Re: [musl] Possible PEBKAC or Bug in musl Semaphores and/or Mutexes

[enh <enh@google.com>]

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On Sat, Mar 12, 2022 at 9:11 AM Gavin Howard <gavin.d.howard@gmail.com>
wrote:

> > This is why condition variables necessarily have an associated
> > predicate (in your case, involving your flag and possibly other
> > state). You can *never* just do pthread_cond_wait. The only correct
> > usage is:
> >
> >         while (!predicate(...)) pthread_cond_wait(mtx,cnd);
> >
> > Correct use of condition variables ensures that you see any relevant
> > changes to the state the predicate is evaluated on, without requiring
> > you to explicitly work out a way to do the same thing with semaphores
> > or other lower-level primitives. It does not matter whatsoever whether
> > there are already waiters when you call signal. If not, they'll
> > necessarily see the state change *before* they wait. The state change
> > cannot happen between evaluation of the predicate and the wait because
> > the calling thread holds the mutex.
>
> Ah, I misunderstood.
>
> > > What this means is that if Main Thread #2 is blocked on waitpid(), then
> > > if another thread creates a child and signals the condition variable,
> > > then after Main Thread #2 returns from waitpid(), it will block on the
> >                                                     ^^^^^^^^^^^^^^^^^^^^
> > > condition variable. If another thread creates another child, sure, it
> >   ^^^^^^^^^^^^^^^^^^
> >
> > No it won't, because it evaluates the predicate that tells it there's
> > something to do before calling wait. If you're not doing that, you're
> > using cond vars in a fundamentally wrong way.
>
> I have now implemented the system using the mutex and changing the
> semaphore to a condition variable with the flag and two other variables
> for how many children there are versus how many children have been
> reaped. We'll see if anything shows up while I run it over and over
> again.
>
> > No, this is absolutely not what's happening. Neither the processor nor
> > the compiler (in general the latter is at least as much of a concern
> > if not more when you have missing/incorrect synchronization) can
> > reorder things like that.
>
> Good to know, thank you.
>
> > On some level they're like any other external function call to a
> > function whose definition the layer optimizing the caller can't see:
> > they must be assumed to be able to store to or load from any memory
> > whose address has been exposed, and thus cannot be reordered
> > whatsoever.
>
> Okay, I wondered if that might be the case.
>
> > Your problem is not mysterious reordering. Your problem is in your
> > program's logic somewhere. Please use the debugging tools at your
> > disposal, especially strace which will probably reveal the problem to
> > you right away (by letting you see the exact sequence of events that
> > happened and trace through it to figure out where it mismatches your
> > expectation).
>
> I did use strace. It revealed nothing out of the ordinary. That's why I
> did not mention it, but I probably should have. I've also used GDB to
> inspect the core dumps. I did try.
>

you might find https://clang.llvm.org/docs/ThreadSanitizer.html useful for
bugs like this.


> Perhaps while I'm learning and making a fool of myself, I'll mention my
> problem with rwlocks.
>
> The relevant code is:
>
> ```
> do
> {
>     bool rel = (strchr((char*) cmd->a, '/') != NULL);
>
>     cont = false;
>
>     // We only need to do something when the cmd is not a relative path.
>     if (!rel)
>     {
>         s = y_strucon_rwlock_rdlock(&r->env.lock);
>         if (y_err(s != y_STATUS_SUCCESS)) goto err;
>
>         exists = y_map_existsStrings_v(&r->env.exec_map, (char*) cmd->a,
>                                        &res);
>
>         // We have to hold the lock until we have copied the result
> because it
>         // could be moved by a write to the map.
>         if (exists)
>         {
>             // Just move the value from res to cmd. I can do this because
>             // the string in res is heap allocated and is not affected by
>             // edits to the map.
>             cmd->len = res->len;
>             cmd->a = res->a;
>
>             // Release the lock as soon as possible.
>             y_strucon_rwlock_rdunlock(&r->env.lock);
>         }
>         else
>         {
>             // Release the lock as soon as possible.
>             y_strucon_rwlock_rdunlock(&r->env.lock);
>
>             <Find executable, error if non-existent, and prepare entry>
>
>             s = y_strucon_rwlock_wrlock(&r->env.lock);
>             if (y_err(s != y_STATUS_SUCCESS))
>             {
>                 y_str_free(&str);
>                 goto err;
>             }
>
>             // Make sure someone didn't get there first.
>             if (!y_map_existsStrings(&r->env.exec_map, (char*) cmd->a))
>             {
>                 s = y_map_insertStrings(&r->env.exec_map, (char*) cmd->a,
>                                         (char*) str.a);
>                 if (s == y_STATUS_ELEM_EXISTS)
>                 {
>                     y_panica("Element already exists");
>                 }
>             }
>
>             y_strucon_rwlock_wrunlock(&r->env.lock);
>
>             // Always free first.
>             y_str_free(&str);
>             y_stackpool_free(pool);
>
>             if (y_err(s != y_STATUS_SUCCESS)) goto err;
>
>             cont = true;
>         }
>     }
> }
> while (cont);
>
> err:
>     <Do some error handling>
> ```
>
> Besides initialization (before any other threads are created) and
> destruction (after all other threads are joined), these are the only
> references to the map in question.
>
> `y_strucon_rwlock_*` functions are just wrappers around POSIX rwlocks.
>
> In this case, I'm not doing anything fancy; it's just rwlocks. However,
> that abort about the element already existing will be triggered
> consistently within about 5 minutes, both on musl and glibc, so probably
> PEBKAC.
>
> If I change the read lock near the beginning with a write lock, I still
> get the same issue. However, if I change the rwlock for a mutex, and
> update all locking and unlocking to match, I don't get the issue.
>
> In this case, the strace shows nothing out of the ordinary that I can
> see.
>
> Yet I can't see how I am doing anything wrong. I've double checked that
> y_map_existsStrings{,_v} do not edit the map at all.
>
> So where's my stupidity on this one?
>
> Gavin Howard
>

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