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Message-ID: <CAF=dzRMjtCkZn0Vx31Bc14-+cnUDi_fP41=tf7eMShAchE-tNw@mail.gmail.com>
Date: Sat, 12 Mar 2022 10:10:47 -0700
From: Gavin Howard <gavin.d.howard@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: Possible PEBKAC or Bug in musl Semaphores and/or Mutexes
> 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.
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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