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


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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