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Re: [PATCH] New condvar implementation that provides stronger ordering guarantees.


On 12/23/2016 11:33 AM, Torvald Riegel wrote:
> On Tue, 2016-06-14 at 22:53 +0200, Torvald Riegel wrote:
>> I've now tested this patch successfully using our existing tests on ppc,
>> ppc64, ppc64le, s390x, and aarch64.  I couldn't test on s390 due to
>> https://www.sourceware.org/ml/libc-alpha/2016-06/msg00545.html.
>>
>> The attached patch is a minor revision that just fixes some formatting
>> and adds an include (revealing by testing on s390x).
> I have attached a followup patch that adds support for the pretty
> printers added in the meantime.  For ease-of-review, I'm sending this as
> a separate path and not a revision of the previous patch; I will commit
> it as a single patch though.

This looks good to me, though the implementation is fairly rudimentary.

I would like us to continue to make this better _before_ the release to
help developers working on the various ports that need to make this work.
When I say "better" I mean expose more algorithm details.

So this should get checked in with the new condvar, but we should cricle
back to this before the release.

Cheers,
Carlos.

> commit 157cc3cc3e7c6656337f23079272d4247dd53814
> Author: Torvald Riegel <triegel@redhat.com>
> Date:   Fri Dec 23 14:52:08 2016 +0100
> 
>     Additional support for pretty printers.
> 
> diff --git a/nptl/nptl-printers.py b/nptl/nptl-printers.py
> index e402f23..c207e9c 100644
> --- a/nptl/nptl-printers.py
> +++ b/nptl/nptl-printers.py
> @@ -293,16 +293,6 @@ class MutexAttributesPrinter(object):
>          elif protocol == PTHREAD_PRIO_PROTECT:
>              self.values.append(('Protocol', 'Priority protect'))
>  
> -CLOCK_IDS = {
> -    CLOCK_REALTIME: 'CLOCK_REALTIME',
> -    CLOCK_MONOTONIC: 'CLOCK_MONOTONIC',
> -    CLOCK_PROCESS_CPUTIME_ID: 'CLOCK_PROCESS_CPUTIME_ID',
> -    CLOCK_THREAD_CPUTIME_ID: 'CLOCK_THREAD_CPUTIME_ID',
> -    CLOCK_MONOTONIC_RAW: 'CLOCK_MONOTONIC_RAW',
> -    CLOCK_REALTIME_COARSE: 'CLOCK_REALTIME_COARSE',
> -    CLOCK_MONOTONIC_COARSE: 'CLOCK_MONOTONIC_COARSE'
> -}
> -
>  class ConditionVariablePrinter(object):
>      """Pretty printer for pthread_cond_t."""
>  
> @@ -313,24 +303,8 @@ class ConditionVariablePrinter(object):
>              cond: A gdb.value representing a pthread_cond_t.
>          """
>  
> -        # Since PTHREAD_COND_SHARED is an integer, we need to cast it to void *
> -        # to be able to compare it to the condvar's __data.__mutex member.
> -        #
> -        # While it looks like self.shared_value should be a class variable,
> -        # that would result in it having an incorrect size if we're loading
> -        # these printers through .gdbinit for a 64-bit objfile in AMD64.
> -        # This is because gdb initially assumes the pointer size to be 4 bytes,
> -        # and only sets it to 8 after loading the 64-bit objfiles.  Since
> -        # .gdbinit runs before any objfiles are loaded, this would effectively
> -        # make self.shared_value have a size of 4, thus breaking later
> -        # comparisons with pointers whose types are looked up at runtime.
> -        void_ptr_type = gdb.lookup_type('void').pointer()
> -        self.shared_value = gdb.Value(PTHREAD_COND_SHARED).cast(void_ptr_type)
> -
>          data = cond['__data']
> -        self.total_seq = data['__total_seq']
> -        self.mutex = data['__mutex']
> -        self.nwaiters = data['__nwaiters']
> +        self.wrefs = data['__wrefs']
>          self.values = []
>  
>          self.read_values()
> @@ -360,7 +334,6 @@ class ConditionVariablePrinter(object):
>  
>          self.read_status()
>          self.read_attributes()
> -        self.read_mutex_info()
>  
>      def read_status(self):
>          """Read the status of the condvar.
> @@ -369,41 +342,22 @@ class ConditionVariablePrinter(object):
>          are waiting for it.
>          """
>  
> -        if self.total_seq == PTHREAD_COND_DESTROYED:
> -            self.values.append(('Status', 'Destroyed'))
> -
> -        self.values.append(('Threads waiting for this condvar',
> -                            self.nwaiters >> COND_NWAITERS_SHIFT))
> +        self.values.append(('Threads known to still execute a wait function',
> +                            self.wrefs >> PTHREAD_COND_WREFS_SHIFT))
>  
>      def read_attributes(self):
>          """Read the condvar's attributes."""
>  
> -        clock_id = self.nwaiters & ((1 << COND_NWAITERS_SHIFT) - 1)
> -
> -        # clock_id must be casted to int because it's a gdb.Value
> -        self.values.append(('Clock ID', CLOCK_IDS[int(clock_id)]))
> +	if (self.wrefs & PTHREAD_COND_CLOCK_MONOTONIC_MASK) != 0:
> +        	self.values.append(('Clock ID', 'CLOCK_MONOTONIC'))
> +	else:
> +        	self.values.append(('Clock ID', 'CLOCK_REALTIME'))
>  
> -        shared = (self.mutex == self.shared_value)
> -
> -        if shared:
> +        if (self.wrefs & PTHREAD_COND_SHARED_MASK) != 0:
>              self.values.append(('Shared', 'Yes'))
>          else:
>              self.values.append(('Shared', 'No'))
>  
> -    def read_mutex_info(self):
> -        """Read the data of the mutex this condvar is bound to.
> -
> -        A pthread_cond_t's __data.__mutex member is a void * which
> -        must be casted to pthread_mutex_t *.  For shared condvars, this
> -        member isn't recorded and has a special value instead.
> -        """
> -
> -        if self.mutex and self.mutex != self.shared_value:
> -            mutex_type = gdb.lookup_type('pthread_mutex_t')
> -            mutex = self.mutex.cast(mutex_type.pointer()).dereference()
> -
> -            self.values.append(('Mutex', mutex))
> -
>  class ConditionVariableAttributesPrinter(object):
>      """Pretty printer for pthread_condattr_t.
>  
> @@ -453,10 +407,12 @@ class ConditionVariableAttributesPrinter(object):
>          created in self.children.
>          """
>  
> -        clock_id = self.condattr & ((1 << COND_NWAITERS_SHIFT) - 1)
> +        clock_id = (self.condattr >> 1) & ((1 << COND_CLOCK_BITS) - 1)
>  
> -        # clock_id must be casted to int because it's a gdb.Value
> -        self.values.append(('Clock ID', CLOCK_IDS[int(clock_id)]))
> +	if clock_id != 0:
> +        	self.values.append(('Clock ID', 'CLOCK_MONOTONIC'))
> +	else:
> +        	self.values.append(('Clock ID', 'CLOCK_REALTIME'))
>  
>          if self.condattr & 1:
>              self.values.append(('Shared', 'Yes'))
> diff --git a/nptl/nptl_lock_constants.pysym b/nptl/nptl_lock_constants.pysym
> index 303ec61..2ab3179 100644
> --- a/nptl/nptl_lock_constants.pysym
> +++ b/nptl/nptl_lock_constants.pysym
> @@ -44,26 +44,13 @@ PTHREAD_PRIO_NONE
>  PTHREAD_PRIO_INHERIT
>  PTHREAD_PRIO_PROTECT
>  
> --- These values are hardcoded as well:
> --- Value of __mutex for shared condvars.
> -PTHREAD_COND_SHARED             (void *)~0l
> -
> --- Value of __total_seq for destroyed condvars.
> -PTHREAD_COND_DESTROYED          -1ull
> -
> --- __nwaiters encodes the number of threads waiting on a condvar
> --- and the clock ID.
> --- __nwaiters >> COND_NWAITERS_SHIFT gives us the number of waiters.
> -COND_NWAITERS_SHIFT
> -
> --- Condvar clock IDs
> -CLOCK_REALTIME
> -CLOCK_MONOTONIC
> -CLOCK_PROCESS_CPUTIME_ID
> -CLOCK_THREAD_CPUTIME_ID
> -CLOCK_MONOTONIC_RAW
> -CLOCK_REALTIME_COARSE
> -CLOCK_MONOTONIC_COARSE
> +-- Condition variable
> +-- FIXME Why do macros prefixed with __ cannot be used directly?
> +PTHREAD_COND_SHARED_MASK          __PTHREAD_COND_SHARED_MASK
> +PTHREAD_COND_CLOCK_MONOTONIC_MASK __PTHREAD_COND_CLOCK_MONOTONIC_MASK
> +COND_CLOCK_BITS
> +-- These values are hardcoded:
> +PTHREAD_COND_WREFS_SHIFT          3
>  
>  -- Rwlock attributes
>  PTHREAD_RWLOCK_PREFER_READER_NP
> diff --git a/nptl/pthreadP.h b/nptl/pthreadP.h
> index 6e0dd09..92a9992 100644
> --- a/nptl/pthreadP.h
> +++ b/nptl/pthreadP.h
> @@ -167,6 +167,13 @@ enum
>  #define __PTHREAD_ONCE_FORK_GEN_INCR	4
>  
>  
> +/* Condition variable definitions.  See __pthread_cond_wait_common.
> +   Need to be defined here so there is one place from which
> +   nptl_lock_constants can grab them.  */
> +#define __PTHREAD_COND_CLOCK_MONOTONIC_MASK 2
> +#define __PTHREAD_COND_SHARED_MASK 1
> +
> +
>  /* Internal variables.  */
>  
>  
> diff --git a/nptl/pthread_cond_init.c b/nptl/pthread_cond_init.c
> index cdd9b7d..c1eac5f 100644
> --- a/nptl/pthread_cond_init.c
> +++ b/nptl/pthread_cond_init.c
> @@ -29,6 +29,10 @@ __pthread_cond_init (pthread_cond_t *cond, const pthread_condattr_t *cond_attr)
>    struct pthread_condattr *icond_attr = (struct pthread_condattr *) cond_attr;
>  
>    memset (cond, 0, sizeof (pthread_cond_t));
> +
> +  /* Update the pretty printers if the internal representation of icond_attr
> +     is changed.  */
> +
>    /* Iff not equal to ~0l, this is a PTHREAD_PROCESS_PRIVATE condvar.  */
>    if (icond_attr != NULL && (icond_attr->value & 1) != 0)
>      cond->__data.__wrefs |= __PTHREAD_COND_SHARED_MASK;
> diff --git a/nptl/pthread_cond_wait.c b/nptl/pthread_cond_wait.c
> index 984f01f..2b43402 100644
> --- a/nptl/pthread_cond_wait.c
> +++ b/nptl/pthread_cond_wait.c
> @@ -293,6 +293,8 @@ __condvar_cleanup_waiting (void *arg)
>       * Bit 1 is the clock ID (0 == CLOCK_REALTIME, 1 == CLOCK_MONOTONIC).
>       * Bit 0 is true iff this is a process-shared condvar.
>       * Simple reference count used by both waiters and pthread_cond_destroy.
> +     (If the format of __wrefs is changed, update nptl_lock_constants.pysym
> +      and the pretty printers.)
>     For each of the two groups, we have:
>     __g_refs: Futex waiter reference count.
>       * LSB is true if waiters should run futex_wake when they remove the
> diff --git a/nptl/test-cond-printers.py b/nptl/test-cond-printers.py
> index af0e12e..9e807c9 100644
> --- a/nptl/test-cond-printers.py
> +++ b/nptl/test-cond-printers.py
> @@ -35,7 +35,7 @@ try:
>  
>      break_at(test_source, 'Test status (destroyed)')
>      continue_cmd() # Go to test_status_destroyed
> -    test_printer(var, to_string, {'Status': 'Destroyed'})
> +    test_printer(var, to_string, {'Threads known to still execute a wait function': '0'})
>  
>      continue_cmd() # Exit
>  
> diff --git a/scripts/test_printers_common.py b/scripts/test_printers_common.py
> index c79d7e3..91cf5ec 100644
> --- a/scripts/test_printers_common.py
> +++ b/scripts/test_printers_common.py
> @@ -157,7 +157,7 @@ def init_test(test_bin, printer_files, printer_names):
>  
>      # Load all the pretty printer files.  We're assuming these are safe.
>      for printer_file in printer_files:
> -        test('source {0}'.format(printer_file))
> +        test('source {0}'.format(printer_file), '')
>  
>      # Disable all the pretty printers.
>      test('disable pretty-printer', r'0 of [0-9]+ printers enabled')
> diff --git a/sysdeps/x86/bits/pthreadtypes.h b/sysdeps/x86/bits/pthreadtypes.h
> index 59b1d0d..38bc454 100644
> --- a/sysdeps/x86/bits/pthreadtypes.h
> +++ b/sysdeps/x86/bits/pthreadtypes.h
> @@ -161,8 +161,6 @@ typedef union
>      unsigned int __g1_orig_size;
>      unsigned int __wrefs;
>      unsigned int __g_signals[2];
> -#define __PTHREAD_COND_CLOCK_MONOTONIC_MASK 2
> -#define __PTHREAD_COND_SHARED_MASK 1
>    } __data;
>    char __size[__SIZEOF_PTHREAD_COND_T];
>    __extension__ long long int __align;


-- 
Cheers,
Carlos.


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