Patchwork nptl: Fix pthread_rwlock_try*lock stalls [BZ #23844]

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Submitter Carlos O'Donell
Date Jan. 22, 2019, 4:11 a.m.
Message ID <c3d0b34e-b0b0-f4bc-4cc9-ea5484c38836@redhat.com>
Download mbox | patch
Permalink /patch/31149/
State New
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Comments

Carlos O'Donell - Jan. 22, 2019, 4:11 a.m.
Torvald, Rik,

Here is the full fix for bug 23844 with two regression test, one
for each of the trylock cases.

May I please use Signed-off-by for both of you? We follow the
same meaning as Linux kernel. The final fix incorporates parts
of both your patches, and mine.

Rik, I don't see an FSF copyright on file for you. In the future
if you want to contribute more to the project this is going to be
a requirement for us to be able to review your patches. As it is
your contributions are below the threshold of ~15 lines, and so
are OK for now. Thank you very much for your help and insight
into the fix.

I would really like to get this in to glibc 2.29, and so would
appreciate a prompt response. I've spent a lot of time reviewing
the fixes you and Torvald proposed and I believe they are correct.
I could not find any other defects in this area of the implementation
and I wrote my entire analysis in comment #14 of bug 23844.

I would appreciate any review of the following commit message,
fixes, and final tests.

~~~~ Commit message ~~~
For a full analysis of both the pthread_rwlock_tryrdlock() stall
and the pthread_rwlock_trywrlock() stall see:
https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14

In the pthread_rwlock_trydlock() function we fail to inspect for
PTHREAD_RWLOCK_FUTEX_USED in __wrphase_futex and wake the waiting
readers.

In the pthread_rwlock_trywrlock() function we write 1 to
__wrphase_futex and loose the setting of the PTHREAD_RWLOCK_FUTEX_USED
bit, again failing to wake waiting readers during unlock.

The fix in the case of pthread_rwlock_trydlock() is to check for
PTHREAD_RWLOCK_FUTEX_USED and wake the readers.

The fix in the case of pthread_rwlock_trywrlock() is to only write
1 to __wrphase_futex if we installed the write phase, since all other
readers would be spinning waiting for this step.

We add two new tests, one exercises the stall for
pthread_rwlock_trywrlock() which is easy to exercise, and one exercises
the stall for pthread_rwlock_trydlock() which is harder to exercise.

The pthread_rwlock_trywrlock() test fails consistently without the fix,
and passes after. The pthread_rwlock_tryrdlock() test fails roughly
5-10% of the time without the fix, and passes all the time after.

Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Torvald Riegel <triegel@redhat.com>
Signed-off-by: Rik Prohaska <prohaska7@gmail.com>
---
 ChangeLog                         |  17 +++++
 nptl/Makefile                     |   3 +-
 nptl/pthread_rwlock_tryrdlock.c   |  25 ++++---
 nptl/pthread_rwlock_trywrlock.c   |   9 ++-
 nptl/tst-rwlock-tryrdlock-stall.c | 112 ++++++++++++++++++++++++++++++
 nptl/tst-rwlock-trywrlock-stall.c | 108 ++++++++++++++++++++++++++++
 support/Makefile                  |   1 +
 support/xpthread_rwlock_destroy.c |  26 +++++++
 support/xthread.h                 |   1 +
 9 files changed, 291 insertions(+), 11 deletions(-)
 create mode 100644 nptl/tst-rwlock-tryrdlock-stall.c
 create mode 100644 nptl/tst-rwlock-trywrlock-stall.c
 create mode 100644 support/xpthread_rwlock_destroy.c
Florian Weimer - Jan. 22, 2019, 12:10 p.m.
* Carlos O'Donell:

> +/* For a full analysis see comment:
> +   https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14

In the past, we had issues with bug trackers going away and such
references becoming stale.

> +
> +   The stall is caused by pthread_rwlock_tryrdlock failing to check
> +   that PTHREAD_RWLOCK_FUTEX_USED is set in the __wrphase_futex futex
> +   and then waking the futex.
> +
> +   The fix for bug 23844 ensures that waiters on __wrphase_futex are
> +   correctly woken.  Before the fix the test stalls as readers can
> +   wait forever on __wrphase_futex.  */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <pthread.h>
> +#include <support/xthread.h>
> +#include <errno.h>
> +
> +/* We need only one lock to reproduce the issue. We will need multiple
> +   threads to get the exact case where we have a read, try, and unlock
> +   all interleaving to produce the case where the readers are waiting
> +   and the try fails to wake them.  */
> +pthread_rwlock_t onelock;
> +
> +/* The number of threads is arbitrary but empirically chosen to have
> +   enough threads that we see the condition where waiting readers are
> +   not woken by a successful tryrdlock.  */
> +#define NTHREADS 32
> +
> +_Atomic int do_exit;
> +
> +void *
> +run_loop (void *arg)
> +{
> +  int i = 0, ret, rw;
> +  while (!do_exit)
> +    {
> +      /* Arbitrarily choose if we are the writer or reader.  Choose a
> +	 high enough ratio of readers to writers to make it likely
> +	 that readers block (and eventually are susceptable to
> +	 stalling).  */
> +      rw = i % 8;
> +      /* If we are a writer, take the write lock, and then unlock.
> +	 If we are a reader, try the lock, then lock, then unlock.  */
> +      if (rw)

rw != 0 (or (i % 8) != 0).  Note sure if the variable actually increases
clarity here.

> +  /* Run for some amount of time.  Empirically speaking exercising
> +     the stall via pthread_rwlock_tryrdlock is much harder, and on
> +     a 3.5GHz 4 core x86_64 VM system it takes somewhere around
> +     20-200s to stall, approaching 100% stall past 200s.  We can't
> +     wait that long for a regression test so we just test for 20s,
> +     and expect the stall to happen with a 5-10% chance (enough for
> +     developers to see).  */
> +  sleep (20);

Use nanosleep or usleep to avoid signals.

> +  /* Then exit.  */
> +  printf ("INFO: Exiting...");
> +  fflush (stdout);

Why no trailing \n?  It will just result in garbled output if there is a
problem.

> +  do_exit = 1;
> +  /* If any readers stalled then we will timeout waiting for them.  */
> +  for (i = 0; i < NTHREADS; i++)
> +    xpthread_join (tids[i]);
> +  printf ("done.\n");
> +  xpthread_rwlock_destroy (&onelock);
> +  printf ("PASS: No pthread_rwlock_tryrdlock stalls detected.\n");
> +  return 0;
> +}
> +
> +#define TIMEOUT (30)

Parentheses are unnecessary.

Most of these comments also apply to ther other test.

I have not reviewed the code.

Thanks,
Florian

Patch

diff --git a/ChangeLog b/ChangeLog
index 834fca6fdc..6ecb020569 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,20 @@ 
+2019-01-21  Carlos O'Donell  <carlos@redhat.com>
+	    Torvald Riegel  <triegel@redhat.com>
+	    Rik Prohaska  <prohaska7@gmail.com>
+
+	[BZ# 23844]
+	* nptl/Makefile (tests): Add tst-rwlock-tryrdlock-stall, and
+	tst-rwlock-trywrlock-stall.
+	* nptl/pthread_rwlock_tryrdlock.c (__pthread_rwlock_tryrdlock):
+	Wake waiters if PTHREAD_RWLOCK_FUTEX_USED is set.
+	* nptl/pthread_rwlock_trywrlock.c (__pthread_rwlock_trywrlock):
+	Set __wrphase_fute to 1 only if we started the write phase.
+	* nptl/tst-rwlock-tryrdlock-stall.c: New file.
+	* nptl/tst-rwlock-trywrlock-stall.c: New file.
+	* support/Makefile (libsupport-routines): Add xpthread_rwlock_destroy.
+	* support/xpthread_rwlock_destroy.c: New file.
+	* support/xthread.h: Declare xpthread_rwlock_destroy.
+
 2019-01-21  H.J. Lu  <hongjiu.lu@intel.com>
 
 	[BZ# 24097]
diff --git a/nptl/Makefile b/nptl/Makefile
index 340282c6cb..0e316edfac 100644
--- a/nptl/Makefile
+++ b/nptl/Makefile
@@ -319,7 +319,8 @@  tests = tst-attr1 tst-attr2 tst-attr3 tst-default-attr \
 	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-rwlock-pwn
+	tst-rwlock-pwn \
+	tst-rwlock-tryrdlock-stall tst-rwlock-trywrlock-stall
 
 tests-internal := tst-rwlock19 tst-rwlock20 \
 		  tst-sem11 tst-sem12 tst-sem13 \
diff --git a/nptl/pthread_rwlock_tryrdlock.c b/nptl/pthread_rwlock_tryrdlock.c
index 368862ff07..2f94f17f36 100644
--- a/nptl/pthread_rwlock_tryrdlock.c
+++ b/nptl/pthread_rwlock_tryrdlock.c
@@ -94,15 +94,22 @@  __pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
       /* Same as in __pthread_rwlock_rdlock_full:
 	 We started the read phase, so we are also responsible for
 	 updating the write-phase futex.  Relaxed MO is sufficient.
-	 Note that there can be no other reader that we have to wake
-	 because all other readers will see the read phase started by us
-	 (or they will try to start it themselves); if a writer started
-	 the read phase, we cannot have started it.  Furthermore, we
-	 cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will
-	 overwrite the value set by the most recent writer (or the readers
-	 before it in case of explicit hand-over) and we know that there
-	 are no waiting readers.  */
-      atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
+	 We have to do the same steps as a writer would when handing over the
+	 read phase to use because other readers cannot distinguish between
+	 us and the writer.
+	 Note that __pthread_rwlock_tryrdlock callers will not have to be
+	 woken up because they will either see the read phase started by us
+	 or they will try to start it themselves; however, callers of
+	 __pthread_rwlock_rdlock_full just increase the reader count and then
+	 check what state the lock is in, so they cannot distinguish between
+	 us and a writer that acquired and released the lock in the
+	 meantime.  */
+      if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
+	  & PTHREAD_RWLOCK_FUTEX_USED) != 0)
+	{
+	  int private = __pthread_rwlock_get_private (rwlock);
+	  futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
+	}
     }
 
   return 0;
diff --git a/nptl/pthread_rwlock_trywrlock.c b/nptl/pthread_rwlock_trywrlock.c
index fd37a71ce4..fae475cc70 100644
--- a/nptl/pthread_rwlock_trywrlock.c
+++ b/nptl/pthread_rwlock_trywrlock.c
@@ -46,8 +46,15 @@  __pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
 	  &rwlock->__data.__readers, &r,
 	  r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
 	{
+	  /* We have become the primary writer and we cannot have shared
+	     the PTHREAD_RWLOCK_FUTEX_USED flag with someone else, so we
+	     can simply enable blocking (see full wrlock code).  */
 	  atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
-	  atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
+	  /* If we started a write phase, we need to enable readers to
+	     wait.  If we did not, we must not change it because other threads
+	     may have set the PTHREAD_RWLOCK_FUTEX_USED in the meantime.  */
+	  if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
+	    atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
 	  atomic_store_relaxed (&rwlock->__data.__cur_writer,
 	      THREAD_GETMEM (THREAD_SELF, tid));
 	  return 0;
diff --git a/nptl/tst-rwlock-tryrdlock-stall.c b/nptl/tst-rwlock-tryrdlock-stall.c
new file mode 100644
index 0000000000..c176350ac1
--- /dev/null
+++ b/nptl/tst-rwlock-tryrdlock-stall.c
@@ -0,0 +1,112 @@ 
+/* Bug 23844: Test for pthread_rwlock_tryrdlock stalls.
+   Copyright (C) 2019 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/>.  */
+
+/* For a full analysis see comment:
+   https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14
+
+   The stall is caused by pthread_rwlock_tryrdlock failing to check
+   that PTHREAD_RWLOCK_FUTEX_USED is set in the __wrphase_futex futex
+   and then waking the futex.
+
+   The fix for bug 23844 ensures that waiters on __wrphase_futex are
+   correctly woken.  Before the fix the test stalls as readers can
+   wait forever on __wrphase_futex.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <support/xthread.h>
+#include <errno.h>
+
+/* We need only one lock to reproduce the issue. We will need multiple
+   threads to get the exact case where we have a read, try, and unlock
+   all interleaving to produce the case where the readers are waiting
+   and the try fails to wake them.  */
+pthread_rwlock_t onelock;
+
+/* The number of threads is arbitrary but empirically chosen to have
+   enough threads that we see the condition where waiting readers are
+   not woken by a successful tryrdlock.  */
+#define NTHREADS 32
+
+_Atomic int do_exit;
+
+void *
+run_loop (void *arg)
+{
+  int i = 0, ret, rw;
+  while (!do_exit)
+    {
+      /* Arbitrarily choose if we are the writer or reader.  Choose a
+	 high enough ratio of readers to writers to make it likely
+	 that readers block (and eventually are susceptable to
+	 stalling).  */
+      rw = i % 8;
+      /* If we are a writer, take the write lock, and then unlock.
+	 If we are a reader, try the lock, then lock, then unlock.  */
+      if (rw)
+	xpthread_rwlock_wrlock (&onelock);
+      else
+	{
+	  if ((ret = pthread_rwlock_tryrdlock (&onelock)) != 0)
+	    {
+	      if (ret == EBUSY)
+		xpthread_rwlock_rdlock (&onelock);
+	      else
+		exit (EXIT_FAILURE);
+	    }
+	}
+      /* Thread does some work and then unlocks.  */
+      xpthread_rwlock_unlock (&onelock);
+      i++;
+    }
+  return NULL;
+}
+
+int
+do_test (void)
+{
+  int i;
+  pthread_t tids[NTHREADS];
+  xpthread_rwlock_init (&onelock, NULL);
+  for (i = 0; i < NTHREADS; i++)
+    tids[i] = xpthread_create (NULL, run_loop, NULL);
+  /* Run for some amount of time.  Empirically speaking exercising
+     the stall via pthread_rwlock_tryrdlock is much harder, and on
+     a 3.5GHz 4 core x86_64 VM system it takes somewhere around
+     20-200s to stall, approaching 100% stall past 200s.  We can't
+     wait that long for a regression test so we just test for 20s,
+     and expect the stall to happen with a 5-10% chance (enough for
+     developers to see).  */
+  sleep (20);
+  /* Then exit.  */
+  printf ("INFO: Exiting...");
+  fflush (stdout);
+  do_exit = 1;
+  /* If any readers stalled then we will timeout waiting for them.  */
+  for (i = 0; i < NTHREADS; i++)
+    xpthread_join (tids[i]);
+  printf ("done.\n");
+  xpthread_rwlock_destroy (&onelock);
+  printf ("PASS: No pthread_rwlock_tryrdlock stalls detected.\n");
+  return 0;
+}
+
+#define TIMEOUT (30)
+#include <support/test-driver.c>
diff --git a/nptl/tst-rwlock-trywrlock-stall.c b/nptl/tst-rwlock-trywrlock-stall.c
new file mode 100644
index 0000000000..f2b5ddb6b4
--- /dev/null
+++ b/nptl/tst-rwlock-trywrlock-stall.c
@@ -0,0 +1,108 @@ 
+/* Bug 23844: Test for pthread_rwlock_trywrlock stalls.
+   Copyright (C) 2019 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/>.  */
+
+/* For a full analysis see comment:
+   https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14
+
+   The stall is caused by pthread_rwlock_trywrlock setting
+   __wrphase_futex futex to 1 and loosing the
+   PTHREAD_RWLOCK_FUTEX_USED bit.
+
+   The fix for bug 23844 ensures that waiters on __wrphase_futex are
+   correctly woken.  Before the fix the test stalls as readers can
+   wait forever on  __wrphase_futex.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <support/xthread.h>
+#include <errno.h>
+
+/* We need only one lock to reproduce the issue. We will need multiple
+   threads to get the exact case where we have a read, try, and unlock
+   all interleaving to produce the case where the readers are waiting
+   and the try clears the PTHREAD_RWLOCK_FUTEX_USED bit and a
+   subsequent unlock fails to wake them.  */
+pthread_rwlock_t onelock;
+
+/* The number of threads is arbitrary but empirically chosen to have
+   enough threads that we see the condition where waiting readers are
+   not woken by a successful unlock.  */
+#define NTHREADS 32
+
+_Atomic int do_exit;
+
+void *
+run_loop (void *arg)
+{
+  int i = 0, ret, rw;
+  while (!do_exit)
+    {
+      /* Arbitrarily choose if we are the writer or reader.  Choose a
+	 high enough ratio of readers to writers to make it likely
+	 that readers block (and eventually are susceptable to
+	 stalling).  */
+      rw = i % 8;
+      /* If we are a writer, take the write lock, and then unlock.
+	 If we are a reader, try the lock, then lock, then unlock.  */
+      if (rw)
+	{
+	  if ((ret = pthread_rwlock_trywrlock (&onelock)) != 0)
+	    {
+	      if (ret == EBUSY)
+		xpthread_rwlock_wrlock (&onelock);
+	      else
+		exit (EXIT_FAILURE);
+	    }
+	}
+      else
+	xpthread_rwlock_rdlock (&onelock);
+      /* Thread does some work and then unlocks.  */
+      xpthread_rwlock_unlock (&onelock);
+      i++;
+    }
+  return NULL;
+}
+
+int
+do_test (void)
+{
+  int i;
+  pthread_t tids[NTHREADS];
+  xpthread_rwlock_init (&onelock, NULL);
+  for (i = 0; i < NTHREADS; i++)
+    tids[i] = xpthread_create (NULL, run_loop, NULL);
+  /* Run for some amount of time.  The pthread_rwlock_tryrwlock stall
+     is very easy to trigger and happens in seconds under the test
+     conditions.  */
+  sleep (10);
+  /* Then exit.  */
+  printf ("INFO: Exiting...");
+  fflush (stdout);
+  do_exit = 1;
+  /* If any readers stalled then we will timeout waiting for them.  */
+  for (i = 0; i < NTHREADS; i++)
+    xpthread_join (tids[i]);
+  printf ("done.\n");
+  xpthread_rwlock_destroy (&onelock);
+  printf ("PASS: No pthread_rwlock_tryrwlock stalls detected.\n");
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/support/Makefile b/support/Makefile
index 432cf2fe6c..c15b93647c 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -129,6 +129,7 @@  libsupport-routines = \
   xpthread_mutexattr_settype \
   xpthread_once \
   xpthread_rwlock_init \
+  xpthread_rwlock_destroy \
   xpthread_rwlock_rdlock \
   xpthread_rwlock_unlock \
   xpthread_rwlock_wrlock \
diff --git a/support/xpthread_rwlock_destroy.c b/support/xpthread_rwlock_destroy.c
new file mode 100644
index 0000000000..6d6e953569
--- /dev/null
+++ b/support/xpthread_rwlock_destroy.c
@@ -0,0 +1,26 @@ 
+/* pthread_rwlock_destroy with error checking.
+   Copyright (C) 2019 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 <support/xthread.h>
+
+void
+xpthread_rwlock_destroy (pthread_rwlock_t *rwlock)
+{
+  xpthread_check_return ("pthread_rwlock_destroy",
+                         pthread_rwlock_destroy (rwlock));
+}
diff --git a/support/xthread.h b/support/xthread.h
index 47c23235f3..9fe1f68b3b 100644
--- a/support/xthread.h
+++ b/support/xthread.h
@@ -84,6 +84,7 @@  void xpthread_rwlockattr_setkind_np (pthread_rwlockattr_t *attr, int pref);
 void xpthread_rwlock_wrlock (pthread_rwlock_t *rwlock);
 void xpthread_rwlock_rdlock (pthread_rwlock_t *rwlock);
 void xpthread_rwlock_unlock (pthread_rwlock_t *rwlock);
+void xpthread_rwlock_destroy (pthread_rwlock_t *rwlock);
 
 __END_DECLS