diff mbox

Fix rwlock stall with PREFER_WRITER_NONRECURSIVE_NP (bug 23861)

Message ID mvmh8gripro.fsf@suse.de
State New
Headers show

Commit Message

Andreas Schwab Nov. 8, 2018, 2:54 p.m. UTC
[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

Carlos O'Donell Nov. 8, 2018, 7:36 p.m. UTC | #1
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>
>
Andreas Schwab Nov. 12, 2018, 9:08 a.m. UTC | #2
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.
Torvald Riegel Dec. 7, 2018, 1:56 p.m. UTC | #3
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.
diff mbox

Patch

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>