the same symptoms.
> >
> > tests-internal := tst-rwlock19 tst-rwlock20 \
> > tst-sem11 tst-sem12 tst-sem13 \
> > diff --git a/nptl/pthread_rwlock_common.c b/nptl/pthread_rwlock_common.c
> > index a290d08332..e95cbe4033 100644
> > --- a/nptl/pthread_rwlock_common.c
> > +++ b/nptl/pthread_rwlock_common.c
> > @@ -314,12 +314,12 @@ __pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
> > harmless because the flag is just about the state of
> > __readers, and all threads set the flag under the same
> > conditions. */
I'd add this to the comment:
"Update r so that the futex call in the loop uses the correct value."
> > - while ((atomic_load_relaxed (&rwlock->__data.__readers)
> > - & PTHREAD_RWLOCK_RWAITING) != 0)
> > + while (((r = atomic_load_relaxed (&rwlock->__data.__readers))
> > + & PTHREAD_RWLOCK_RWAITING) != 0)
> > {
> > int private = __pthread_rwlock_get_private (rwlock);
> > int err = futex_abstimed_wait (&rwlock->__data.__readers,
> > - r, abstime, private);
> > + r, abstime, private);
>
> Why is this change correct?
The fix is fine (and the formatting too, I guess). I think the comment
above should explain the fix. The futex call needs to wait on the based on
the value that lead to calling futex_wait in the first place.
@@ -318,7 +318,8 @@ tests = tst-attr1 tst-attr2 tst-attr3 tst-default-attr \
tst-minstack-throw \
tst-cnd-basic tst-mtx-trylock tst-cnd-broadcast \
tst-cnd-timedwait tst-thrd-detach tst-mtx-basic tst-thrd-sleep \
- tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock
+ tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock \
+ tst-rwlock-pwn
tests-internal := tst-rwlock19 tst-rwlock20 \
tst-sem11 tst-sem12 tst-sem13 \
@@ -314,12 +314,12 @@ __pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
harmless because the flag is just about the state of
__readers, and all threads set the flag under the same
conditions. */
- while ((atomic_load_relaxed (&rwlock->__data.__readers)
- & PTHREAD_RWLOCK_RWAITING) != 0)
+ while (((r = atomic_load_relaxed (&rwlock->__data.__readers))
+ & PTHREAD_RWLOCK_RWAITING) != 0)
{
int private = __pthread_rwlock_get_private (rwlock);
int err = futex_abstimed_wait (&rwlock->__data.__readers,
- r, abstime, private);
+ r, abstime, private);
/* We ignore EAGAIN and EINTR. On time-outs, we can just
return because we don't need to clean up anything. */
if (err == ETIMEDOUT)
new file mode 100644
@@ -0,0 +1,78 @@
+/* Test rwlock with PREFER_WRITER_NONRECURSIVE_NP.
+ Copyright (C) 2018 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <support/xthread.h>
+
+#define LOOPS 10
+#define NTHREADS 3
+
+volatile int do_exit;
+pthread_rwlockattr_t mylock_attr;
+pthread_rwlock_t mylock;
+
+void *
+wrloop (void *a)
+{
+ while (!do_exit)
+ {
+ if (random () & 1)
+ {
+ xpthread_rwlock_wrlock (&mylock);
+ xpthread_rwlock_unlock (&mylock);
+ }
+ else
+ {
+ xpthread_rwlock_rdlock (&mylock);
+ xpthread_rwlock_unlock (&mylock);
+ }
+ }
+ return NULL;
+}
+
+int
+do_test (void)
+{
+ xpthread_rwlockattr_init (&mylock_attr);
+ xpthread_rwlockattr_setkind_np (&mylock_attr,
+ PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
+ xpthread_rwlock_init (&mylock, &mylock_attr);
+
+ for (int n = 0; n < LOOPS; n++)
+ {
+ pthread_t tids[NTHREADS];
+ do_exit = 0;
+ for (int i = 0; i < NTHREADS; i++)
+ tids[i] = xpthread_create (NULL, wrloop, NULL);
+ sleep (1);
+ printf ("Exiting..."); fflush (stdout);
+ do_exit = 1;
+ for (int i = 0; i < NTHREADS; i++)
+ xpthread_join (tids[i]);
+ printf ("done.\n");
+ }
+ pthread_rwlock_destroy (&mylock);
+ pthread_rwlockattr_destroy (&mylock_attr);
+ return 0;
+}
+
+#define TIMEOUT 3 * LOOPS
+#include <support/test-driver.c>