From: Jeff Hostetler <git@jeffhostetler.com>
Date: Tue, 28 May 2019 14:57:50 GMT
Subject: Re: [PATCH 0/1] trace2: fix tracing when NO_PTHREADS is defined
Message-ID: <47db8f30-846b-12d0-250e-4bccf1e19c45@jeffhostetler.com>
In-Reply-To: <20190528062804.GE7946@sigill.intra.peff.net>



On 5/28/2019 2:28 AM, Jeff King wrote:
> On Sat, May 25, 2019 at 05:43:55PM +0700, Duy Nguyen wrote:
> 
>>> +typedef struct {
>>> +       void *data;
>>> +       /* extra indirection because setspecific is passed key by value */
>>> +       void **vdata;
>>
>> Ha! I was thinking a separate key->value mapping which is complicated
>> in C. But this works pretty well for a single thread, and it even
>> supports multiple keys.
> 
> I really wish that all of the functions passed the pthread_key_t by
> reference. That would make it possible to define the key as a single
> pointer.
> 
> I'm not sure if pthread_key_t's are meant to be shallow-copyable. I.e.,
> should this work:
> 
>    void foo(pthread_key_t *out)
>    {
> 	pthread_key_t tmp;
> 	pthread_key_create(&tmp, NULL);
> 	*out = tmp;
>    }
>    ...
>    pthread_key_t k;
>    foo(&k);
>    pthread_setspecific(k, some_ptr);
> 
> It does not with my proposed plan, because the pointer in tmp.data went
> out of scope, leaving tmp.vdata (and thus k.vdata) as a dangling
> pointer.
> 
> The code above seems like a vaguely crazy thing to do. But if we want to
> be absolutely paranoid, we'd have to malloc() an extra pointer in the
> create() function, instead of carrying it inside the key. Or just make a
> global "void *thread_specific_data[PTHREAD_KEYS_MAX]" and make each key
> an integer index into it.
> 
> It's pretty clear that they expect one of those two implementations,
> given that POSIX says key creation can report either ENOMEM, or EAGAIN
> if we exceed PTHREAD_KEYS_MAX. :)
> 
> -Peff
> 


Yes, a fixed global void* array should be fine.
Besides there aren't that many calls to pthread_key_create()  (My use in
Trace2 is the second, right?), so just create an arbitrary value for
PTHREAD_KEYS_MAX and be done.


Jeff