[PATCH] Fix rwlock stall with PREFER_WRITER_NONRECURSIVE_NP (bug 23861)

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[PATCH] Fix rwlock stall with PREFER_WRITER_NONRECURSIVE_NP (bug 23861)

Andreas Schwab
        [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

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
 
 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);
   /* 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.
+   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>
--
2.19.1


--
Andreas Schwab, SUSE Labs, [hidden email]
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."
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Re: [PATCH] Fix rwlock stall with PREFER_WRITER_NONRECURSIVE_NP (bug 23861)

Carlos O'Donell-6
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>
>


--
Cheers,
Carlos.
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Re: [PATCH] Fix rwlock stall with PREFER_WRITER_NONRECURSIVE_NP (bug 23861)

Andreas Schwab
On Nov 08 2018, Carlos O'Donell <[hidden email]> 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.

--
Andreas Schwab, SUSE Labs, [hidden email]
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."